My observations indicate, that this is from the Enhancer. Telerik Data Access uses special Enhancer
program, to read and process compiled .NET assemblies for adding persistence capability. Is possible that Enhancer is changing your symbol file (.pdb) somehow.
Our recommendation is to isolate the model in a separate class library project, in which case Enhancer will work only on it. Also, you can see your variable, in the Watch window, after your yield iteration ended.
I hope this information helps.
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