Using a Delegate to call a constructor

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

Try this -

Action myCtor = CreateCtor(t, Type.EmptyTypes, typeof(Action));

public static Delegate CreateCtor(Type type, Type[] parameterTypes, Type delegateType, string typeParameterName)
{
    var ctorInfo = type.GetConstructor(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance, null, parameterTypes, null);
    if (ctorInfo == null)
    {
        string parameterString = string.Empty;
        if(parameterTypes.Length > 0)
        {
            string[] parameterStrings = new string[parameterTypes.Length];
            for(int i = 0; i < parameterTypes.Length; ++i)
            {
                parameterStrings[i] = parameterTypes[i].ToString();
            }
            parameterString = string.Join(",", parameterStrings);
        }
        throw new ArgumentException(string.Format("Type '{0}' does not define .ctor({1}).", type, parameterString), typeParameterName);
    }

    bool isVisible = type.IsVisible && (ctorInfo.IsPublic && !ctorInfo.IsFamilyOrAssembly);

    DynamicMethod dynamicCtor = new DynamicMethod(Guid.NewGuid().ToString("N"), type, parameterTypes, ctorInfo.Module, !isVisible);
    var il = dynamicCtor.GetILGenerator();
    for (int i = 0; i < parameterTypes.Length; ++i)
    {
        switch (i)
        {
            case 0: il.Emit(OpCodes.Ldarg_0); break;
            case 1: il.Emit(OpCodes.Ldarg_1); break;
            case 2: il.Emit(OpCodes.Ldarg_2); break;
            case 3: il.Emit(OpCodes.Ldarg_3); break;
            default: il.Emit(OpCodes.Ldarg, i); break;
        }
    }
    il.Emit(OpCodes.Newobj, ctorInfo);
    il.Emit(OpCodes.Ret);
    return dynamicCtor.CreateDelegate(delegateType);
}

Answer 2

There are no arguments to the constructor so you should not load arguments on the stack ilgen.Emit(OpCodes.Ldarg_0):

class Program
{
    static void Main()
    {
        var t = typeof(Program);
        var dm = new DynamicMethod("MyCtor", t, new Type[0], t.Module);
        var ctor = t.GetConstructor(new Type[0]);
        ILGenerator ilgen = dm.GetILGenerator();
        ilgen.Emit(OpCodes.Newobj, ctor);
        ilgen.Emit(OpCodes.Ret);
        var del = (Func<Program>)dm.CreateDelegate(typeof(Func<Program>));
        var instance = del();
        Console.WriteLine(instance);
    }
}

Answer 3

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).