Hi. I've identified a difference of DLR between .NET 4.0 Beta 2 and the last release of .NET 4.0. In .NET 4.0 Beta 2, this code perfectly works at runtime : var dateTimeList = new List(); dynamic myDynamicObject = dateTimeList; object value = DateTime.Now; myDynamicObject.Add(value); Now, with last release of .NET 4.0, I have an exception at run time (to solve myDynamicObject.Add(value);) :-( In my real code, 'myDynamicObject' is a dynamic (but I know that it is always an ObservableCollection where T can be anything). 'value' is an instance which was got by some reflexions. As 'value' can have any type, the type of 'value' is Object. Do you see how can I solve this new limitation of .NET 4.0 ? Thanks