Xaml compiler and designer treating clr-namespace differently

Question

I have a very simple wpf app I built to demo a class library I built, but I am coming up on some strange behavior:

xmlns:lcl="clr-namespace:testApp;assembly=testApp"

This above is the important line, but I am including everything to be complete.

<Window x:Class="testApp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:lcl="clr-namespace:testApp;assembly=testApp"
        xmlns:evt="clr-namespace:EventToCommand;assembly=EventToCommand"
        Title="MainWindow" Height="350" Width="525">
    <Window.DataContext>
        <lcl:ViewModel />
    </Window.DataContext>
    <Label Name="lblOutput" VerticalAlignment="Center" HorizontalAlignment="Center" Content="{Binding labelOutput}" />
</Window>

When I include the assembly, the designer works, and the binding on the label text works properly. However, when I go to build and run the program, I get the following error:

Error   1   The tag 'ViewModel' does not exist in XML namespace 'clr-namespace:testApp;assembly=testApp'

When I remove the assembly portion of that line, the designer stops showing me anything, but the program builds and runs perfectly.

What is going on and how do I stop it?

EDIT:

Here is the top of the AssemblyInfo.cs file to show that that is the correct name:

[assembly: AssemblyTitle("testApp")]
[assembly: AssemblyDescription("")]
[assembly: AssemblyConfiguration("")]
[assembly: AssemblyCompany("Microsoft")]
[assembly: AssemblyProduct("testApp")]
[assembly: AssemblyCopyright("Copyright © Microsoft 2013")]
[assembly: AssemblyTrademark("")]
[assembly: AssemblyCulture("")]

Answer 1

Try changing the reference xmlns:lcl="clr-namespace:testApp;assembly=testApp" to xmlns:lcl="clr-namespace:testApp" .

When classes are in the same project view, it is not necessary to use "assembly="