# C# Quick Reference

*Types, LINQ, async/await, collections, OOP essentials*

> Source: Microsoft C# Documentation (learn.microsoft.com) · MIT

## Basics

### Hello World

```
Console.WriteLine("Hello, World!");  // top-level (C# 10+)
// Classic: class Program { static void Main() { ... } }
```

### Build & Run

```
dotnet new console -n MyApp   # create project
dotnet run                     # compile and run
dotnet build                   # compile only
```

### Variables & Constants

```
int x = 42;
var name = "Alice";           // type inference
const double Pi = 3.14159;
readonly int maxRetries = 3;  // set once, in ctor
```

## Types

### Value Types

| Command | Description |
|---------|-------------|
| `int` | 32-bit signed integer |
| `long` | 64-bit signed integer |
| `float` | 32-bit floating point (suffix `f`) |
| `double` | 64-bit floating point |
| `decimal` | 128-bit high precision (suffix `m`) |
| `bool` | `true` / `false` |
| `char` | 16-bit Unicode character |

### Reference Types

| Command | Description |
|---------|-------------|
| `string` | Immutable UTF-16 text |
| `object` | Base type for all types |
| `dynamic` | Bypasses compile-time type checking |
| `int[]` | Array of integers |
| `List<T>` | Generic list (System.Collections.Generic) |

### Nullable & Tuples

```
int? age = null;               // nullable value type
string? name = null;           // nullable reference (C# 8+)
var point = (X: 1, Y: 2);     // named tuple
Console.WriteLine(point.X);
```

### String Features

```
string name = "World";
string msg = $"Hello, {name}!";       // interpolation
string path = @"C:\Users\file.txt";   // verbatim
string raw = """raw "string" here""";  // raw (C# 11+)
```

## Control Flow

### If / Else

```
if (x > 0) Console.WriteLine("positive");
else if (x == 0) Console.WriteLine("zero");
else Console.WriteLine("negative");
```

### Switch & Pattern Matching

```
string label = x switch {
    > 0 => "positive", 0 => "zero", _ => "negative"
};
if (obj is string s && s.Length > 0) { }  // pattern match
```

### Loops

```
for (int i = 0; i < 10; i++) { }
foreach (var item in collection) { }
while (condition) { }
do { } while (condition);
```

## Classes

### Class Definition

```
public class Person {
    public string Name { get; set; }
    public int Age { get; init; }       // init-only (C# 9+)
    public Person(string name, int age) { Name = name; Age = age; }
}
```

### Records (C# 9+)

```
public record Point(double X, double Y);
var p1 = new Point(1, 2);
var p2 = p1 with { X = 3 };  // non-destructive copy
// auto: Equals, GetHashCode, ToString, deconstruct
```

### Inheritance

```
public abstract class Shape { public abstract double Area(); }
public class Circle(double r) : Shape {
    public override double Area() => Math.PI * r * r;
}
```

### Access Modifiers

| Command | Description |
|---------|-------------|
| `public` | Accessible from anywhere |
| `private` | Same class only (default for members) |
| `protected` | Same class and derived classes |
| `internal` | Same assembly only (default for classes) |
| `protected internal` | Same assembly or derived classes |

## Interfaces

### Interface Definition

```
public interface IShape {
    double Area();
    double Perimeter() => 0;  // default impl (C# 8+)
}
public class Rect(double w, double h) : IShape { public double Area() => w * h; }
```

### Common Interfaces

| Command | Description |
|---------|-------------|
| `IEnumerable<T>` | Iteration support (foreach, LINQ) |
| `IDisposable` | Deterministic cleanup (`using` statement) |
| `IComparable<T>` | Natural ordering for sorting |
| `IEquatable<T>` | Value equality comparison |
| `ICloneable` | Object cloning |

## LINQ

### Method Syntax

```
var result = numbers
    .Where(n => n > 3)
    .OrderBy(n => n)
    .Select(n => n * 2)
    .ToList();
```

### Query Syntax

```
var result = from n in numbers
             where n > 3
             orderby n
             select n * 2;
```

### Common LINQ Methods

| Command | Description |
|---------|-------------|
| `.Where(pred)` | Filter elements |
| `.Select(func)` | Project / transform elements |
| `.OrderBy(key)` | Sort ascending |
| `.GroupBy(key)` | Group elements by key |
| `.First() / .FirstOrDefault()` | First element (or default) |
| `.Any(pred)` | `true` if any element matches |
| `.Count()` | Number of elements |
| `.Sum() / .Average()` | Aggregate numeric values |
| `.Distinct()` | Remove duplicates |
| `.SelectMany(func)` | Flatten nested collections |

## Async/Await

### Async Method

```
public async Task<string> FetchAsync(string url) {
    using var client = new HttpClient();
    return await client.GetStringAsync(url);
}
```

### Task Combinators

```
var results = await Task.WhenAll(task1, task2, task3);
var first = await Task.WhenAny(task1, task2);
```

### Async Patterns

| Command | Description |
|---------|-------------|
| `Task` | Async void return (no result) |
| `Task<T>` | Async return with result of type T |
| `ValueTask<T>` | Lightweight task for sync-fast paths |
| `await foreach` | Async iteration over `IAsyncEnumerable<T>` |
| `CancellationToken` | Cooperative cancellation for async ops |

## Collections

### Common Collections

| Command | Description |
|---------|-------------|
| `List<T>` | Dynamic array, fast index access |
| `Dictionary<K,V>` | Hash map, O(1) lookup by key |
| `HashSet<T>` | Unique elements, O(1) lookup |
| `Queue<T>` | FIFO collection |
| `Stack<T>` | LIFO collection |
| `LinkedList<T>` | Doubly-linked list |
| `SortedDictionary<K,V>` | Sorted by key (tree-based) |

### Dictionary Usage

```
var dict = new Dictionary<string, int> {
    ["Alice"] = 90, ["Bob"] = 85
};
dict.TryGetValue("Alice", out int score);
foreach (var (key, val) in dict) { }
```

### Immutable Collections

```
using System.Collections.Immutable;
var list = ImmutableList.Create(1, 2, 3);
var newList = list.Add(4);  // returns new list
```

## Properties

### Property Syntax

```
public string Name { get; set; }
public int Age { get; private set; }
public string Email { get; init; }        // init-only
public string Display => $"{Name} ({Age})"; // computed
```

### Indexers

```
public double this[int row, int col] {
    get => data[row, col];
    set => data[row, col] = value;
}
```

### Property Patterns

| Command | Description |
|---------|-------------|
| `{ get; set; }` | Read-write auto-property |
| `{ get; }` | Read-only (set in constructor only) |
| `{ get; init; }` | Read-only after initialization (C# 9+) |
| `{ get; private set; }` | Publicly readable, privately writable |
| `=> expression` | Expression-bodied (computed) property |

## Exceptions

### Try / Catch / Finally

```
try { int result = int.Parse(input); }
catch (FormatException ex) { Console.Error.WriteLine(ex.Message); }
catch (Exception ex) when (ex is not OutOfMemoryException) { }
finally { /* always executes */ }
```

### Using Statement

```
using var file = File.OpenRead("data.txt");
// file.Dispose() called automatically at scope end
// equivalent to try/finally with Dispose()
```

### Common Exceptions

| Command | Description |
|---------|-------------|
| `ArgumentNullException` | Null argument passed to method |
| `ArgumentOutOfRangeException` | Argument outside valid range |
| `InvalidOperationException` | Operation invalid for current state |
| `NullReferenceException` | Dereference of null object |
| `KeyNotFoundException` | Key not found in dictionary |
| `NotImplementedException` | Method not yet implemented |

### Custom Exception

```
public class AppException : Exception {
    public int Code { get; }
    public AppException(string msg, int code)
        : base(msg) { Code = code; }
}
```