.NET NUnit test - Assembly.GetEntryAssembly() is null

You could do something like this with Rhino Mocks: Encapsulate the Assembly.GetEntryAssembly() call into a class with interface IAssemblyLoader and inject it into the class your are testing. This is not tested but something along the lines of this:

[Test] public void TestSomething() {
  // arrange
  var stubbedAssemblyLoader = MockRepository.GenerateStub<IAssemblyLoader>();
  stubbedAssemblyLoader.Stub(x => x.GetEntryAssembly()).Return(Assembly.LoadFrom("assemblyFile"));

  // act      
  var myClassUnderTest = new MyClassUnderTest(stubbedAssemblyLoader);
  var result = myClassUnderTest.MethodToTest();

  // assert
  Assert.AreEqual("expected result", result);
}

public interface IAssemblyLoader {
  Assembly GetEntryAssembly();
}
public class AssemblyLoader : IAssemblyLoader {
  public Assembly GetEntryAssembly() {
    return Assembly.GetEntryAssembly();
  }
}

Implement the SetEntryAssembly(Assembly assembly) method given in

http://frostwave.googlecode.com/svn-history/r75/trunk/F2DUnitTests/Code/AssemblyUtilities.cs

to your unit test project.

         /// <summary>
        /// Use as first line in ad hoc tests (needed by XNA specifically)
        /// </summary>
        public static void SetEntryAssembly()
        {
            SetEntryAssembly(Assembly.GetCallingAssembly());
        }

        /// <summary>
        /// Allows setting the Entry Assembly when needed. 
        /// Use AssemblyUtilities.SetEntryAssembly() as first line in XNA ad hoc tests
        /// </summary>
        /// <param name="assembly">Assembly to set as entry assembly</param>
        public static void SetEntryAssembly(Assembly assembly)
        {
            AppDomainManager manager = new AppDomainManager();
            FieldInfo entryAssemblyfield = manager.GetType().GetField("m_entryAssembly", BindingFlags.Instance | BindingFlags.NonPublic);
            entryAssemblyfield.SetValue(manager, assembly);

            AppDomain domain = AppDomain.CurrentDomain;
            FieldInfo domainManagerField = domain.GetType().GetField("_domainManager", BindingFlags.Instance | BindingFlags.NonPublic);
            domainManagerField.SetValue(domain, manager);
        }