c# - The type or namespace name 'ObservableCollection' could not be found in .net 2.0

Question

I am getting the below error when try to use ObservableCollection in .net 2.0 framework

Error 3 The type or namespace name 'ObservableCollection' could not be found (are you missing a using directive or an assembly reference?)

i knew following assemblies are needed to use ObservableCollection. but am using .net 2.0 framework. so no windowsBase.dll there in 2.0 framework.

in WindowsBase.dll (.NET 3.x)
in System.dll (.NET 4.x)
in System.Windows.dll (Silverlight/Windows Phone 7)
in System.ObjectModel.dll (WinRT) (included in ".NET for Windows Store apps")

how can i resolve the issue .net 2.0 framework. is there any way ?

Thanks,

Siva