C# Extension Methods

How it works

An extension method provides instance-style call syntax for a static method in an appropriate static class. It does not modify the target type’s implementation, gain private access or change ownership. Traditional this-parameter extension syntax remains useful in this C# 14 course; newer extension-block syntax is a separate authoring choice, not required to understand the contract. Keep extension names specific so they do not hide surprising side effects behind an apparently harmless property-like operation.

Run it locally

Use the .NET 10 SDK with C# 14 in a separate console lab. Create an empty project once with dotnet new console --framework net10.0, then replace Program.cs with this complete source and add any supporting files shown. Keep only one lesson entry point in the project. C# does not currently run in Bhylu’s browser playground. Apache, PHP and MySQL are not needed.

dotnet build -p:LangVersion=14.0 -p:Nullable=enable -p:TreatWarningsAsErrors=true
dotnet run --no-build

Use the SDK’s project workflow; the review compiles against its matching reference assemblies and executes the results without external NuGet packages. File-writing examples belong in a private empty lab directory.

Download the complete C# source as Program.cs.

Example

using System;

internal static class Program
{
    private static void Main(string[] args)
    {
        string code = "W-17";
        Console.WriteLine(code.AsBookingLabel());
    }
}

internal static class BookingLabels
{
    public static string AsBookingLabel(this string code)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(code);
        return "Booking " + code;
    }
}

Expected result

Booking W-17

The static helper validates its input before constructing a new string. It does not mutate code. A public identifier type would be a better home for syntax and length invariants shared by many operations. Keep extensions out of unrelated global namespaces when they would create ambiguous or misleading APIs for callers.