Question

I found this but tried to use it and failed.

How can i create an object using reflections and make it fast by putting it in a delegate?

        DynamicMethod dm = new DynamicMethod("MyCtor", t, new Type[] { });            
        var ctor = t.GetConstructor(new Type[] { });
        ILGenerator ilgen = dm.GetILGenerator();
        ilgen.Emit(OpCodes.Ldarg_0);
        ilgen.Emit(OpCodes.Newobj, ctor);
        ilgen.Emit(OpCodes.Ret);
        var d = (Func<T>)dm.CreateDelegate(t);
        dm.Invoke(null, new object[] { });

Before putting it in a deleage i tried to at least invoke it and when i did above i get the error

An unhandled exception of type 'System.Reflection.TargetInvocationException' occurred in mscorlib.dll

Additional information: Exception has been thrown by the target of an invocation.

If i call d() instead i get the exception

An unhandled exception of type 'System.ArgumentException' occurred in mscorlib.dll

Additional information: Type must derive from Delegate.

How do i put a no param constructor into a delegate and call it?

Answer 1

If you have access to .NET 3.5 (which your use of Func<T> suggests), you may find Expression easier than ILGenerator:

class Foo { }
static void Main() {
    Func<Foo> func = GetCtor<Foo>(); // cache this somewhere!
    Foo foo = func();
}
static Func<T> GetCtor<T>() {
    Type type = typeof(T);
    Expression body = Expression.New(type);
    return Expression.Lambda<Func<T>>(body).Compile();        
}

Pretty easy to extend that to use a specific constructor, passing arguments, or adding post-constructor property bindings; casts, conversions, etc (see this related answer). If you have a specific scenario, I’ll happily add an example.

Note also that you should cache and re-use any such constructors – otherwise you lose the benefit (i.e. don’t recreate the delegate per-call).