C# Cheat Sheet

Free C# cheat sheet: syntax, LINQ, async/await, properties, and OOP essentials in one quick reference.

Basics

Concept	Syntax	Example
Variable	`var x = 5; int y = 10;`	`var name = "Alice";`
String interpolation	`$"text {expr}"`	`$"Hello {name}, age {age}"`
Null coalescing	`a ?? b`	`var result = input ?? "default";`
Pattern matching	`is pattern`	`if (obj is string s) Console.Write(s);`

Learn it in a lesson: Variables & Data Types · Nullable Types & Null Operators · Pattern Matching & Switch Expressions

LINQ

Concept	Syntax	Example
Filter	`.Where(x => ...)`	`var adults = people.Where(p => p.Age >= 18);`
Project	`.Select(x => ...)`	`var names = people.Select(p => p.Name);`
Sort	`.OrderBy / .ThenBy`	`people.OrderBy(p => p.Last).ThenBy(p => p.First);`
One item First throws on empty; FirstOrDefault returns null. Single throws if there is more than one.	`.First / .FirstOrDefault / .Single`	`var ada = people.FirstOrDefault(p => p.Name == "Ada");`
Aggregate	`.Count() .Sum() .Average() .Max()`	`var total = orders.Sum(o => o.Amount);`
Group	`.GroupBy(x => key)`	`orders.GroupBy(o => o.Country);`
Run it now LINQ is lazy — nothing runs until you enumerate it.	`.ToList() / .ToArray()`	`var list = query.ToList();`

Learn it in a lesson: LINQ

OOP

Concept	Syntax	Example
Class	`class Name { }`	`class Dog { public string Name { get; set; } }`
Inheritance	`class B : A { }`	`class Puppy : Dog { }`
Interface	`interface IName { }`	`interface IDrawable { void Draw(); }`
Record	`record Name(params)`	`record Person(string Name, int Age);`

Learn it in a lesson: Object-Oriented Programming · Inheritance & Polymorphism · Records

Collections

Concept	Syntax	Example
List	`List<T>`	`var names = new List<string> { "Ada" }; names.Add("Alan");`
Dictionary	`Dictionary<K,V>`	`var ages = new Dictionary<string,int> { ["Ada"] = 36 };`
HashSet	`HashSet<T>`	`var set = new HashSet<int> { 1, 2, 3 };`
Safe lookup Indexing a missing key throws; TryGetValue returns false instead.	`TryGetValue(key, out var v)`	`if (ages.TryGetValue("Ada", out var a)) { }`
Immutable view Hands callers the data without handing them Add and Remove.	`IReadOnlyList<T>`	`public IReadOnlyList<Item> Items => _items;`

Learn it in a lesson: Collections · Generic Lists (List<T>) · Dictionary (Key-Value Pairs) · Domain-Driven Design (Aggregates, Value Objects, Repositories)

Async

Concept	Syntax	Example
Async method	`async Task<T> Method()`	`async Task<string> GetAsync() { ... }`
Await	`await asyncMethod()`	`var data = await GetDataAsync();`
Task.WhenAll	`await Task.WhenAll(t1, t2)`	`await Task.WhenAll(task1, task2);`

Learn it in a lesson: Async & Await

Null Safety

Concept	Syntax	Example
Nullable type With nullable reference types on, the compiler warns when you might dereference null.	`string? name`	`string? middle = null;`
Null-conditional Short-circuits to null instead of throwing.	`obj?.Prop`	`var n = user?.Address?.City;`
Null-coalescing	`a ?? b`	`var city = user?.City ?? "Unknown";`
Assign if null	`a ??= b`	`_cache ??= Build();`
Guard a parameter .NET 6+. Fails at the boundary with the parameter's name in the message.	`ArgumentNullException.ThrowIfNull(x)`	`ArgumentNullException.ThrowIfNull(order);`

Learn it in a lesson: Nullable Types & Null Operators