The type or namespace name 'ObservableCollection' could not be found

I am working with Entity Framework in asp.net c# framework 3.5 I have ge开发者_JAVA百科nerated entity classes using poco generator template. But I am getting following error:

The type or namespace name ObservableCollection could not be found (are you missing a using directive or an assembly reference?)

FYI System.Collections.ObjectModel is also added in class.

What could be wrong? How to resolve it?

using System.Collections.ObjectModel;

Add WindowsBase to your reference.

@Riz Please ensure that your project has a reference to System.Windows as it appears that ObservableColletion is contained in System.Collections.ObjectModel namespace which is in System.Windows in .Net 4.0 and System.Collection.ObjectModel in .Net 3.0 & 3.5. It wasn't available before that. In silverlight it can be found in System.Collections.ObjectModel.

Make sure System.Data.Entity Dll is version 4... not 2. Looks like it exist only on .net framework 4.0

I think ObservableCollection only exists in .NET 4.0.