WPF - globally add xaml namespace declaration

I have quite a huge WPF application with a lot of XAML files. Every single XAML file has from 5 to 10 clr to xml namespace mappings xmlns:abc="clr-namespace:Abcdef".
It looks awful and is a pain to write in each and every file.

Is there a way to define those globally?

There is no way to define them globally across files. This is a limitation of XML; XAML is a subset of it.

However you can clean them up a bit using XmlnsDefinition

See this article: http://zachbonham.blogspot.com/2010/04/organize-xaml-namespace-declarations.html

If you started with this XAML:

<UserControl
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:treeView="clr-namespace:MaryKay.SamPortal.Common.UI.TreeView.Views;assembly=MaryKay.SamPortal.Common.UI"
    xmlns:infoBar="clr-namespace:MaryKay.SamPortal.Common.UI.InfoBar.Views;assembly=MaryKay.SamPortal.Common.UI">
  <infoBar:InformationBar DataContext="{Binding InfoBar}"/>
</UserControl>

And added these XmlnsDefinition attributes:

[assembly: XmlnsDefinition("urn:marykay-samportal-common-ui", "MaryKay.SamPortal.Common.UI.InfoBar.Views")]
[assembly: XmlnsDefinition("urn:marykay-samportal-common-ui", "MaryKay.SamPortal.Common.UI.RoleGroupPicker.Views")]
[assembly: XmlnsDefinition("urn:marykay-samportal-common-ui", "MaryKay.SamPortal.Common.UI.BetterPopup")]
[assembly: XmlnsDefinition("urn:marykay-samportal-common-ui", "MaryKay.SamPortal.Common.UI.TextEditor")]
[assembly: XmlnsDefinition("urn:marykay-samportal-common-ui", "MaryKay.SamPortal.Common.UI.Converters")]
[assembly: XmlnsDefinition("urn:marykay-samportal-common-ui", "MaryKay.SamPortal.Common.UI.Documents")]
[assembly: XmlnsDefinition("urn:marykay-samportal-common-ui", "MaryKay.SamPortal.Common.UI.SplashScreen")]
[assembly: XmlnsDefinition("urn:marykay-samportal-common-ui", "MaryKay.SamPortal.Common.UI.TemplateSelector")]
[assembly: XmlnsDefinition("urn:marykay-samportal-common-ui", "MaryKay.SamPortal.Common.UI.ModalDialog")]
[assembly: XmlnsDefinition("urn:marykay-samportal-common-ui", "MaryKay.SamPortal.Common.UI.ConsultantSearch.Views")]
// etc...

You could end up with this XAML instead:

<UserControl
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:commonUI="urn:marykay-samportal-common-ui">
  <commonUI:InformationBar DataContext="{Binding InfoBar}"/>
</UserControl>