Implementing INotifyPropertyChanged via reflection

Question

Using C#/.Net 4.0, I'm storing data in a BindingList where dataRow is being defined at run time via Reflection.Emit. (The structure of the incoming data varies and is defined by an external source.) After struggling a bit with my first foray into the world of reflection and IL, I have been able to create my dataRow fill it with values, fill my BindingList and display the result in a grid. I now am trying to implement the INotifyPropertyChanged interface and the PropertyChangedEventHandler when changes are made to the data. Using this as a guide, I've got code that runs but it doesn't appear that the RaisePropertyChanged event is firing or it just isn't doing anything. When I compare my dynamic version with a normal/static version via ildasm.exe I see major differences in the remove_PropertyChanged and add_PropertyChanged methods. Could anyone provide some tips or examples of implementing the INotifyPropertyChanged interface via reflection.

After further review it seems that the event field must be null so the PropertyChangedEventHandler is not getting called. I added some message boxes in the RaiseProprtyChanged method builder and I discovered that the equivalent of if (PropertyChanged != null) is returning zero/false, so nothing happens. If I change the OpCodes.Brtrue to OpCodes.Brfalse I get a "Object reference not set to an instance of an object" message. It feels like I missing something simple but I can't find it.

            //implement IINotifyPropertyChanged interface
        tb.AddInterfaceImplementation(typeof(INotifyPropertyChanged));

        //property changed event handler
        FieldBuilder eventField = tb.DefineField("PropertyChanged", typeof(PropertyChangedEventHandler), FieldAttributes.Private);

        EventBuilder eb = tb.DefineEvent("PropertyChanged", EventAttributes.None, typeof(PropertyChangedEventHandler));

        MethodBuilder mbEV = tb.DefineMethod("remove_PropertyChanged", MethodAttributes.Public |
            MethodAttributes.SpecialName | MethodAttributes.NewSlot |
            MethodAttributes.HideBySig | MethodAttributes.Virtual |
            MethodAttributes.Final, null, new[] { typeof(PropertyChangedEventHandler) });

        MethodImplAttributes eventMethodFlags = MethodImplAttributes.Managed; //| MethodImplAttributes.Synchronized;
        mbEV.SetImplementationFlags(eventMethodFlags);
        il = mbEV.GetILGenerator();
        il.Emit(OpCodes.Ldarg_0);
        il.Emit(OpCodes.Ldarg_0);
        il.Emit(OpCodes.Ldfld, eventField);
        il.Emit(OpCodes.Ldarg_1);
        il.EmitCall(OpCodes.Call, typeof(Delegate).GetMethod("Remove", new[] { typeof(Delegate), typeof(Delegate) }), null);
        il.Emit(OpCodes.Castclass, typeof(PropertyChangedEventHandler));
        il.Emit(OpCodes.Stfld, eventField);
        il.Emit(OpCodes.Ret);
        MethodInfo miRemoveEvent = typeof(INotifyPropertyChanged).GetMethod("remove_PropertyChanged");
        tb.DefineMethodOverride(mbEV, miRemoveEvent);
        eb.SetRemoveOnMethod(mbEV);

        mbEV = tb.DefineMethod("add_PropertyChanged", MethodAttributes.Public |
            MethodAttributes.SpecialName | MethodAttributes.NewSlot |
            MethodAttributes.HideBySig | MethodAttributes.Virtual |
            MethodAttributes.Final, null, new[] { typeof(PropertyChangedEventHandler) });

        mbEV.SetImplementationFlags(eventMethodFlags);
        il = mbEV.GetILGenerator();
        il.Emit(OpCodes.Ldarg_0);
        il.Emit(OpCodes.Ldarg_0);
        il.Emit(OpCodes.Ldfld, eventField);
        il.Emit(OpCodes.Ldarg_1);
        il.EmitCall(OpCodes.Call, typeof(Delegate).GetMethod("Combine", new[] { typeof(Delegate), typeof(Delegate) }), null);
        il.Emit(OpCodes.Castclass, typeof(PropertyChangedEventHandler));
        il.Emit(OpCodes.Stfld, eventField);
        il.Emit(OpCodes.Ret);
        MethodInfo miAddEvent = typeof(INotifyPropertyChanged).GetMethod("add_PropertyChanged");
        tb.DefineMethodOverride(mbEV, miAddEvent);
        eb.SetAddOnMethod(mbEV);

        MethodInfo msgboxMethodInfo = typeof(System.Windows.Forms.MessageBox).GetMethod("Show", BindingFlags.Public | BindingFlags.Static, null, CallingConventions.Standard, new Type[] { typeof(String) }, null);

        MethodBuilder mbRaisePropertyChanged = tb.DefineMethod("RaisePropertyChanged", MethodAttributes.Virtual, null, new Type[] { typeof(string) });
        il = mbRaisePropertyChanged.GetILGenerator();
        System.Reflection.Emit.Label labelExit = il.DefineLabel();
        il.Emit(OpCodes.Ldarg_0);
        il.Emit(OpCodes.Ldfld, eventField);
        il.Emit(OpCodes.Ldnull);
        il.Emit(OpCodes.Ceq); //this is returning false
        il.Emit(OpCodes.Brtrue, labelExit);
        il.Emit(OpCodes.Nop); //I never get here
        il.Emit(OpCodes.Ldstr, "After If");
        il.EmitCall(OpCodes.Call, msgboxMethodInfo, null);
        il.Emit(OpCodes.Pop);
        il.Emit(OpCodes.Ldarg_0);
        il.Emit(OpCodes.Ldfld, eventField);
        il.Emit(OpCodes.Ldarg_0);
        il.Emit(OpCodes.Ldarg_1);
        il.Emit(OpCodes.Newobj, typeof(PropertyChangedEventArgs).GetConstructor(new[] { typeof(string) }));
        il.EmitCall(OpCodes.Callvirt, typeof(PropertyChangedEventHandler).GetMethod("Invoke"), null);
        il.Emit(OpCodes.Nop);
        il.Emit(OpCodes.Nop);
        il.MarkLabel(labelExit);
        il.Emit(OpCodes.Ret);

Answer 1

If you are trying to modify the IL of an assembly I would stear clear of reflection. You may run into assembly locking issues.

As for a solution try these links.

Custom weaving

• http://code.google.com/p/notifypropertyweaver/
• http://justinangel.net/AutomagicallyImplementingINotifyPropertyChanged

Using postsharp

• http://www.lesnikowski.com/blog/index.php/inotifypropertychanged-with-postsharp/
• http://www.sharpcrafters.com/solutions/ui