The wackiest bug I've ever seen

I have a situation I can't explain. Look at the following code and notice how the variables c and d are defined and their values are strange. c equals null but d doesn't. Surely this is impossible(right?) given the following things I'm 100% certain of:

• a.value is a field, not a property
• the type qualifier is a sealed reference type, with base type object
• The application is just single threaded.
• I've done 'Clean Solution'.

Now the weirdest thing is that when I stop the debug run, replace the variable reference c in the lambda expression in the bottom line by d and build and run again, d is null butc` isn't!

private static object CreateConstraints(CompositeElement constraintsElement)
{
    Contract.Requires(constraintsElement != null);

    var constraintTypes = from e in constraintsElement where e.DefinitionName.IsAnyOf("ConstraintType", "ConstraintTypeNamePrependedWithComma") select (Qualifier)e.Value;
    var declarer = (MemberDeclaration)constraintsElem开发者_StackOverflow社区ent.Parent.Value;
    GenericTypeParameterCollection genericTypeParameters = declarer.Name.Suffixes.OfType<GenericTypeParameterCollection>().First();

    Element a = constraintsElement["TypeName"];
    a.SetValue();
    var c = a.value as Qualifier;
    var d = a.value as Qualifier;

    genericTypeParameters.First(gp => gp.Type == c).Constraints.AddRange(constraintTypes);

    return null;
}

I've also made screenshots of the disassemblies of both runs. Personally I don't know for sure whether they're incorrect.

So I want to ask if I'm missing something which could make this perfectly normal behavior, or is there something really weird going on here?

Call me daft but I don't really see a problem.

• from the description, there is no 'bug' . (Or does your programs logic go astray?)
• all we have is a variable c appearing twice in a Debugger window.
• one instance of c has the right type (and I assume the right value).
• the other one is null. This must be a view of the captured c and since the lambda is not executing it is null.

So what is supposed to be the problem?

You could try putting a breakpoint inside the lambda.

Looking at your debugger screenshot from before the edit, I notice that the local "c" is listed twice, once with value null, once with a value as an instance of a qualifier. This seems weird, and makes me suspect a bug in C#'s compilation of the lambda expression. As a workaround, try using an explicit delegate instead.

Also, there is an error squiggle on the double equals in the lambda expression. Hovering over that to get the error message may provide a clue as to what is going on.

Perhaps there is an exception being thrown deep in the call stack and the exception handling is being done in such a way as to prevent the assignments from being done properly.