1/// <summary>
2/// Redirects Debug Assert dialog messages to console output.
3/// </summary>
4/// <remarks>
5/// Debug assertions are written to NUnit's Console output tab.
6/// </remarks>
7[TestFixtureSetUp]
8public void TestFixtureSetUp()
9{
10    // Disable Debug traces
11    Trace.Listeners.Clear();
12
13    // Disable Debug assert message boxes
14    using (DefaultTraceListener listener = new DefaultTraceListener())
15    {
16        listener.AssertUiEnabled = false;
17        Trace.Listeners.Add(listener);
18    }
19
20    // Restore Debug traces to NUnit's Console.Out tab.
21    Trace.Listeners.Add(new TextWriterTraceListener(Console.Out));
22}
23

Add to TestFixtureSetup method to enable this functionality for your Test fixture.