Add "leave" keyword for escaping exception frames

[asyncawaitmvvm]

Microsoft's C++ has a __leave keyword for Structured Exception Handling (SEH) which is essentially a "break" for try-catch scopes. It immediately causes flow to exit the try scope and enter the finally scope. It's fantastic for setting up rigid C-style error cleanup without using a goto. This keyword if brought to C# would have strong precedent and get a lot of use.

For reference:
https://msdn.microsoft.com/en-us/library/9xtt5hxz.aspx

The __leave keyword is valid only within the guarded section of a try-finally statement, and its effect is to jump to the end of the guarded section. Execution continues at the first statement in the termination handler.
A goto statement can also jump out of the guarded section, but it degrades performance because it invokes stack unwinding. The __leave statement is more efficient because it does not cause stack unwinding.

[Neme12]

It's fantastic for setting up rigid C-style error cleanup without using a goto

Ok... so what is the advantage over goto?

[stakx]

It's fantastic for setting up rigid C-style error cleanup without using a goto. This keyword if brought to C# would have strong precedent and get a lot of use.

"C-style error cleanup" doesn't exactly sound like a tempting thing to have in C#. (Wouldn't error cleanup typically happen inside catch, anyways?)

Could you perhaps show an example of what a typical, good use of leave might look like?

[jnm2]

If I'm understanding right, inverting an if in a method results in if (!condition) return;. Within a loop it results in if (!condition) continue;. With this proposal, if you're within a try (or using?) block, inverting an if would result in if (!condition) leave;?

[HaloFour]

A goto statement can also jump out of the guarded section, but it degrades performance because it invokes stack unwinding. The __leave statement is more efficient because it does not cause stack unwinding.

That sounds like a pretty specific optimization for Visual C++. Considering that C# already emits leave/leave.s IL opcodes to exit from a protected region is there any reason to believe that a separate leave C# keyword would offer any benefits, beyond acting like a break statement out of the try/catch block?