Code to inject add and remove methods for the event handler.

EventHandlerHookupMethods.cs

private MethodDefinition CreateAddPropertyChangedMethod()
{
  return CreatePropertyChangedEventHookupMethod(
    "add_PropertyChanged", 
    "Combine");
}

private MethodDefinition CreateRemovePropertyChangedMethod()
{
  return CreatePropertyChangedEventHookupMethod(
    "remove_PropertyChanged", 
    "Remove");
}

private MethodDefinition CreatePropertyChangedEventHookupMethod(
  string eventHookupMethodName, 
  string delegateMethodName)
{
  // .method public final hidebysig specialname newslot virtual 
  //   instance void add_PropertyChanged (
  //     class [System]System.ComponentModel.PropertyChangedEventHandler 'value'
  //   ) cil managed 
  var methodDef = new MethodDefinition(eventHookupMethodName,
    MethodAttributes.Public |
    MethodAttributes.Final |
    MethodAttributes.HideBySig |
    MethodAttributes.SpecialName |
    MethodAttributes.NewSlot |
    MethodAttributes.Virtual,
    _types.Void);
  var paramDef = new ParameterDefinition("value", 
    ParameterAttributes.None, 
    _types.PropertyChangedEventHandler);
  methodDef.Parameters.Add(paramDef);
  methodDef.Body.MaxStackSize = 3;
  for (int i = 0; i < 3; i++)
  {
    var v = new VariableDefinition(_types.PropertyChangedEventHandler);
    methodDef.Body.Variables.Add(v);
  }
  methodDef.Body.InitLocals = true;
  var il = methodDef.Body.GetILProcessor();
  Action<OpCode> op = x => il.Append(il.Create(x));
  // IL_0000: ldarg.0
  op(OpCodes.Ldarg_0);
  // IL_0001: ldfld class [System]System.ComponentModel.PropertyChangedEventHandler Voodoo.ViewModel.GoalViewModel::PropertyChanged
  var eventHandlerFieldDef = _typeDef.Fields
    .FirstOrDefault(f => f.Name == PropertyChangedFieldName);
  il.Append(il.Create(OpCodes.Ldfld, eventHandlerFieldDef));
  // IL_0006: stloc.0
  op(OpCodes.Stloc_0);
  // // loop start (head: IL_0007)
  //    IL_0007: ldloc.0
  var loopTargetInsn = il.Create(OpCodes.Ldloc_0);
  il.Append(loopTargetInsn);
  //    IL_0008: stloc.1
  op(OpCodes.Stloc_1);
  //    IL_0009: ldloc.1
  op(OpCodes.Ldloc_1);
  //    IL_000a: ldarg.1
  op(OpCodes.Ldarg_1);
  //    IL_000b: call class [mscorlib]System.Delegate [mscorlib]System.Delegate::Combine(class [mscorlib]System.Delegate, class [mscorlib]System.Delegate)
  var combineMethodReference = new MethodReference(
    delegateMethodName, 
    _types.Delegate, 
    _types.Delegate);
  var delegateParamDef = new ParameterDefinition(_types.Delegate);
  combineMethodReference.Parameters.Add(delegateParamDef);
  combineMethodReference.Parameters.Add(delegateParamDef);
  il.Append(il.Create(OpCodes.Call, combineMethodReference));
  //    IL_0010: castclass [System]System.ComponentModel.PropertyChangedEventHandler
  il.Append(il.Create(OpCodes.Castclass, 
    _types.PropertyChangedEventHandler));
  //    IL_0015: stloc.2
  op(OpCodes.Stloc_2);
  //    IL_0016: ldarg.0
  op(OpCodes.Ldarg_0);
  //    IL_0017: ldflda class [System]System.ComponentModel.PropertyChangedEventHandler Voodoo.ViewModel.GoalViewModel::PropertyChanged
  il.Append(il.Create(OpCodes.Ldflda, eventHandlerFieldDef));
  //    IL_001c: ldloc.2
  op(OpCodes.Ldloc_2);
  //    IL_001d: ldloc.1
  op(OpCodes.Ldloc_1);
  //    IL_001e: call !!0 [mscorlib]System.Threading.Interlocked::CompareExchange<class [System]System.ComponentModel.PropertyChangedEventHandler>(!!0&, !!0, !!0)
  //    var declaringTypeRef = _typeDef.Module.Import(typeof(Interlocked));
  var declaringTypeRef = _types.Interlocked;
  var elementMethodRef = new MethodReference(
    "CompareExchange", 
    _types.Void, 
    declaringTypeRef);
  var genParam = new GenericParameter("!!0", elementMethodRef);
  elementMethodRef.ReturnType = genParam;
  elementMethodRef.GenericParameters.Add(genParam);
  var firstParamDef = new ParameterDefinition(
    new ByReferenceType(genParam));
  var otherParamDef = new ParameterDefinition(genParam);
  elementMethodRef.Parameters.Add(firstParamDef);
  elementMethodRef.Parameters.Add(otherParamDef);
  elementMethodRef.Parameters.Add(otherParamDef);
  var genInstanceMethod = new GenericInstanceMethod(elementMethodRef);
  genInstanceMethod.GenericArguments.Add(
    _types.PropertyChangedEventHandler);
  il.Append(il.Create(OpCodes.Call, genInstanceMethod));
  //    IL_0023: stloc.0
  op(OpCodes.Stloc_0);
  //    IL_0024: ldloc.0
  op(OpCodes.Ldloc_0);
  //    IL_0025: ldloc.1
  op(OpCodes.Ldloc_1);
  //    IL_0026: bne.un.s IL_0007
  il.Append(il.Create(OpCodes.Bne_Un_S, loopTargetInsn));
  // // end loop
  // IL_0028: ret
  op(OpCodes.Ret);
    
  return methodDef;
}