Catch two exceptions in the same catch block?

If you can upgrade your application to C# 6 you are lucky. The new C# version has implemented Exception filters. So you can write this:

catch (Exception ex) when (ex is CommunicationException || ex is SystemException) {
    //handle it
}

Some people think this code is the same as

catch (Exception ex) {                
    if (ex is CommunicationException || ex is SystemException) {
        //handle it
    }
    throw;
}

But it´s not. Actually this is the only new feature in C# 6 that is not possible to emulate in prior versions. First, a re-throw means more overhead than skipping the catch. Second, it is not semantically equivalent. The new feature preserves the stack intact when you are debugging your code. Without this feature the crash dump is less useful or even useless.

See a discussion about this on CodePlex. And an example showing the difference.