Strange error: 'DockingManager' does not exist in XML namespace ...' - Unblock DLL

C# .NET Framework 4.0

I have had a strange error in my WPF application when I referenced an external library (Xceed.Wpf.AvalonDock.dll).

This error

 Error The tag 'DockingManager' does not exist in XML namespace 'http://avalondock.codeplex.com'. Line 41 Position 10.

 appeared at design time but not at run-time, so after a few hours I’ve founded that this was a “security error”, because Windows protects you from components downloaded from the Internet.

After that I clicked on “Unblock” and the error is missed.

Thanks Microsoft … :-|