>Don't forget that intellisense is listing in namespace order. Double check that the namespaces used in the assembly are what you thought/are looking for (I've made that mistake before.....)
>>

I checked my Members.Library project properties and it had the Assembly Name and default namespace set to Members. I've changed both to Members.Library, but it hasn't made a difference. All my code in my class library all had Members.Library as the namespace.

>>>To narrow things down a bit I'd try temporarily adding a new project and trying to reference the Members.Library assembly from there (initially without actually making use of the assembly elswhere in XAML)
>>>

I added a new Winforms project and put using Members.Library in the Program.cs and it didn't complain (it also showed me the intellisense for Library after I had typed in Members.)

I then added a new WPF Application and in the default Window1.xaml tried to add the xmlns:Members= and Members.Library was not in the intellisense list.

This is the code:

<Window x:Class="TestWpf.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:Members="Mem"
    Title="Window1" Height="300" Width="300">
    <Grid>
        
    </Grid>
</Window>