“is”表达式的结果在运行时返回false，但在检查时为true

Question

I have the following code. The CustomControlHelper generates an instance of an object via reflection. At this stage we don't know what type of object we are dealing with. We do know it will be a CustomControl , but we don't know if it implements any particular interface or if it extends any other classes. The following code is trying to establish whether the loaded control implements the IRichAdminCustomControl interface.

Object obj = CustomControlHelper.GetControl(cc.Id, cc.ControlClass);            
if(obj != null)
{
    bool isWhatWeWant = (obj is IRichAdminCustomControl);
    return isWhatWeWant;
}

That's all fine, but I've noticed that when I know I have an object that implements IRichAdminCustomControl , the expression evaluates to false.

Okay, this is where it gets really weird. If I inspect the code when debugging, the expression evaluates to true, but then if I immediately let the code run and inspect the result, it evaluates to false (I've attached an animated gif below to illustrate).

Has anyone come across weirdness like this before and if so, what on earth is causing it?

Incidentally, I believe the product I'm using uses Spring.NET to provide dependency injection in the CustomControlHelper.

Answer 1

If you are using Visual Studio 2010 SP1, I came across this bug:

Misreporting of variable values when debugging x64 code

There is a workaround on that page, posted by Microsoft:

You can either set all projects to compile to x86, or create an intermediate initialised variable declaration to ensure the debugger reports the correct value of the variable being examined.

Try this as a workaround:

bool isWhatWeWant = true;
isWhatWeWant &= (obj is IRichAdminCustomControl);
bool finalValue = isWhatWeWant; // this line should fix isWhatWeWant too in the debugger
return finalValue;

EDIT: seems like VS2012 also encounters similar problems in specific conditions.

Answer 2

This happened to me once and even though I never came to a conclusion as to why it was happening I believed the PDB files that were being loaded with the debugging symbols where out of sync. So, by "cleaning" the solution and then rebuilding the solution this weird issue went away.

Answer 3

Two possibilities come to mind. The first is that your interface name is generic enough that it could already be in the namespace somewhere. Try fully qualifying the interface in the is clause. The second possibility is that you might be running the code as part of a constructor, or being called indirectly by a constructor. Any reflection like stuff needs to be done after we are certain the application has fully loaded.

Answer 4

So I found the answer. It was because I had two copies of the dll in different locations. I had one copy in the bin of my back-end application and one in a shared external directory that gets dynamically loaded by the backend app.

I should explain; this application consists of two apps running in tandem, a frontend app and a backend app. Ordinarily, you place "Custom Controls" into your frontend app. These controls are then copied on application start to a external directory that is accessible to the backend app.

In this case, I had logic in my Custom Control library that needed to be accessed in the backend app - so I had to make a reference to it... which ended up with the backend app having two references to the same class. D'oh! And OF COURSE that's going to work when you're debugging.

Solution was to split my extra logic into its own project and reference THAT in the backend app.

I'm not going to "accept" my own answer here because, although it solved my specific problem, the solution is a bit TOO specific to the apps I'm working with and would be unlikely to help anyone else.