ThreadStateException thown in WatiN with Internet Explorer

I was using NUnit to Assert some responses from a suite of WatiN tests whilst using the IE browser. Running them caused a ThreadStateException with an error message of 'The CurrentThread needs to have its ApartmentState set to ApartmentState.STA to be able to automate Internet Explorer'

Internet Explorer is not thread safe so WatiN should use a single threaded apartment (STA). To ensure you abide by this WatiN will throw a ThreadStateException if it is not. This can be rectified by setting the ApartmentState to ApartmentState.STA as the error message suggests for Internet Explorer.

How set your CurrentThread to ApartmenState.STA for WatiN in NUnit

If you've haven't already create an App.config in your NUnit testing assembly. You will need to add the following to it:

<?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" />
   </TestRunner>
  </NUnit>
</configuration>

If you are having problems with different runners other than NUnit you can find tips on how to set STA on them too.