Why is TargetInvocationException treated as uncaug

问题:

I have some code that is using reflection to pull property values from an object. In some cases the properties may throw exceptions, because they have null references, etc.

object result;
try
{
    result = propertyInfo.GetValue(target, null);

}
catch (TargetInvocationException ex)
{
    result = ex.InnerException.Message;
}
catch (Exception ex)
{
    result = ex.Message;
}

Ultimately the code works correctly, however when I am running under the debugger:

When the property throws an exception, the IDE drops into the debugger as if the exception was uncaught. If I just hit run, the program flows through and the exception comes out as a TargetInvocationException with the real exception in the InnerException property.

How can I stop this from happening?

回答1:

This seems to be "by design". What happens is that you likely have menu Tools → Options → Debugging → General → Enable Just My Code enabled.

As How to: Break on User-Unhandled Exceptions states:

The Debug → Exceptions dialog shows an additional column (Break when an exception is User-unhandled) when "Enable Just My Code" is on.

Essentially this means that whenever the exception is leaving the boundary of your code (and in this case, it falls through down to the .NET framework reflection code), Visual Studio breaks because it thinks that the exception has left the user code. It doesn't know that it will return into the user code later in the stack.

So there are two workarounds: Disable Just My Code in menu Tools → Options → Debugging → General or Remove the check box from the User-unhandled .NET Framework exceptions in menu Debug → Exceptions dialog.

回答2:

EDIT: I've just tried this myself, and it looks like reflection is treated slightly differently. You might want to think of a reflection call as starting a new level of "handled" as far as the debugger is concerned: nothing is catching that exception before it gets translated and rethrown as a TargetInvocationException, so it breaks in. I don't know if there's any way of inhibiting that - but does it happen very often? If you're regularly performing lots of operations which result in exceptions, you might want to reconsider your design.

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Original answer

Go to Debug / Exceptions... and see what the settings are. You'll see this behaviour if TargetInvocationException (or anything higher in the hierarchy) has the "Thrown" tickbox checked.