This piece of code has a huge bug in it. Can you see it?
I’ll give you a hint, what is going to happen if Key, IV or Mode are going to throw an exception? Well, let us check the actual generated code from Reflector.
As you can see, we first set the value to a local variable, we set the properties and only then we enter the using clause. If any of the properties will throw out an exception, we will never enter the using block, and the object will never be disposed.
Personally, I think that this is an issue with the compiler, that it should be able to generate safe code for this scenario. As it is, the code looks fine, but the subtle bug can come and bite you.