Unit tests using StructurMap ObjectFactory fails - unit-testing

Question

I have a class that I want to write unit tests for. I'm using StructureMap (2.6.3) in the project and I have some problems with that.
For testing I use Nunit (2.6.0.12054) and RhinoMocks (3.6).
Normally i inject my dependencies in the constructor of my classes and then it's easy to substitute the dependencies with mocks in my unit tests. But there are a few cases where I can't do that, where I have no control over when the class under test is created. In these cases I use ObjectFactory.GetInstance() to get the dependencies.
public class MyClass
{
public int MyMethod(string parameter)
{
var myDependency = ObjectFactory.GetInstance<IMyDependency>();
try
{
return myDependency.CalculateValue(parameter);
}
catch (Exception ex)
{
//Suppress exception.
return 0;
}
}
}
For this class and method I want to write two tests. First I want to to test that the dependency is called and the value it calculates is returned by the class under test. Second, the dependency might throw an exception and I want to test that this exception is suppressed by the class under test and that it return zero in that case.
[TestFixture]
public class MyClassTests()
{
[Test]
public void MyMethod_DependencyReturnsValue_ReturnsValueFromDependency
{
//Arrange.
const int valueFromDependencyStub = 333;
var myDependencyStub = MockRepository.GenerateStub<IMyDependency>();
myDependencyStub.Stub(x => x.CalculateValue()).Return(valueFromDependencyStub);
ObjectFactory.Inject<IMyDependency>(myDependencyStub);
var target = new MyClass();
//Act.
var result = target.MyMethod("test");
//Assert.
Assert.AreEqual(valueFromDependencyStub, result);
}
[Test]
public void MyMethod_DependencyThrowsException_ReturnsZero
{
//Arrange.
var myDependencyStub = MockRepository.GenerateStub<IMyDependency>();
myDependencyStub.Stub(x => x.CalculateValue()).Throw(new Exception());
ObjectFactory.Inject<IMyDependency>(myDependencyStub);
var target = new MyClass();
//Act.
var result = target.MyMethod("test");
//Assert.
Assert.AreEqual(0, result);
}
}
Both these tests work if I run them individually, but if I run the both it does not work. I my real case the second test, where the stubbed dependency throws an exception, runs first. When the other test runs the stubbed dependency still throws an exception.
The problem, as I understand it, is that I'm using the global ObjectFactory and inject my stub into that. That would probably work if I could clear the ObjectFactory after each test, but I found no way to do that. ObjectFactory.EjectAllInstancesOf() sounded like something that would work, but it doesn't.
How do I solve this? Either by changing my test or by actually rewriting the class under test.