I'm testing some code that does work whenever assemblies are loaded into an appdomain. For unit testing (in VS2k8's built-in test host) I spin up a new, uniquely-named appdomain prior to each test with the idea that it should be "clean":

[TestInitialize()]  
public void CalledBeforeEachTestMethod()  
{  
 AppDomainSetup appSetup = new AppDomainSetup();  
 appSetup.ApplicationBase = @"G:\<ProjectDir>\bin\Debug";
 Evidence baseEvidence = AppDomain.CurrentDomain.Evidence;  
 Evidence evidence = new Evidence( baseEvidence );  
 _testAppDomain = AppDomain.CreateDomain( "myAppDomain" + _appDomainCounter++, evidence, appSetup );  
}  

[TestMethod]
public void MissingFactoryCausesAppDomainUnload()
{
 SupportingClass supportClassObj = (SupportingClass)_testAppDomain.CreateInstanceAndUnwrap(
GetType().Assembly.GetName().Name,
typeof( SupportingClass ).FullName );  
 try  
 {  
  supportClassObj.LoadMissingRegistrationAssembly();  
  Assert.Fail( "Should have nuked the app domain" );  
 }  
 catch( AppDomainUnloadedException ) { }  
}

[TestMethod]
public void InvalidFactoryMethodCausesAppDomainUnload()
{
 SupportingClass supportClassObj = (SupportingClass)_testAppDomain.CreateInstanceAndUnwrap(
GetType().Assembly.GetName().Name,
typeof( SupportingClass ).FullName );  
 try  
 {  
  supportClassObj.LoadInvalidFactoriesAssembly();  
  Assert.Fail( "Should have nuked the app domain" );  
 }  
 catch( AppDomainUnloadedException ) { }  
}

public class SupportingClass : MarshalByRefObject
{
 public void LoadMissingRegistrationAssembly()  
 {  
  MissingRegistration.Main();  
 }  
 public void LoadInvalidFactoriesAssembly()  
 {  
  InvalidFactories.Main();  
 }  
}

If every test is run individually I find that it works correctly; the appdomain is created and has only the few intended assemblies loaded. However, if multiple tests are run in succession then each _testAppDomain already has assemblies loaded from all previous tests. Oddly enough, the two tests get appdomains with different names. The test assemblies that define MissingRegistration and InvalidFactories (two different assemblies) are never loaded into the unit test's default appdomain. Can anyone explain this behavior?