Control from WPFToolkit doesn't exist in namespace

I had the exact same problem.

If I skipped the unblock step and simply unzipped, the xaml preview window would not load and VS would keep giving me the 'IntegerUpDown component does not exist in the namespace http://schemas.xceed.com/wpf/xaml/toolkit' error, even though auto-complete would happily list all the components in that namespace.

However if I unblock the zip file first, then extract, then reference the dll in VS, it all works correctly.

TL;DR: follow the installation instructions exactly, particularly unblocking the zip file first.

NumericUpDown is not part of the basic WPF Toolkit but part of the Extended WPF Toolkit

Use the IntegerUpDown (or any of the provided derived classes) and be sure to use the appropriate DLL in your application. Here is an example using the IntegerUpDown when the Extended WPF Toolkit DLL (Xceed.Wpf.Toolkit.Dll) is referenced by your project:

<Window x:Class="WpfApplication4.Window1"        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"        xmlns:toolkit="http://schemas.xceed.com/wpf/xaml/toolkit"        Title="Window1" Height="300" Width="300">    <Grid>      <toolkit:IntegerUpDown Value="10" Increment="1" Minimum="0" Maximum="10" />   </Grid></Window>

try

xmlns:toolkit="http://schemas.xceed.com/wpf/xaml/toolkit"