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C#
1
Print a literal string on standard output
Console.WriteLine("Hello, World!");
2
Loop to execute some code a constant number of times
for (int i = 0; i < 10; i++)
{
    Console.WriteLine("Hello");
}
Alternative implementation:
Console.WriteLine(string.Join(Environment.NewLine, Enumerable.Repeat("Hello", 10)));
Alternative implementation:
Console.WriteLine( string.Concat(Enumerable.Repeat("Hello\n", 10)) );
3
Like a function which doesn't return any value, thus has only side effects (e.g. Print to standard output)
void Finish(string name)
{
    System.Console.WriteLine($"My job here is done. Goodbye, {name}");
}
Alternative implementation:
void Finish(string name) 
    => System.Console.WriteLine($"My job here is done. Goodbye, {name}");
4
Create a function which returns the square of an integer
int Square(int x)
{
    return x * x;
}
Alternative implementation:
int Square(int x) => (int)Math.Pow(x, 2);
Alternative implementation:
double Square(int value) => System.Math.Pow(value, 2);
5
Declare a container type for two floating-point numbers x and y
struct Point
{
    public double x;
    public double y;
};
Alternative implementation:
record Point(double x, double y);
Alternative implementation:
(double x, double y)
6
Do something with each item x of an array-like collection items, regardless indexes.
foreach (var x in items)
{
    DoSomething(x);
}
7
Print each index i with its value x from an array-like collection items
for (int i = 0; i < items.Length; i++)
{
    Console.WriteLine($"{i} {items[i]}");
}
Alternative implementation:
foreach (var (i, x) in items.AsIndexed())
{
    System.Console.WriteLine($"{i}: {x}");
}

public static class Extensions
{
    public static IEnumerable<(int, T)> AsIndexed<T>(
        this IEnumerable<T> source)
    {
        var index = 0;
        foreach (var item in source)
        {
            yield return (index++, item);
        }
    }
}
Alternative implementation:
foreach (var (x, i) in items.Select((x, i)=>(x, i)))
{
    System.Console.WriteLine($"{i}: {x}");
}
8
Create a new map object x, and provide some (key, value) pairs as initial content.
var x = new Dictionary<string, int> {
   {"year", 2019}, {"month", 12}
};
Alternative implementation:
var x = new Dictionary<string, int> {
   ["year"] = 2019,
   ["month"] = 12
};
9
The structure must be recursive because left child and right child are binary trees too. A node has access to children nodes, but not to its parent.
class BinaryTree<T>:IComparable<T>
{
 T value;
 BinaryTree<T> leftChild;
 BinaryTree<T> rightChild;
}
10
Generate a random permutation of the elements of list x
private static Random rng = new Random();  

public static void Shuffle<T>(this IList<T> x)  
{  
    int n = x.Count;  
    while (n > 1) {  
        n--;  
        int k = rng.Next(n + 1);  
        T value = x[k];  
        x[k] = x[n];  
        x[n] = value;  
    }  
}
Alternative implementation:
public static IEnumerable<T> Shuffle<T>(this IEnumerable<T> input)
    => input.OrderBy(_ => Guid.NewGuid());
11
The list x must be non-empty.
x[new Random().Next(x.Count)]
Alternative implementation:
x[Random.Shared.Next(x.Count)]
12
Check if the list contains the value x.
list is an iterable finite container.
list.Contains(item);
13
Access each key k with its value x from an associative array mymap, and print them.
foreach(var entry in map)
{
    Console.WriteLine("Key=" + entry.Key + ", Value=" + entry.Value);
}
14
Pick a random number greater than or equals to a, strictly inferior to b. Precondition : a < b.
Random rng = new();
double pick(double a, double b)
{
    return rng.NextDouble() * (b - a) + a;
}
15
Pick a random integer greater than or equals to a, inferior or equals to b. Precondition : a < b.
Random r = new Random();
return r.Next(a, b + 1);
Alternative implementation:
Random.Shared.Next(a, b + 1)
16
Call a function f on every node of binary tree bt, in depth-first infix order
void Dfs(BinaryTree bt)
{
    if (bt.Left != null) 
        Dfs(bt.Left);

    f(bt);

    if (bt.right != null) 
        Dfs(bt.Right);
}
17
The structure must be recursive. A node may have zero or more children. A node has access to its children nodes, but not to its parent.
class Node<T>
{
 T value;
 List<Node<T>> childNodes;
}
18
Call a function f on every node of a tree, in depth-first prefix order
public static void Dfs(Action<Tree> f, Tree root) {
    f(root);
    foreach(var child in root.Children)
        Dfs(f, child);
} 
19
Reverse the order of the elements of list x.
This may reverse "in-place" and destroy the original ordering.
x.Reverse();
20
Implement a function search which looks for item x in a 2D matrix m.
Return indices i, j of the matching cell.
Think of the most idiomatic way in the language to return the two values at the same time.
(int, int) Search(int[,] m, int x)
{
    for (var i = 0; i <= m.GetUpperBound(0); i++)
        for (var j = 0; j <= m.GetUpperBound(1); j++)
            if (m[i, j] == x)
                return (i, j);

    return (-1, -1);
}
21
Swap the values of the variables a and b
var tmp = a;
a = b;
b = tmp;
Alternative implementation:
(a, b) = (b, a);
22
Extract the integer value i from its string representation s (in radix 10)
long i = Convert.ToInt64(s);
Alternative implementation:
int i = int.Parse(s);
23
Given a real number x, create its string representation s with 2 decimal digits following the dot.
string s = $"{x:F2}";
24
Declare a new string s and initialize it with the literal value "ネコ" (which means "cat" in japanese)
string s = "ネコ";
25
Share the string value "Alan" with an existing running process which will then display "Hello, Alan"
ConcurrentQueue<int> coll = new ConcurrentQueue<int>();
var ts = new CancellationTokenSource();
CancellationToken ct = ts.Token;

Task t2 = Task.Factory.StartNew(() => {
	while (true) {
		Thread.Sleep(250);
		if (ct.IsCancellationRequested) break;
			bool isDequeued = coll.TryDequeue(out int result);
			if (isDequeued) Console.WriteLine(result);
	}});
while (true) {
	int val = Convert.ToInt32(Console.ReadLine());
	if (val == -1) break;
	coll.Enqueue(val);
}
ts.Cancel();
Alternative implementation:
using(var readyEvent = new ManualResetEvent(false))
{
    var thread = new Thread(() =>
    {
        readyEvent.WaitOne();
        Console.WriteLine(value);
    });

    thread.Start();

    value = "Alan";
    readyEvent.Set();

    thread.Join();
}
26
Declare and initialize a matrix x having m rows and n columns, containing real numbers.
var x = new double[m, n];
27
Declare and initialize a 3D array x, having dimensions boundaries m, n, p, and containing real numbers.
var x = new double[m, n, p];
28
Sort the elements of the list (or array-like collection) items in ascending order of x.p, where p is a field of the type Item of the objects in items.
System.Array.Sort(items, Comparer<Item>.Create((a, b) => a.p - b.p));
Alternative implementation:
items.OrderBy(x => x.p)
29
Remove i-th item from list items.
This will alter the original list or return a new list, depending on which is more idiomatic.
Note that in most languages, the smallest valid value for i is 0.
items.RemoveAt(i);
30
Launch the concurrent execution of procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool.
Parallel.For(1, 1001, f);
31
Create the recursive function f which returns the factorial of the non-negative integer i, calculated from f(i-1)
int f(int i)
{
    if (i == 0) return 1;
    return i * f(i - 1);
}
Alternative implementation:
int f(int i) => i == 0 ? 1 : i * f(i - 1)
32
Create function exp which calculates (fast) the value x power n.
x and n are non-negative integers.
long exp(long x, long n){
    if (n == 0)
        return 1;
    if (n == 1)
        return x;
    if (n % 2 == 0)
        return exp(x * x, n / 2);
    else
        return x * exp(x * x, (n - 1) / 2);
}
Alternative implementation:
long exp(long x, long n) {
    return (long) Math.Pow(x, n);
}
33
Assign to the variable x the new value f(x), making sure that no other thread may modify x between the read and the write.
lock (x) {
  x = f(x);	
}
Alternative implementation:
private readonly object m_Sync = new object();

lock(m_Sync)
{
    x = f(x);
}
34
Declare and initialize a set x containing unique objects of type T.
HashSet<T> x = new HashSet<T>();
35
Implement a function compose (A -> C) with parameters f (A -> B) and g (B -> C), which returns composition function g ∘ f
Func<A, C> compose(Func<A, B> f, Func<B, C> g) => a => g(f(a));
36
Implement a function compose which returns composition function g ∘ f for any functions f and g having exactly 1 parameter.
Func<T1, T3> compose<T1, T2, T3>(Func<T1, T2> f, Func<T2, T3> g) => x => g(f(x));
37
Transform a function that takes multiple arguments into a function for which some of the arguments are preset.
Func<A, C> curry<A, B, C>(Func<A, B, C> f, B b) => (A a) => f(a, b);
38
Find substring t consisting in characters i (included) to j (excluded) of string s.
Character indices start at 0 unless specified otherwise.
Make sure that multibyte characters are properly handled.
var t = s.Substring(i, j - i);
39
Set boolean ok to true if string word is contained in string s as a substring, or to false otherwise.
var ok = s.Contains(word);
41
Create string t containing the same characters as string s, in reverse order.
Original string s must remain unaltered. Each character must be handled correctly regardless its number of bytes in memory.
char[] charArray = s.ToCharArray();
Array.Reverse(charArray);
string t = new string(charArray);
Alternative implementation:
string t = string.Create(s.Length, s, static (span, s) =>
{
    s.AsSpan().CopyTo(span);
    span.Reverse();
});
42
Print each item v of list a which is not contained in list b.
For this, write an outer loop to iterate on a and an inner loop to iterate on b.
int gb = 0;
foreach (int v in a)
{
  foreach (int w in b)
  {
    gb = w;
    if (w == v)
      break;
  }
  if (gb == v)
    continue;
  System.Console.WriteLine(v);
}
43
Look for a negative value v in 2D integer matrix m. Print it and stop searching.
bool keepLooping = true;

for(int i = 0; i < m.length && keepLooping; i++)
{
	for(int j = 0; j < m[i].length && keepLooping; j++)
	{
		if(m[i][j] < 0)
		{
			Console.WriteLine(m[i][j]);
			keepLooping = false;
		}
	}
}
Alternative implementation:
foreach (int v in m)
{
    if (v < 0)
    {
        Console.WriteLine(v);
        break;
    }
}
Alternative implementation:
foreach (int[] row in m)
{
    foreach (int v in row)
    {
        if (v < 0)
        {
            Console.WriteLine(v);
            goto DONE;
        }
    }
}
DONE: ;
Alternative implementation:
for (
        int i = 0,
        rows = m.GetLength(0),
        cols = m.GetLength(1); i < rows; i++)
{
    for (int j = 0; j < cols; j++)
    {
        if (m[i, j] < 0)
        {
            Console.WriteLine(m[i, j]);
            i = int.MaxValue - 1; // Break outer loop.
            break;
        }
    }
}
44
Insert element x at position i in list s. Further elements must be shifted to the right.
s.Insert(i, x);
45
Sleep for 5 seconds in current thread, before proceeding with the next instructions.
Thread.Sleep(5000);
Alternative implementation:
Task.Delay(5000);
46
Create string t consisting of the 5 first characters of string s.
Make sure that multibyte characters are properly handled.
string t = s.Substring(0, 5);
47
Create string t consisting in the 5 last characters of string s.
Make sure that multibyte characters are properly handled.
var t = s.Substring(s.Length - 5);
48
Assign to variable s a string literal consisting in several lines of text, including newlines.
string s = @"Huey
Dewey
Louie";
49
Build list chunks consisting in substrings of input string s, separated by one or more space characters.
string[] chunks = s.Split(' ');
50
Write a loop that has no end clause.
while (true)
{
    // Do something
}
51
Determine whether the map m contains an entry for the key k
bool keyExists = m.ContainsKey(key)
52
Determine whether the map m contains an entry with the value v, for some key.
m.ContainsValue(v)
53
Concatenate elements of string list x joined by the separator ", " to create a single string y.
string y = string.Join(", ", x);
54
Calculate the sum s of the integer list or array x.
var s = x.Sum();
55
Create the string representation s (in radix 10) of the integer value i.
string s = i.ToString()
56
Fork-join : launch the concurrent execution of procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool.
Wait for the completion of the 1000 tasks and then print "Finished".
int numTasks = 1000;
Task<int>[] output = new System.Threading.Tasks.Task<int>[numTasks];
for (int i = 0; i < numTasks; i++)
{
  output[i] = Task.Factory.StartNew(
                   new Func <object, int>(LongRunningOperation), 2000);
}
Task.WaitAll(output);

int LongRunningOperation(object objMs)
{
  int ms = (int)objMs;
  Thread.Sleep(ms);
  return ms;
}
Alternative implementation:
await Task.WhenAll(Enumerable.Range(0, 1000).Select(i => Task.Run(() => f(i))));
Console.WriteLine("Finished");

static async Task f(int value)
{
    // Do something with value. Delay to simulate work
    await Task.Delay(500);
}
57
Create the list y containing the items from the list x that satisfy the predicate p. Respect the original ordering. Don't modify x in-place.
var y = x.FindAll(p);
Alternative implementation:
var y = x.Where(p).ToList();
58
Create string lines from the content of the file with filename f.
string lines = File.ReadAllText(f);
59
Print the message "x is negative" to standard error (stderr), with integer x value substitution (e.g. "-2 is negative").
Console.Error.WriteLine($"{x} is negative");
60
Assign to x the string value of the first command line parameter, after the program name.
static void Main(string[] args)
{
    string x = args[0];
}
61
Assign to variable d the current date/time value, in the most standard type.
DateTime d = DateTime.Now;
62
Set i to the first position of string y inside string x, if exists.

Specify if i should be regarded as a character index or as a byte index.

Explain the behavior when y is not contained in x.
var i = x.IndexOf(y);
63
Assign to x2 the value of string x with all occurrences of y replaced by z.
Assume occurrences of y are not overlapping.
string x2 = x.Replace(y, z, StringComparison.Ordinal);
64
Assign to x the value 3^247
var x = BigInteger.Pow(3, 247);
65
From the real value x in [0,1], create its percentage string representation s with one digit after decimal point. E.g. 0.15625 -> "15.6%"
string s = $"{x:p1}";
66
Calculate the result z of x power n, where x is a big integer and n is a positive integer.
var z = BigInteger.Pow(x, n);
67
Calculate binom(n, k) = n! / (k! * (n-k)!). Use an integer type able to handle huge numbers.
public BigInteger binom(int n, int k)
{
	return factorial(n)/(factorial(k) * factorial(n-k));
}

public BigInteger factorial(int x)
{
	BigInteger result = 1;
	for(int i=1;i<=x;i++)
	{
	result = result * i;
	}
	return result;
}
68
Create an object x to store n bits (n being potentially large).
new BitArray(n);
69
Use seed s to initialize a random generator.

If s is constant, the generator output will be the same each time the program runs. If s is based on the current value of the system clock, the generator output will be different each time.
var random = new Random(s);
70
Get the current datetime and provide it as a seed to a random generator. The generator sequence will be different at each run.
Random rng = new Random(DateTime.Now.Day);
71
Basic implementation of the Echo program: Print all arguments except the program name, separated by space, followed by newline.
The idiom demonstrates how to skip the first argument if necessary, concatenate arguments as strings, append newline and print it to stdout.
public class Program
{
    public static void Main(string[] args)
    {
        Console.WriteLine(string.Join(" ", args));
    }
}
74
Compute the greatest common divisor x of big integers a and b. Use an integer type able to handle huge numbers.
int gcd(int a, int b)
{
  while (b != 0)
  {
    int t = b;
    b = a % t;
    a = t;
  }
  return a;
}
Alternative implementation:
int gcd(int a, int b)
{
  if (b == 0) 
    return a;
  else 
    return gcd(b, a % b);
}
75
Compute the least common multiple x of big integers a and b. Use an integer type able to handle huge numbers.
int gcd(int a, int b)
{
  while (b != 0)
  {
    int t = b;
    b = a % t;
    a = t;
  }
  return a;
}

int lcm(int a, int b)
{
  if (a == 0 || b == 0)
    return 0;
  return (a * b) / gcd(a, b);
}

int x = lcm(140, 72);
76
Create the string s of integer x written in base 2.

E.g. 13 -> "1101"
String s = Convert.ToString(x,2);
Alternative implementation:
String s = Convert.ToString(x,2).PadLeft(16, '0');
77
Declare a complex x and initialize it with value (3i - 2). Then multiply it by i.
var x = new Complex(-2, 3);
x *= Complex.ImaginaryOne;
78
Execute a block once, then execute it again as long as boolean condition c is true.
do
{
    stuff();
} while(c);
79
Declare the floating point number y and initialize it with the value of the integer x .
float y = x;
80
Declare integer y and initialize it with the value of floating point number x . Ignore non-integer digits of x .
Make sure to truncate towards zero: a negative x must yield the closest greater integer (not lesser).
int y = (int)x;
81
Declare integer y and initialize it with the rounded value of floating point number x .
Ties (when the fractional part of x is exactly .5) must be rounded up (to positive infinity).
long y = (long)Math.Round(x);
82
Find how many times string s contains substring t.
Specify if overlapping occurrences are counted.
int SubstringCount(string s, string t, bool allowOverlap = false)
{
  int p = 0;
  int tl = allowOverlap ? 1 : t.Length;
  int cnt = 0;

  while (1 == 1)
  {
    p = s.IndexOf(t, p);
    if (p == -1) break;
    p += tl;
    cnt++;
  }
  return cnt;
}
83
Declare regular expression r matching strings "http", "htttp", "httttp", etc.
var r = new Regex("htt+p");
84
Count number c of 1s in the integer i in base 2.

E.g. i=6 → c=2
var c = BitOperations.PopCount((uint)i);
Alternative implementation:
public static int BitCount(int i)
{
    var c = 0;
    while (n != 0)
    {
        c++;
        n &= (n - 1); //walking through all the bits which are set to one
    }

    return c;
}
85
Write boolean function addingWillOverflow which takes two integers x, y and return true if (x+y) overflows.

An overflow may be above the max positive value, or below the min negative value.
bool addingWillOverflow(int x, int y)
{
  bool willOverflow = false;

  if (x > 0 && y > 0)
    if (y > (int.MaxValue - x)) willOverflow = true;

  if (x < 0 && y < 0)
    if (y < (int.MinValue - x)) willOverflow = true;

  return willOverflow;
}
86
Write boolean function multiplyWillOverflow which takes two integers x, y and return true if (x*y) overflows.

An overflow may be above the max positive value, or below the min negative value.
public bool WillOverwflow(int x, int y) => int.MaxValue / x < y;
Alternative implementation:
public bool multiplyWillOverflow(int x, int y) 
{
	if (x == 0)
		return false;

	if (y > int.MaxValue / x)
		return true;

	if (y < int.MinValue / x)
		return true;

	return false;
}
87
Exit immediately.
If some extra cleanup work is executed by the program runtime (not by the OS itself), describe it.
Environment.Exit(0)
88
Create a new bytes buffer buf of size 1,000,000.
var buf = new byte[1000000];
89
You've detected that the integer value of argument x passed to the current function is invalid. Write the idiomatic way to abort the function execution and signal the problem.
throw new ArgumentException(nameof(×));
90
Expose a read-only integer x to the outside world while being writable inside a structure or a class Foo.
class Foo
{
	public int x { get; private set; }
}
91
Read from the file data.json and write its content into the object x.
Assume the JSON data is suitable for the type of x.
JObject x = JObject.Parse(File.ReadAllText("data.json"));
92
Write the contents of the object x into the file data.json.
await File.WriteAllTextAsync("data.json", JsonSerializer.Serialize(x));	
93
Implement procedure control which receives one parameter f, and runs f.
T control(Func<T> f) {
	return f();
}
94
Print the name of the type of x. Explain if it is a static type or dynamic type.

This may not make sense in all languages.
System.Console.WriteLine( x.GetType() );
95
Assign to variable x the length (number of bytes) of the local file at path.
var filePath = "Sample.txt";
var fileInfo = new FileInfo(filePath);
var _x = fileInfo.Length;

Alternative implementation:
long? FileSize(string path) {
    if (!File.Exists(path)) {
        return null;
    }
    return (new FileInfo(filePath)).Length;
}
96
Set boolean b to true if string s starts with prefix prefix, false otherwise.
bool b = s.StartsWith(prefix);
97
Set boolean b to true if string s ends with string suffix, false otherwise.
bool b = s.EndsWith(suffix);
Alternative implementation:
var _b = _s.EndsWith(_suffix);
98
Convert a timestamp ts (number of seconds in epoch-time) to a date with time d. E.g. 0 -> 1970-01-01 00:00:00
var d = DateTimeOffset.FromUnixTimeSeconds(ts).UtcDateTime;
99
Assign to the string x the value of the fields (year, month, day) of the date d, in format YYYY-MM-DD.
string x = d.ToString("yyyy-MM-dd");
100
Sort elements of array-like collection items, using a comparator c.
var orderdEnumerable = x.OrderBy(y => y, c);
Alternative implementation:
Array.Sort(items, c);
Alternative implementation:
items.Sort(c);
101
Make an HTTP request with method GET to URL u, then store the body of the response in string s.
var client = new HttpClient();
s = await client.GetStringAsync(u);
103
Read from the file data.xml and write its contents into the object x.
Assume the XML data is suitable for the type of x.
XDocument x = XDocument.Load("data.xml");

105
Assign to the string s the name of the currently executing program (but not its full path).
var path=Environment.CommandLine;
var s=Path.GetFileName(path);
106
Assign to string dir the path of the working directory.
(This is not necessarily the folder containing the executable itself)
string path = Directory.GetCurrentDirectory();
107
Assign to string dir the path of the folder containing the currently running executable.
(This is not necessarily the working directory, though.)
dir = AppDomain.CurrentDomain.BaseDirectory;
109
Set n to the number of bytes of a variable t (of type T).
int n = sizeof(T);
Alternative implementation:
int n;
unsafe
{
    n = sizeof(T);
}
Alternative implementation:
int n = Marshal.SizeOf(t);
110
Set the boolean blank to true if the string s is empty, or null, or contains only whitespace ; false otherwise.
bool blank = string.IsNullOrWhiteSpace(s);
Alternative implementation:
bool blank = string.IsNullOrWhiteSpace(s);
111
From current process, run program x with command-line parameters "a", "b".
string[] args = { "a", "b"};
Process.Start(x, string.Join(" ", args));
112
Print each key k with its value x from an associative array mymap, in ascending order of k.
SortedDictionary<string, string> myMap;
foreach (var item in myMap)
{
  Console.WriteLine($"{item.Key}={item.Value}");
}
	
113
Print each key k with its value x from an associative array mymap, in ascending order of x.
Note that multiple entries may exist for the same value x.
Dictionary<String, String> mymap = new Dictionary<String, String>();
  
foreach(KeyValuePair<string, string> a in mymap.OrderBy(x => x.Value))
{
  Console.WriteLine("Key = {0}, Value = {1}", a.Key, a.Value);
}
115
Set boolean b to true if date d1 is strictly before date d2 ; false otherwise.
bool b = d1 < d2;
116
Remove all occurrences of string w from string s1, and store the result in s2.
string s2 = s1.Replace(w, string.Empty);
117
Set n to the number of elements of the list x.
int n = x.Count;
Alternative implementation:
int n = x.Length;
118
Create the set y from the list x.
x may contain duplicates. y is unordered and has no repeated values.
var y = new HashSet<T>(x);
119
Remove duplicates from the list x.
Explain if the original order is preserved.
var uniques = x.Distinct().ToList();
120
Read an integer value from the standard input into the variable n
n = int.Parse(Console.ReadLine());
122
Create an enumerated type Suit with 4 possible values SPADES, HEARTS, DIAMONDS, CLUBS.
enum Suit
{
	Spades,
	Hearts,
	Diamonds,
	Clubs
}
123
Verify that predicate isConsistent returns true, otherwise report assertion violation.
Explain if the assertion is executed even in production environment or not.
var result = isConsistent();
Debug.Assert(result);
Alternative implementation:
var result = isConsistent();
Trace.Assert(result);
124
Write the function binarySearch which returns the index of an element having the value x in the sorted array a, or -1 if no such element exists.
public static int binarySearch<T>(List<T> a, T x)
{
    var result = a.BinarySearch(x);
    return result >= 0 ? result : -1;
}
Alternative implementation:
public static int binarySearch<T>(T[] a, T x)
{
    var result = Array.BinarySearch<T>(a, x);
    return result >= 0 ? result : -1;
}
125
measure the duration t, in nanoseconds, of a call to the function foo. Print this duration.
var stopwatch = new Stopwatch();
stopwatch.Start();
foo();
stopwatch.Stop();
var t = stopwatch.ElapsedMilliseconds;
126
Write a function foo that returns a string and a boolean value.
public Tuple<string, bool> foo_PreCSharp7()
{
    // Only accessed via .Item1 and .Item2
    return new Tuple<string, bool>("string", true);
}

public (string, bool) foo_CSharp7UnnamedTuples()
{
    // Only accessed via .Item1 and .Item2
    return ("string", true);
}

public (string NamedStringArg, bool NamedBooleanArg) foo_CSharp7()
{
    // Can be accessed via .NamedStringArg or .NamedBooleanArg
    return ("string", true);
}
131
Execute f1 if condition c1 is true, or else f2 if condition c2 is true, or else f3 if condition c3 is true.
Don't evaluate a condition when a previous condition was true.
if (c1)
{
    f1();
}
else if (c2)
{
    f2();
}
else if (c3)
{
    f3();
}
132
Run procedure f, and return the duration of the execution of f.
var sw=new Stopwatch();
sw.Start();
f();
sw.Stop();
var duration=sw.Elapsed;

133
Set boolean ok to true if string word is contained in string s as a substring, even if the case doesn't match, or to false otherwise.
bool ok = s.ToLower().Contains(word.ToLower());
Alternative implementation:
ok = s.Contains(word, StringComparison.CurrentCultureIgnoreCase);
134
Declare and initialize a new list items, containing 3 elements a, b, c.
var items = new List<T>{a,b,c};
Alternative implementation:
T[] items = new T[] { a, b, c };
135
Remove at most 1 item from list items, having value x.
This will alter the original list or return a new list, depending on which is more idiomatic.
If there are several occurrences of x in items, remove only one of them. If x is absent, keep items unchanged.
items.Remove(x);
136
Remove all occurrences of value x from list items.
This will alter the original list or return a new list, depending on which is more idiomatic.
items.RemoveAll(r => r == x);
137
Set the boolean b to true if the string s contains only characters in the range '0'..'9', false otherwise.
bool b = s.All(char.IsDigit);
138
Create a new temporary file on the filesystem.
string file = Path.GetTempFileName();
139
Create a new temporary folder on filesystem, for writing.
string newDir = Path.GetTempPath() + Guid.NewGuid();
Directory.CreateDirectory(newDir);
140
Delete from map m the entry having key k.

Explain what happens if k is not an existing key in m.
map.Remove(key);
141
Iterate in sequence over the elements of the list items1 then items2. For each iteration print the element.
items1.ForEach(Console.WriteLine);
items2.ForEach(Console.WriteLine);
Alternative implementation:
foreach (var item in items1.Concat(items2))
{
    Console.WriteLine(item);
}
142
Assign to string s the hexadecimal representation (base 16) of integer x.

E.g. 999 -> "3e7"
String s = x.ToString("x")
143
Iterate alternatively over the elements of the list items1 and items2. For each iteration, print the element.

Explain what happens if items1 and items2 have different size.
for(int i = 0; i < Math.Max(items1.Count, items2.Count); i++)
{
  if (i < items1.Count) Console.WriteLine(items1[i]);
  if (i < items2.Count) Console.WriteLine(items2[i]);
}
144
Set boolean b to true if file at path fp exists on filesystem; false otherwise.

Beware that you should never do this and then in the next instruction assume the result is still valid, this is a race condition on any multitasking OS.
bool b = File.Exists(fp);
145
Print message msg, prepended by current date and time.

Explain what behavior is idiomatic: to stdout or stderr, and what the date format is.
Console.WriteLine($"[{DateTime.Now}] {msg}");
146
Extract floating point value f from its string representation s
var f=float.Parse(s);
147
Create string t from string s, keeping only ASCII characters
string t = Regex.Replace(s, @"[^\u0000-\u007F]+", string.Empty);
148
Read a list of integer numbers from the standard input, until EOF.
string input = Console.ReadLine();
string[] intlist = input.Split(new char[] {',', ' '});
	
foreach(string item in intlist)
{
  Console.WriteLine(Convert.ToInt32(item));
}
149
As an exception, this content is not under license CC BY-SA 3.0 like the rest of this website.
 
150
Remove last character from string p, if this character is a slash /.
p = p.TrimEnd('/');
151
Remove last character from string p, if this character is the file path separator of current platform.

Note that this also transforms unix root path "/" into the empty string!
p.TrimEnd(Path.DirectorySeparatorChar);
152
Create string s containing only the character c.
string s = c.ToString();
Alternative implementation:
var s = new string(c, 1);
153
Create the string t as the concatenation of the string s and the integer i.
var t = $"{s} {i}";
154
Find color c, the average between colors c1, c2.

c, c1, c2 are strings of hex color codes: 7 chars, beginning with a number sign # .
Assume linear computations, ignore gamma corrections.
Color color1 = ColorTranslator.FromHtml(c1);
Color color2 = ColorTranslator.FromHtml(c2);
c = string.Format($"#{((color1.R + color2.R) / 2):X2}{((color1.G + color2.G) / 2):X2}{((color1.B + color2.B) / 2):X2}");
155
Delete from filesystem the file having path filepath.
File.Delete(filepath);
156
Assign to the string s the value of the integer i in 3 decimal digits. Pad with zeros if i < 100. Keep all digits if i1000.
string s = string.Format("{0:000}",i);
157
Initialize a constant planet with string value "Earth".
const string _planet = "Earth";
158
Create a new list y from randomly picking exactly k elements from list x.

It is assumed that x has at least k elements.
Each element must have same probability to be picked.
Each element from x must be picked at most once.
Explain if the original ordering is preserved or not.
Random rnd = new Random();
List<int> y = x.OrderBy(r => rnd.Next()).Take(k).ToList();
160
Execute f32() if platform is 32-bit, or f64() if platform is 64-bit.
This can be either a compile-time condition (depending on target) or a runtime detection.
if(Environment.Is64BitOperatingSystem)
{
    f64();
}
else
{
    f32();
}
161
Multiply all the elements of the list elements by a constant c
elements.Select(x => x*c)
162
execute bat if b is a program option and fox if f is a program option.
void Main(string[] args)
{
  if (args.Contains("b")) bat();
  else if (args.Contains("f")) fox();
}
163
Print all the list elements, two by two, assuming list length is even.
for(int i = 0; i < list.Count; i += 2) {
  Console.WriteLine(string.Format("{0}, {1}", list[i], list[i + 1]));
}
Alternative implementation:
foreach (var chunk in list.Chunk(2))
{
    Console.WriteLine(string.Join(' ', chunk));
}
164
Open the URL s in the default browser.
Set boolean b to indicate whether the operation was successful.
var b = true;
try
{
    Process.Start(new ProcessStartInfo()
    {
        FileName = s,
        UseShellExecute = true,
    });
}
catch { b = false; }
165
Assign to variable x the last element of list items.
var x = items.LastOrDefault();
Alternative implementation:
var x = items[items.Count-1];
Alternative implementation:
var x = items[^1];
166
Create the list ab containing all the elements of the list a, followed by all the elements of the list b.
var list1 = new List<int>(){1,2,3};
var list2 = new List<int>(){4,5,6};

var list3 = list1.Concat(list2);
167
Create string t consisting of string s with its prefix p removed (if s starts with p).
var t = s.TrimStart(p);
168
Create string t consisting of string s with its suffix w removed (if s ends with w).
string t = s.TrimEnd(w);
169
Assign to integer n the number of characters of string s.
Make sure that multibyte characters are properly handled.
n can be different from the number of bytes of s.
int n = s.Length;
170
Set n to the number of elements stored in mymap.

This is not always equal to the map capacity.
n = mymap.Count;
171
Append element x to the list s.
s.Add(x);
172
Insert value v for key k in map m.
m[k] = v;
173
Number will be formatted with a comma separator between every group of thousands.
$"{1000:n}"
174
Make a HTTP request with method POST to URL u
new HttpClient().PostAsync(u, content);
175
From array a of n bytes, build the equivalent hex string s of 2n digits.
Each byte (256 possible values) is encoded as two hexadecimal characters (16 possible values per digit).
string s = BitConverter.ToString(a);
s = s.Replace("-", string.Empty);
176
From hex string s of 2n digits, build the equivalent array a of n bytes.
Each pair of hexadecimal characters (16 possible values per digit) is decoded into one byte (256 possible values).
byte[] a = new byte[s.Length/2];
for (int i = 0, h = 0; h < s.Length; i++, h += 2)
{
  a[i] = (byte) Int32.Parse(s.Substring(h, 2), System.Globalization.NumberStyles.HexNumber);
}
177
Construct a list L that contains all filenames that have the extension ".jpg" , ".jpeg" or ".png" in directory D and all it's subdirectories.
string D = @"C:\The\Search\Path";

// Assumes that the file system is case insensitive
HashSet<string> exts = new HashSet<string>(StringComparer.CurrentCultureIgnoreCase) { "jpg", "jpeg", "png" };
IEnumerable<string> L =
Directory
    .EnumerateFiles(D, "*", SearchOption.AllDirectories)
    .Where(currentFile => exts.Contains(Path.GetExtension(currentFile)));
178
Set boolean b to true if if the point with coordinates (x,y) is inside the rectangle with coordinates (x1,y1,x2,y2) , or to false otherwise.
Describe if the edges are considered to be inside the rectangle.
b = new Rect(new Point(x1, y1), new Point(x2, y2)).Contains(x, y);
179
Return the center c of the rectangle with coördinates(x1,y1,x2,y2)
class Point
{
	public float X { get; }
	public float Y { get; }

	public Point(float x, float y)
	{
		X = x;
		Y = y;
	}
}

class Rectangle
{
	Point Point1;
	Point Point2;

	public Rectangle(Point point1, Point point2)
	{
		Point1 = point1;
		Point2 = point2;
	}

	public Point GetCenter()
	{
		return new Point(
		(Point1.X + Point2.X) / 2,
		(Point1.Y + Point2.Y) / 2
		);
	}
}
180
Create list x containing the contents of directory d.

x may contain files and subfolders.
No recursive subfolder listing.
var files = Directory.GetFiles(dirPath);
182
Output the source of the program.
public class Quine{public static void Main(){var s="public class Quine{{public static void Main(){{var s={0}{1}{0};System.Console.Write(s,(char)34,s);}}}}";System.Console.Write(s,(char)34,s);}}
183
Make a HTTP request with method PUT to URL u
new HttpClient().PutAsync(u, content);
184
Assign to variable t a string representing the day, month and year of the day after the current date.
var t = DateTime.Today.AddDays(1).ToShortDateString();
185
Schedule the execution of f(42) in 30 seconds.
Task.Delay(TimeSpan.FromSeconds(30))
    .ContinueWith(_ => f(42));
186
Exit a program cleanly indicating no error to OS
Environment.Exit(0);
189
Produce a new list y containing the result of function T applied to all elements e of list x that match the predicate P.
var y = x.Where(P).Select(T).ToList();
191
Given a one-dimensional array a, check if any value is larger than x, and execute the procedure f if that is the case
if (a.Any(e => e > x))
    f();
192
Declare a real variable a with at least 20 digits; if the type does not exist, issue an error at compile time.
decimal a = 1234567890.12345678901m;
196
Given an integer array a of size n, pass the first, third, fifth and seventh, ... up to the m th element to a routine foo which sets all these elements to 42.
void Foo(out int element)
{
    element = 42;
}

for (int i = 0; i < m; i += 2)
{
    Foo(out a[i]);
}
197
Retrieve the contents of file at path into a list of strings lines, in which each element is a line of the file.
        public List<string> GetLines(string _path)
        {
            return File.ReadAllLines(_path).ToList();
        }
198
Abort program execution with error condition x (where x is an integer value)
System.Environment.Exit(x);
199
Truncate a file F at the given file position.
var F = new FileStream("F", FileMode.Open)
// advance into F here
F.SetLength(F.Position);
200
Returns the hypotenuse h of the triangle where the sides adjacent to the square angle have lengths x and y.
double hypo(double x, double y)
{
    return Math.Sqrt(Math.Pow(x, 2) + Math.Pow(y, 2));
}
202
Calculate the sum of squares s of data, an array of floating point values.
var s = data.Sum(x => x * x);
203
Calculate the mean m and the standard deviation s of the list of floating point values data.
var m = data.Average();
var s = CalculateStdDev(data);

float CalculateStdDev(IEnumerable<float> values)
{
	double ret = 0;

	if (values.Count() > 0)
	{
		double avg = values.Average();
	      	double sum = values.Sum(d => Math.Pow(d - avg, 2));
	      	ret = Math.Sqrt((sum) / values.Count()-1);
	}
	return (float)ret;
}
205
Read an environment variable with the name "FOO" and assign it to the string variable foo. If it does not exist or if the system does not support environment variables, assign a value of "none".
string foo = Environment.GetEnvironmentVariable("FOO");
if (string.IsNullOrEmpty(foo)) foo = "none";
206
Execute different procedures foo, bar, baz and barfl if the string str contains the name of the respective procedure. Do it in a way natural to the language.
switch (str)
{
    case nameof(Foo):
        Foo();
        break;
    case nameof(Bar):
        Bar();
        break;
    case nameof(Baz):
        Baz();
        break;
    case nameof(Barfl):
        Barfl();
        break;
}
211
Create the folder at path on the filesystem
Directory.CreateDirectory(path)
212
Set boolean b to true if path exists on the filesystem and is a directory; false otherwise.
bool b = Directory.Exists(path);
214
Append extra character c at the end of string s to make sure its length is at least m.
The length is the number of characters, not the number of bytes.
s = s.PadRight(m, c);
215
Prepend extra character c at the beginning of string s to make sure its length is at least m.
The length is the number of characters, not the number of bytes.
s = s.PadLeft(m, c);
218
Create the list c containing all unique elements that are contained in both lists a and b.
c should not contain any duplicates, even if a and b do.
The order of c doesn't matter.
c = a.Intersect(b).ToList();
219
Create string t from the value of string s with each sequence of spaces replaced by a single space.

Explain if only the space characters will be replaced, or the other whitespaces as well: tabs, newlines.
string t = Regex.Replace(s, " +", " ");
Alternative implementation:
string t = Regex.Replace(s, @"\s+", " ");
220
Create t consisting of 3 values having different types.

Explain if the elements of t are strongly typed or not.
var t = (2.5f, "foo", true);
221
Create string t from string s, keeping only digit characters 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.
var t = string.Concat(s.Where(c => char.IsDigit(c)));
222
Set i to the first index in list items at which the element x can be found, or -1 if items does not contain x.
var i = items.IndexOf(x)
223
Loop through list items checking a condition. Do something else if no matches are found.

A typical use case is looping through a series of containers looking for one that matches a condition. If found, an item is inserted; otherwise, a new container is created.

These are mostly used as an inner nested loop, and in a location where refactoring inner logic into a separate function reduces clarity.
if (!items.Any(i => MatchesCondition(i)))
{
	DoSomethingElse();
}
224
Insert element x at the beginning of list items.
items.Insert(0, x);
Alternative implementation:
items = items.Prepend(x).ToList();
225
Declare an optional integer argument x to procedure f, printing out "Present" and its value if it is present, "Not present" otherwise
void f(int? x = null)
{
    Console.WriteLine(x.HasValue ? $"Present {x}" : "Not Present");
}
226
Remove the last element from list items.
items.RemoveAt(items.Count - 1);
227
Create the new list y containing the same elements as the list x.

Subsequent modifications of y must not affect x (except for the contents referenced by the elements themselves if they contain pointers).
List<T> y = x.ToList();
Alternative implementation:
List<Int32> y = new List<Int32>(x);
228
Copy the file at path src to dst.
File.Copy(src, dst, true); 
231
Set b to true if the byte sequence s consists entirely of valid UTF-8 character code points, false otherwise.
var encoding = new UTF8Encoding(false, true);
bool b;
try
{
    encoding.GetCharCount(s);
    b = true;
}
catch (DecoderFallbackException)
{
    b = false;
}
234
Assign to string s the standard base64 encoding of the byte array data, as specified by RFC 4648.
string s = Convert.ToBase64String(data);
235
Assign to byte array data the bytes represented by the base64 string s, as specified by RFC 4648.
byte[] bytes = Convert.FromBase64String(s);
237
Assign to c the result of (a xor b)
int c = a ^ b;
238
Write in a new byte array c the xor result of byte arrays a and b.

a and b have the same size.
var c = a.Zip(b, (l, r) => (byte)(l ^ r)).ToArray();
239
Assign to string x the first word of string s consisting of exactly 3 digits, or the empty string if no such match exists.

A word containing more digits, or 3 digits as a substring fragment, must not match.
string x = Regex.Match(s, @"\b\d\d\d\b").Value;
Alternative implementation:
var re = new Regex(@"\b\d\d\d\b");
string x = re.Match(s).Value;
241
Explicitly decrease the priority of the current process, so that other execution threads have a better chance to execute now. Then resume normal execution and call function busywork.
Thread.Yield();
busywork();
242
Call a function f on each element e of a set x.
foreach(var e in x)
    f(e);
243
Print the contents of the list or array a on the standard output.
a.ForEach(Console.WriteLine);
Alternative implementation:
Console.WriteLine( string.Join(", ", a) );
244
Print the contents of the map m to the standard output: keys and values.
Console.WriteLine(string.Join(Environment.NewLine, m));
246
Set c to the number of distinct elements in list items.
int c = items.Distinct().Count();
247
Remove all the elements from list x that don't satisfy the predicate p, without allocating a new list.
Keep all the elements that do satisfy p.

For languages that don't have mutable lists, refer to idiom #57 instead.
x.RemoveAll(item => !p(item));
249
Define variables a, b and c in a concise way.
Explain if they need to have the same type.
var (a, b, c) = (42, "hello", 5.0);
252
Assign to the variable x the value "a" if calling the function condition returns true, or the value "b" otherwise.
x = condition() ? "a" : "b";
255
Print the values of the set x to the standard output.
The order of the elements is irrelevant and is not required to remain the same next time.
foreach (var el in x)
{
    Console.WriteLine(el);
}
Alternative implementation:
Console.WriteLine(string.Join(Environment.NewLine, x));
256
Print the numbers 5, 4, ..., 0 (included), one line per number.
for (int i = 5; i >= 0; i--)
{
    Console.WriteLine(i);
}
257
Print each index i and value x from the list items, from the last down to the first.
for(int i = items.Count - 1; i >= 0; i--)
{
    Console.WriteLine($"Index = {i}, Item = {items[i]}");
}
258
Convert the string values from list a into a list of integers b.
var b = a.Select(i => int.Parse(i)).ToList();
259
Build the list parts consisting of substrings of the input string s, separated by any of the characters ',' (comma), '-' (dash), '_' (underscore).
var parts = Regex.Split(s, "[,_-]");
Alternative implementation:
var parts = s.Split(',', '-', '_');
260
Declare a new list items of string elements, containing zero elements
var items = new List<string>();
Alternative implementation:
var items = Enumerable.Empty<string>();
261
Assign to string x the value of fields (hours, minutes, seconds) of date d, in format HH:MM:SS.
var x = date.ToString("HH:mm:ss");
262
Assign to t the number of trailing 0 bits in the binary representation of the integer n.

E.g. for n=112, n is 1110000 in base 2 ⇒ t=4
var t = Convert.ToString(n, 2)
    .Reverse()
    .TakeWhile(i => i == '0')
    .Count();
Alternative implementation:
int t = 0;

if(n != 0)
{
    while((n & 1) == 0)
    {
        t++;
        n >>= 1;
    }
}
else
{
    t = 8 * sizeof(int);
}
266
Assign to the string s the value of the string v repeated n times, and write it out.

E.g. v="abc", n=5 ⇒ s="abcabcabcabcabc"
var sb = new StringBuilder();
for(var i = 0; i < n; i++)
	sb.Append(v);

var s = sb.ToString();
Alternative implementation:
var s = string.Concat(Enumerable.Repeat(v, n));
Alternative implementation:
string s = string.Create(v.Length * n, (v, n), static (span, state) =>
{
    var (v, n) = state;
    var originSpan = v.AsSpan();
    for (var i = 0; i < n; i++)
    {
        var subSpan = span[(i * originSpan.Length)..];
        originSpan.CopyTo(subSpan);
    }
});
267
Declare an argument x to a procedure foo that can be of any type. If the type of the argument is a string, print it, otherwise print "Nothing."

Test by passing "Hello, world!" and 42 to the procedure.
void foo(object x)
{
    if (x is string s)
    {
        Console.WriteLine(s);
    }
    else
    {
        Console.WriteLine("Nothing.");
    }
}

foo("Hello, world!");
foo(42);
Alternative implementation:
void foo<T>(T x)
{
    if (x is string s)
    {
        Console.WriteLine(s);
    }
    else
    {
        Console.WriteLine("Nothing.");
    }
}

foo("Hello, world!");
foo(42);
268
Define a type vector containing three floating point numbers x, y, and z. Write a user-defined operator x that calculates the cross product of two vectors a and b.
record Vector(double X, double Y, double Z)
{
    public static Vector operator *(Vector a, Vector b)
    {
        return new(
            a.Y*b.Z - a.Z*b.Y,
            a.Z*b.X - a.X*b.Z,
            a.X*b.Y - a.Y*b.X
        );
    }
}
269
Given the enumerated type t with 3 possible values: bike, car, horse.
Set the enum value e to one of the allowed values of t.
Set the string s to hold the string representation of e (so, not the ordinal value).
Print s.
T e = T.Horse;
string s = e.ToString();
Console.WriteLine(s);
271
If a variable x passed to procedure tst is of type foo, print "Same type." If it is of a type that extends foo, print "Extends type." If it is neither, print "Not related."
public static void tst(Object x) {
  if (x.GetType() == typeof(foo))
    Console.WriteLine("Same type.");
  else if (x.GetType().IsAssignableTo(typeof(foo)))
    Console.WriteLine("Extends type.");
  else
    Console.WriteLine("Not related.");			
}
272
Fizz buzz is a children's counting game, and a trivial programming task used to affirm that a programmer knows the basics of a language: loops, conditions and I/O.

The typical fizz buzz game is to count from 1 to 100, saying each number in turn. When the number is divisible by 3, instead say "Fizz". When the number is divisible by 5, instead say "Buzz". When the number is divisible by both 3 and 5, say "FizzBuzz"
string FizzBuzzOrNumber(int number) => number switch {
    var n when n % 15 == 0 => "FizzBuzz",
    var n when n % 3 == 0 => "Fizz",
    var n when n % 5 == 0 => "Buzz",
    var n => n.ToString()
};

for (var i = 1; i <= 100; i++)
{
    Console.WriteLine(FizzBuzzOrNumber(i));
}
274
Create the string t from the string s, removing all the spaces, newlines, tabulations, etc.
var t = new string(s.Where(c => !Char.IsWhiteSpace(c)).ToArray());
275
From the string s consisting of 8n binary digit characters ('0' or '1'), build the equivalent array a of n bytes.
Each chunk of 8 binary digits (2 possible values per digit) is decoded into one byte (256 possible values).
var a = Enumerable.Range(0, s.Length / 8)
        .Select(i => s.Substring(i * 8, 8).ToCharArray())
        .Select(block => (byte)block.Aggregate(0, (acc, c) => (acc << 1) + (c - '0')))
        .ToArray();
299
Write a line of comments.

This line will not be compiled or executed.
// This is a comment
302
Give the integer x = 8, assign to the string s the value "Our sun has 8 planets", where the number 8 was evaluated from x.
var s = $"Our sun has {x} planets";
304
Create the array of bytes data by encoding the string s in UTF-8.
byte[] data = Encoding.UTF8.GetBytes(s);