Syntax and types

The project layout, top-level statements, the value and reference split, and nullability — the decisions that shape everything else.

A project, not just a file

C# compiles to an assembly that runs on the .NET runtime. The build is driven by a .csproj file, so the unit of work is a project rather than a single source file.

dotnet new console -o Hello
cd Hello
dotnet run                       # restore, build and execute
dotnet build -c Release          # compile only
dotnet add package Humanizer     # add a NuGet dependency
dotnet publish -c Release -r linux-x64 --self-contained

// Program.cs — top-level statements, C# 9 onwards; no Main method required
using System;
using System.Collections.Generic;
using System.Linq;

var scores = new List<int> { 90, 85, 77 };
var average = scores.Average();

Console.WriteLine($"Average: {average:F1} across {scores.Count} scores");

record Score(int Value, string Subject);      // an immutable data type
var best = new Score(90, "Maths");
Console.WriteLine(best);                      // Score { Value = 90, Subject = Maths }

Value types and reference types

A value type holds its data inline and is copied on assignment. A reference type holds a reference to an object on the heap, so assignment copies the reference and both names end up looking at the same object.

struct Point { public int X, Y; }

var a = new Point { X = 1, Y = 2 };
var b = a;               // a full copy
b.X = 99;
Console.WriteLine(a.X);  // 1 — a is untouched

var list1 = new List<int> { 1, 2 };
var list2 = list1;       // the same object
list2.Add(3);
Console.WriteLine(list1.Count);   // 3

// boxing turns a value into an object; unboxing casts it back and can throw
object boxed = 42;
int unboxed = (int)boxed;

// value types can be nullable, and ?? supplies a default
int? missing = null;
int actual = missing ?? 0;

Text and formatting

var name = "Ada";
var greeting = $"Hello, {name}! The time is {DateTime.Now:HH:mm}.";

// a verbatim string treats its contents literally: a quote is written twice
var title = @"She said ""hello"" and left.";

// a raw string literal (C# 11) needs no doubling at all
var json = """
           { "name": "Ada", "active": true }
           """;

var upper = name.ToUpperInvariant();     // culture-independent
var same = string.Equals(name, "ada", StringComparison.OrdinalIgnoreCase);

var sb = new System.Text.StringBuilder();
for (var i = 0; i < 100; i++) sb.Append(i).Append(',');
var built = sb.ToString();               // one buffer, not a hundred strings

• $ interpolates a string, @ makes it verbatim, and the two combine as $@.
• Strings are immutable, so every + inside a loop allocates a new one; use StringBuilder for repeated concatenation.
• Compare with an explicit StringComparison: Ordinal or OrdinalIgnoreCase for identifiers and protocol values, and a culture-aware overload only for text shown to people.
• List<int> and other generic types are declared with angle brackets, and the compiler checks every use.

FAQ

Related

Classes and interfaces LINQ and async/await

Last refreshed 2026-09-18.