Question: I have a class that I want to extend the WPF Window class. I cant find the class to extend. I think it's in the `System.Windows` namespace, but I have no access to it. Can anyone help. 

Please let me know if you require anymore information.

I am building the project as a class library.

Current references are:

```
Microsoft.CSharp
System
System.Core
System.Data
System.Data.DataSetExtentions
System.Xml
System.Xml.Linq

```

I am trying to extend the `System.Windows.Window` class, but when I type `System.`, there is no `.Windows`.

Answer: 
You need to add a reference to PresentationFramework (in PresentationFramework.dll)
The [Window](http://msdn.microsoft.com/en-us/library/system.windows.window.aspx) class is declared in the PresentationFramework.dll assembly, so you need to add a reference to that.