Examine an Exception in a Catch() Block

Found a useful nugget in Richter’s recent CLR via C# book I want to share with you. But first some background.

Sometimes when I write a catch block, I don’t really have any plans for the caught exception. The following is a contrived example that is somewhat realistic.

try
{
    DoSomething();
}
catch(System.SomeException)
{
    DoSomethingElse();
    throw;
}

In the above code, I only want DoSomethingElse() to execute if DoSomething() throws an exception of type SomeException. I can’t put DoSomethingElse() in a finally block because then it would always get called and not just when the exception is thrown. I don’t need to do anything with SomeException because I am propagating it up the callstack via the throw keyword to let some other method handle it.

But now, as I am stepping through the code in the debugger, I may actually want to examine SomeException when the debugger reaches the line DoSomethingElse(). Typically I would have to rewrite the code like so:

try
{
    DoSomething();
}
catch(System.FormatException e)
{
    DoSomethingElse();
    throw;
}

Just so I can examine the exception now stored in the variable e. This is plain dumb and Richter points out why in a little tip in his book. You can use the debugger variable $exception provided by the Visual Studio.NET Debugger to examine the exception in a catch block. I wish I had known about this a while ago.