Partial declarations, must not specify different base classes? - wpf

Question

When I changed this:
public partial class FrmCategories : UserControl
to this:
public partial class FrmCategories : MyUserControl
Where MyUserControl inherits from UserControl.
I got this error:
Error 2 Partial declarations of 'WpfTest.FrmCategories' must not
specify different base classes
\Projects\WpfTest\WpfTest\FrmCategories.xaml.cs 21 26 WpfTest
XAML:
<UserControl x:Class="WpfTest.FrmCategories"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:we="clr-namespace:WpfTest"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
mc:Ignorable="d" Background="Azure" Height="131" Width="229">
<StackPanel Margin="5,24,5,0" Name="catFrm" Height="75" VerticalAlignment="Top">
I'm beginning WPF (as the name of the project implies), so I expect a trivial error here

Answer 1

You need to change the root element of the XAML file:
<we:MyUserControl x:Class="WpfTest.FrmCategories"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:we="clr-namespace:WpfTest"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
mc:Ignorable="d" Background="Azure" Height="131" Width="229">
<StackPanel Margin="5,24,5,0" Name="catFrm" Height="75" VerticalAlignment="Top">
</we:MyUserControl>

Answer 2

partial only means that you can split the class definition into different files. But still all the partial definitions define only one class which as such may only have one base class.
If you want to inherit from MyUserControl, you have to change the xaml code, too:
<we:MyUserControl x:Class="WpfTest.FrmCategories"
...>
...
</we:MyUserControl>
Also, notice that you only have to specify the base class once, i.e. you could even change the C# code to
public partial class FrmCategories
because the base class has been defined in the xaml.