C# Reference

int x = 42;

32-bit signed integer.

int count = 0;

long x = 9999999999L;

64-bit signed integer.

double d = 3.14;

64-bit floating-point.

decimal d = 1.99m;

128-bit high-precision decimal — use for money.

bool flag = true;

Boolean true/false.

string s = "hello";

Immutable sequence of characters.

char c = 'A';

Single Unicode character.

var x = 42;

Implicitly typed local variable — type inferred at compile time.

int? x = null;

Value type that can also hold null.

string? name = null;

object obj = 42;

Base type of all .NET types.

var list = new List<int>();

Dynamic array — O(1) index, O(n) insert.

list.Add(1); list.Remove(1);

var d = new Dictionary<string, int>();

Hash-map — O(1) lookup by key.

d["key"] = 42;

var set = new HashSet<string>();

Unordered collection of unique values.

set.Add("a"); set.Contains("a");

var q = new Queue<int>();

FIFO queue.

q.Enqueue(1); q.Dequeue();

var st = new Stack<int>();

LIFO stack.

st.Push(1); st.Pop();

IEnumerable<int> nums = list;

Base interface for all sequences — supports foreach.

int[] arr = new int[5];

Fixed-size sequential collection.

var arr = new[] { 1, 2, 3 };

list.Where(x => x > 0)

Filters a sequence by a predicate.

nums.Where(n => n % 2 == 0)

list.Select(x => x * 2)

Projects each element into a new form (map).

list.Select(s => s.ToUpper())

list.OrderBy(x => x.Name)

Sorts ascending by key.

people.OrderBy(p => p.Age)

list.GroupBy(x => x.Category)

Groups elements by a key.

items.GroupBy(i => i.Type)

list.FirstOrDefault(x => x > 5)

Returns first matching element or default(T).

list.Any(x => x > 0)

Returns bool — any/all elements satisfy predicate.

list.Count(x => x > 0)

Returns count of matching elements.

query.ToList()

Materialises a lazy query into a concrete collection.

public async Task<T> FooAsync()

Defines an asynchronous method that can be awaited.

var result = await GetDataAsync();

Task<int> t = DoWorkAsync();

Represents an in-progress async operation.

await Task.WhenAll(t1, t2)

Awaits multiple tasks concurrently.

.ConfigureAwait(false)

Avoids capturing the synchronisation context — use in library code.

CancellationToken ct

Propagates cancellation signal to async operations.

await task.WaitAsync(ct);

str.Contains("sub")

Returns true if the string contains the substring.

str.StartsWith("He")

Tests the beginning or end of a string.

str.Substring(start, length)

Extracts a portion of the string.

str.Replace("old", "new")

Returns a new string with occurrences replaced.

str.Split(',')

Splits string into an array by a separator.

str.Trim()

Removes leading/trailing whitespace.

$"Hello {name}"

String interpolation — embeds expressions in string literals.

string.IsNullOrEmpty(str)

Returns true if string is null or has zero length.

try { } catch (Exception e) { } finally { }

Structured exception handling block.

throw new ArgumentNullException(nameof(param));

Throws an exception.

new ArgumentNullException(nameof(x))

Thrown when a null argument is passed to a method that doesn't accept it.

throw new InvalidOperationException(msg)

Method call invalid for the current state of the object.

catch (Exception e) when (e.Message.Contains("x"))

Filters exceptions without unwinding the stack.