Previously if an assembly (module) wished to perform initialization then it must be specifically called by the application or else the assembly author must resort to external tooling to inject a .NET module initializer.

Now with C# 9.0 developers can specify the ModuleInitializer attribute instead to have their assembly perform necessary initialization at load time.

Code

Now

class Profiler
{
    [ModuleInitializer]
    internal static void Initialize()
    {
        SetupCollectors();
        WireUpEvents();
    }
}

Before

class Profiler
{
    public static void Initialize()
    {
        SetupCollectors();
        WireUpEvents();
    }
}

class Program
{
    static void Main()
    {
        Profiler.Initialize(); // Hope callers do this before using the profiler!
    }
}

Notes

• The method must be static, parameterless, return void, be non-generic, not be in a generic type, and be internal or public
• The ModuleInitializerAttribute is only available from .NET 5 onwards but creating your own with the same name is enough to make it work on earlier targets:

#if !NET5_0_OR_GREATER
namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Method, AllowMultiple = false)]
    public sealed class ModuleInitializerAttribute : Attribute { }
}
#endif

More information

• Specification
• Module initialization before C# 9 @ Coen Goedegebure's Blog