WPF xmlns: The 'clr-namespace' URI refers to a namespace that is not included in the assembly

I am learning data binding in xaml and this xmlns problem is stopping me. My cs code is in namespace "Ping". Here is my MainWindow.xaml:

<Window x:Class="Ping.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:custom="clr-namespace:WPFDemo;assembly=WPFDemo"
    xmlns:local="clr-namespace:Ping"

The last line complains as i开发者_开发知识库n the title. Why is it not working? I am using "Ping" and it's the only namespace I got.

Thanks

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If Ping is the only namespace you're using, you should remove the reference to WPFDemo