How to have a common superclass for xaml controls?

How to have a common superclass for xaml controls?

For your requirement, you could make abstract base class for UserControl. And the key procedure is add the base class namesapce into the xaml.

For example:

namespace ButtonResourceTest
{
    public sealed partial class CustomUserControl : UserControlBase
    {
        public CustomUserControl()
        {
            this.InitializeComponent();
        }
    }
    public  abstract class UserControlBase : UserControl
    {


    }


}

Xaml

modify the default UserControl root node to local:UserControlBase.

<local:UserControlBase
    x:Class="ButtonResourceTest.CustomUserControl"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:ButtonResourceTest"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"

    d:DesignHeight="300"
    d:DesignWidth="400">

    <Grid>

    </Grid>
</local:UserControlBase>