C# Cheatsheet

Updated September 2026 · Quick reference

C# is a modern, strongly typed language commonly used with .NET for apps, APIs, and tools.

Focus on classes/records, nullability, and async/await—they appear throughout real projects.

Basics

Hello world

Top-level statements keep small programs short; larger apps use namespaces and classes.

Console.WriteLine("Hello");

Variables

Declare with explicit types or var when the type is obvious.

int count = 0;
var name = "Ada";

String interpolation

Embed expressions in strings with $"...".

Console.WriteLine($"Hi, {name}");

Nullable reference types

With nullable enabled, string? may be null; plain string should not.

string? maybe = null;
string sure = maybe ?? "default";

dotnet CLI

Create, run, and test projects with the .NET SDK.

dotnet new console -n Hello
cd Hello
dotnet run

Types & OOP

Class

Reference type with fields, properties, and methods.

public class User {
  public int Id { get; set; }
  public string Name { get; set; } = "";
}

Record

Immutable-friendly type with value-like equality—great for DTOs.

public record Book(string Title, int Year);

Interface

Contract of members a type must implement.

public interface ILogger {
  void Log(string message);
}

Inheritance

Derive with :. Prefer composition when inheritance is unclear.

public class Admin : User { }

Collections & LINQ

List

Dynamic array of items.

var nums = new List<int> { 1, 2, 3 };
nums.Add(4);

Dictionary

Key/value lookups.

var ages = new Dictionary<string, int> { ["Ada"] = 36 };

LINQ Where / Select

Query collections declaratively.

var evens = nums.Where(n => n % 2 == 0).ToList();
var names = users.Select(u => u.Name);

FirstOrDefault

Safely get one item or a default when missing.

var user = users.FirstOrDefault(u => u.Id == 1);

Async & errors

async / await

Write asynchronous code that reads like synchronous code.

public async Task<string> LoadAsync() {
  return await File.ReadAllTextAsync("a.txt");
}

Task

Represents an asynchronous operation. Prefer returning Task/Task<T> from async methods.

await Task.Delay(100);

try / catch

Handle exceptions; be specific when possible.

try {
  // work
} catch (FileNotFoundException ex) {
  Console.WriteLine(ex.Message);
}

using

Dispose resources automatically.

using var reader = new StreamReader("a.txt");
var text = reader.ReadToEnd();

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