I have written before about running Windows Forms GUI tests on a compiled application exe here.

And i must say that it works GREAT!

I had a few problems of which the most annoying was the Drag and Drop functionality. If you have it enabled with AllowDrop = true 

on your controls or any other 3rd party controls the moment your control gets loaded you will get this error:

System.InvalidOperationException: DragDrop registration did not succeed. ---> 
System.Threading.ThreadStateException: Current thread must be set to single thread apartment (STA) mode 
                                       before OLE calls can be made. 
                                       Ensure that your Main function has STAThreadAttribute marked on it.
                                       at System.Windows.Forms.Control.SetAcceptDrops(Boolean accept)

So what to do? At first i disabled all DragDrop in my app so i can start writing test, but this was a hack.

The problem is because my app was running under Single-Threaded Apartment (STA) and NUnit framwork runs under Multithreaded Apartment (MTA)

I asked on the SourceForge NunitForms Help Forums and a fellow named Steve Clark pointed me in the right direction.

NUnit has a configuration section where you can define the apartment model you wish to run it in.

This is a simple config file of the startup project for NUnitForms test running with the STA fix:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
    <configSections>
        <sectionGroup name="NUnit">
            <section name="TestRunner" type="System.Configuration.NameValueSectionHandler"/>
        </sectionGroup>
    </configSections>
    <NUnit>
        <TestRunner>
            <!-- Valid values are STA,MTA. Others ignored. -->
            <add key="ApartmentState" value="STA" />
            <!-- See ThreadPriority enum for other valid values -->
            <add key="ThreadPriority" value="Normal" />
        </TestRunner>
    </NUnit>
</configuration>

Hope this helps.