System.MissingMethodException was unhandled

I have been creating a "string array" for use in a BizTalk orchestration. My first version was quick and dirty, and I didn't implement the IEnumerator interface.

Later, I decided to implement IEnumerator (and IEnumerable) so that it would be more recognizable to the next person reading my code. Everything compiled and a test program started, but the above exception was thrown when the client tried to call the Reset() method.

One thing that I didn't realize was related was that the debugger wouldn't step into my code. I saw the message below when I tried to do that:

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Microsoft Visual Studio
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The following module was built either with optimizations enabled or without debug information:

C:\WINDOWS\assembly\GAC_MSIL\AtlasReo.BizTalk.Utils\1.0.0.0__799aa29801fe6d60\AtlasReo.BizTalk.Utils.dll

To debug this module, change its project build configuration to Debug mode. To
suppress this message, disable the 'Warn if no user code on launch' debugger option.
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OK
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As many people reading this have probably figured out by now, I had an older version of my string array class in the GAC. Oops.

Pulling the assembly out of the GAC solved the first issue, and then I could also run the debugger on the assembly code.