I have been facing a silly bug related to AppDomain, so stupid that it is worth mentionned in a blog.

The symptoms

Load an assembly in a separate AppDomain, create an instance of a type. Things work correctly. Then unload the domain, and recreate the type. Somehow, strange bugs (Custom Attributes not detected, the debugger messed up) arise.

How-to recreate it:

Assume that we have the Assembly MbUnit.Core.dll, MbUnit.Framework.dll (that references Core) and Test.dll that references both. Core defines an abstract Custom Attribute TestPatternFixtureAttribute,

public abstract TestFixturePatternAttribute :Attribute{...}

public class TestFixtureAttribute : TestFixturePatternAttribute{...}

[TestFixture]
public class MyTest{...}

1. Create a separate AppDomain using AppDomain.CreateDomain,
2. Create an instance of RemoteTestTree using AppDomain.CreateInstanceFromAndUnWrap
3. Load Test.dll in the domain and find fixtures. At this point, everything works and we found the fixtures.
4. Unload the separate AppDomain using AppDomain.UnLoad,
5. Re-Create an instance of RemoteTestTree (at this point, weird things happen because the debugger seems to tell: RemoteTestTree is not initialized)
6. Load Test.dll in the domain and find fixtures. This time, no fixture is found because the framework seems to tell that TestFixtureAttribute does not inherit from TestPatternAttribute.

The fix

In point 2,5, I have been using AppDomain.CreateInstanceFromAndUnWrap, this is the error. I should have been using AppDomain.CreateInstanceAndUnWrap. The difference is subtle, 4 letters.

The explanation

I'm still unsure of the explanation.... so I'm waiting for someone to come up with a relevant explanation :)