Hooking into an “OnLoad” for class library

Question

Does anyone know if there's a way to hook into an "OnLoad" event to run some operations when an assembly loads?

Specifically, I am creating a plug-in for an application. The plug-in's DLL gets loaded and objects start being used, but the problem is I need to load another assembly dynamically before anything happens. This assembly can't be copied to the application's directory and must remain invisible to it.

Answer 1

It is really sad that writing a Main() function in an Assembly DLL is never called by the .NET framework. It seems that Microsoft forgot that.

But you can easily implement it on your own:

In the DLL assembly you add this code:

using System.Windows.Forms;

public class Program
{
    public static void Main()
    {
        MessageBox.Show("Initializing");
    }
}

Then in the Exe Assembly that loads this DLL you add this function:

using System.Reflection;

void InitializeAssembly(Assembly i_Assembly)
{
    Type t_Class = i_Assembly.GetType("Program");
    if (t_Class == null)
        return; // class Program not implemented

    MethodInfo i_Main = t_Class.GetMethod("Main");
    if (i_Main == null)
        return; // function Main() not implemented

    try 
    {
        i_Main.Invoke(null, null);
    }
    catch (Exception Ex)
    {
        throw new Exception("Program.Main() threw exception in\n" 
                            + i_Assembly.Location, Ex);
    }
}

Obviously you should call this function at the very beginning before doing anything else with that Assembly.

Answer 2

You need to hook on to AssemblyLoad event.

Refer- http://msdn.microsoft.com/en-us/library/system.appdomain.assemblyload.aspx

Answer 3

C# does not provide a way to do that but the underlying IL code does via module initializers. You can use tools like Fody/ModuleInit to turn a specially named static C# class to run as a module initializer which will be run when your dll is loaded.