Roslyn: Proposal: If / while with multiple statements condition

[vbcodec]

Add ability to put multiple statements into condition for if and while.
Example:

if (
var p = GetPoint();
p.X += 10;
p.Length > 20)
{
...
}

Last statement (required) is expression that provide bool value.

Benefits:

• better semantically corelated preparing and checking
• less effort to avoid conflicts with names of temporaryy variables

[orthoxerox]

There's an issue about "statement expressions" somewhere around.

[HaloFour]

This would be the "semi-colon expressions" that were proposed on CodePlex for C# 6.0 but were finally cut. Here's a mention: C# Design Notes for Dec 16, 2013

Those expressions would have extended to anywhere in the language:

bool b = (var p = GetPoint(); p.X += 10; p.Length > 20);

However considering the variable scoping of out and pattern variables to remain consistent the declared variable p would have to remain in the enclosing scope:

if ((var p = GetPoint(); p.X += 10; p.Length > 20)) {
   ...
}
// p is still in scope here

[HaloFour]

Here it is, Sequence Expressions: #6182

[vbcodec]

@HaloFour
The difference is that last statement (bool expression) allow for typeswitch, while #6182 probably not

[HaloFour]

@vbcodec

Is that in reference to the scope statement?

[vbcodec]

@HaloFour
Corrected, I meant #6182

[HaloFour]

@vbcodec

I don't see why sequence expressions would preclude type switch. The last statement in the sequence can be any expression, including a bool expression. Other than the fact that this proposal limits the sequence to within if and while conditions what differentiates it?

[alrz]

@HaloFour A pair of parentheses. I think the same thing for tuples is to be considered inside switch, default, method invocations etc, because double parentheses look confusing IMO,

This feature is also proposed for C++17 (link).

[HaloFour]

@alrz

That could be "fixed" by changing the spec to allow the if, while and do statements to accept a Boolean sequence expression directly:

_if-statement_:
if ( _boolean-expression_ ) _embedded-statement_
if ( _boolean-expression_ ) _embedded-statement_ else _embedded-statement_
if _boolean-sequence-expression_ _embedded-statement_
if _boolean-sequence-expression_ _embedded-statement_ else _embedded-statement_

Then both of the following would work:

bool b = (var p1 = GetPoint(); p1.X += 10; p1.Length > 20);
if (var p2 = GetPoint(); p2.X += 10; p2.Length > 20) {
   ...
}

[vbcodec]

@HaloFour
With #6182 you cannot use typeswitch while assigning value to variable, because you can write

bool b = (var p = GetPoint(); p.X += 10; p.Length > 20 && p is Point3d p3d);

which is incorrect. And that double braces.
Whils similar grammatically and semantically, they need different implementations.

[HaloFour]

@vbcodec

I don't see why not? A type-switch is just a Boolean expression that also assigns a variable. You can use it as follows:

bool b = p is Point3d p3d;

Of course p3d wouldn't be definitely assigned after that statement, but that's no different than various other situations with type switch:

if (x == y || p is Point3d p3d) {
    // oops, p3d not definitely assigned here
}