Probably, this is because you are confusing C#/.NET namespaces with XAML namespaces. Yes, they are mapped together, but it should be specified in your XAML top element's namespace declarations. Intellisense helps you to choose the write one: when you start typing in the attribute starting with "xmlns:x " (where x is any unique
namespace prefix you are planning to use, Intellisense will show you the list of choices on the right of text insertion point. Just try it to see. The attribute value mapped to correct .NET namespace will be written in correct syntax for you.
For background, please see this MSDN article:
https://msdn.microsoft.com/en-us/library/ms747086%28v=vs.110%29.aspx[
^].
—SA