(The second in what seems to be a day of obscure .NET facts)

While looking at some C++/CLI code in Reflector today at work, we encountered the try ... fault construct.

What’s a fault block?

It’s like a finally block, but it’s only entered in the event of an exception.

So it’s like a catch block, then?

Not exactly:

• fault blocks don’t have filters like catch blocks do
• At the end of a fault block the exception is implicitly rethrown

Supposedly the benefit of a fault block is performance: that throw; statement at the end of a catch block has the same overhead as any other throw statement, whereas finally and fault blocks incur no runtime overhead. The stack is not unwound when a fault block is entered: fault blocks won’t appear in the exception stack trace.

Where do fault blocks show up?

In the code the C++/CLI compiler emits in order to make sure that local variables have their destructors called in the event of an exception.