C# extension methods: how they work and where .NET uses them (LINQ, DI)

// ======================================================================
// Q: What are extension methods? Where does .NET use them?
//
// Static methods in a static class, with this on the first parameter. They add
// methods to types without modifying them. All of LINQ is extension methods, as
// are all the builder.Services.AddXxx() calls. C# 14 also has extension members
// (extension(...) blocks with properties).
// ======================================================================

public static class DependencyInjection
{
    public static IServiceCollection AddApplication(this IServiceCollection services)
    {
        services.AddScoped<PlaceOrderHandler>();
        return services;               // fluent: builder.Services.AddApplication().AddInfrastructure()
    }
}