How to write code to be executed BEFORE any method in an assembly?

Question

There is an event AppDomain.CurrentDomain.DomainUnload which allows to clean up static resources which are independent of any particular instance of class and even of any particular class. I consider it as the code which is executed AFTER any code in my current assembly (Am I right by the way?).

But is there a way to write a piece of code (for the current assembly) which is to be executed BEFORE any other code in the current assembly (class library)? Or should I look for a more tricky way to initialize some resources before any code started execution?

I know about static constructors, but the order of their calls is not well defined. In other words, there is no guarantee that a particular static constructor will be executed before other static constructors of other classes.

There is also an AppDomain.AssemblyLoad. I am not sure this is the thing I am looking for. This event occurs when OTHER assemblies are loaded, not the current.

Answer 1

I had the same problem and solved it this way. I define an IAssembyInitializer interface with just the method void Initialize(). In every assembly for which I want to execute some code just after it is loaded, I define a class implementing this interface. The I define an attribute to specify the classes in the assembly that implements this interface (otherwise you could find them out by Reflection, but I preferred it this way):

[AttributeUsage(AttributeTargets.Assembly, AllowMultiple = true)]
public class AssemblyInitializerAttribute : Attribute
{
    AssemblyInitializerAttribute ()
    {
    }
    AssemblyInitializerAttribute (string typeName)
    {
        TypeName = typeName;
    }
    public string TypeName;
}

The attribute is set in the AssemblyInfo in this way:

[assembly: AssemblyInitializerAttribute ("MyNamespace.AnAssemblyInitializer")]

Finally, in the main assembly of the application I register to the AssemblyLoad event a method that executes all the initializations:

AppDomain.CurrentDomain.AssemblyLoad += new AssemblyLoadEventHandler(NewAssemblyLoaded);

        static void NewAssemblyLoaded(object sender, AssemblyLoadEventArgs args)
        {
            Assembly anAssembly = args.LoadedAssembly;
            AssemblyInitializerAttribute[] initializers = (AssemblyInitializerAttribute[])anAssembly .GetCustomAttributes(typeof(AssemblyInitializerAttribute), false);
            foreach (AssemblyInitializerAttribute anInit in initializers)
            {
                Type initType = anInit.TypeName != null ? anAssembly.GetType(anInit.TypeName) : null;
                if (initType != null && initType.GetInterface("IAssemblyInitializer") != null)
                {
                    IAssemblyInitializer anInitializer = (IAssemblyInitializer)Activator.CreateInstance(initType);
                    anInitializer.Initialize();
                }
            }
        }

Answer 2

You can use instances of Lazy<T> held in static fields to control static initialization order yourself. The code body of those Lazy<T> initializer objects can reference other Lazy<T> instances which automatically orchestrates a DAG of initialization. You cannot have cycles, obviously.

With C++/CLI you can indeed have code executed when the assembly loads (module initializers). You probably don't want to go that route.

With C# this is not possible. Static ctors and lazy initialization patterns are the best you get.