Vista - Inheriting controls in WPF

I'm trying to have something like this in WPF:
Control_A: a control that inherits from GenericControl
GenericControl: an abstract control that inherits from UserControl.

I can not set Control_A to inherit from GenericControl:
I've changed the XAML in this way:

<src:GenericControl
x:Class="Control_A"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:src="clr-namespace:bdWebsiteManagerWPF"
Width="300"
Height="300">
<Grid>
</Grid>
</src:GenericControl>

I've also changed the type of the partial source class in this way:

Partial Public Class Control_A
Inherits GenericControl

But I got this error: "The tag 'GenericControl' does not exist in XML
namespace 'clr-namespace:bdWebsiteManagerWPF'."

So this is not a problem about the fact that GenericControl is
abstract, but a problem in the inheritance.

Thanks in advance.
Sid.

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