XAML Namespace Beginning with a Number - wpf

Question

I have inheritted Button and am trying to add it to my main window.
It doesn't show up in the Toolbox. I have tried rebuilding several times.
I have tried adding xmlns:dc="clr-namespace:123Letters and xmlns:dc="clr-namespace:123Letters;assembly=123Letters to my MainWindow.
My MainWindow is:
<Window x:Class="MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
x:Name="MainWin"
Title="123 Letters">
</Window>
My WeatherStationButton is:
<Button x:Class="WeatherStationButton"
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:d="http://schemas.microsoft.com/expression/blend/2008"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="300">
<Grid>
</Grid>
</Button>
and
Public Class WeatherStationButton
Inherits Button
Public Property WeatherStation As tblWeather
End Class
This is super simple stuff. I believe it's because XAML doesn't allow numbers in the first part of namespaces, but I can not find any reference to that anywhere, so I am asking if I am doing something wrong or is this one of XAML's "features"?

Answer 1

Just realized I could add my answer.
If your project name is 123Letters, you need to add an underscore before the first number when referencing it in XAML like, _123Letters.
Your clr-namespace declaration would then be:
xmlns:dc="clr-namespace:_123Letters"
and you could add your inherited button to your MainWindow like this:
<dc:WeatherStationButton x:Name="btnWS" />