Variable Aliasing "Out var" style

[TonyValenti]

Hi C# Team,
I love how you can create, assign, and use variables with out var in situations like this:

if(int.TryParse(StringValue, out var x) && x >= 21){
  Console.WriteLine("Have a beer!");
}

I would like to propose that the language be extended to allow similar constructs where ever variables are used. Specifically comparison evaluations.

Here are some examples of what I would like to be able to do:

if(Company.CEO.Age CEOAge >= 21  && CEOAge < 45){
  Console.WriteLine("This example assigns CEOAge the value of Company.CEO.Age");
}

if(Company.Financials.CalculateIncome(x,y,z) CompanyIncome > Threshhold){
  Console.WriteLine($"The company made {CompanyIncome} which exceeded {Threshhold}");
}

I think that scoping rules should be identical to Out-Var.
This has the following benefits:

• I don't have to create variables outside of my scope in order to access them.
• Aliases allow performance benefits because the value is evaluated once and reused.
• Cleaner code.

[HaloFour]

You can use is var:

if (Company.CEO.Age is var CEOAge && CEOAge >= 21 && CEOAge < 45) {
}

[jnm2]

@HaloFour beat me by seconds :D

[Logerfo]

@HaloFour as far as I know, is var always returns true. If that's right, it's weird that the compiler still flag checks it: https://sharplab.io/#v2:EYLgtghgzgLgpgJwDQBMQGoA+ABADAAmwEYBuAWAChsBmQgJkKIHZ8BvS/Tw24gNkIAsjABwAKAJRsu0mdI6z8ASwBmohoqj4AbhAT4AXuPkKuAYQD2AOyjmANnAB0AdQSL4owyRPH8AX0q+QA==

[svick]

@Logerfo

Only in Debug mode: https://sharplab.io/#v2:EYLgtghgzgLgpgJwD4AEAMACFBGA3AWACgUBmLAJi2wHYMBvIjJrMnANiwBYqAOACgCU9ZiNEjGYjAEsAZn0pSoGAG4QEGAF4CJk5gGEA9gDsoBgDZwAdAHUEU+Hy25dOjAF8iboA===

[DavidArno]

@HaloFour

You can use is var:

Or, using my suggestion of an "as pattern":

if (Company.CEO.Age as var CEOAge >= 21 && CEOAge < 45) {
}

(which is basically the same as this suggestion and neatly avoids the need to introduce the variable and perform the comparison as two separate expressions).

[Logerfo]

@svick if I build a solution in VS with debug variant, it will still use the release version of the compiler, right?

[svick]

@Logerfo How you compile the compiler doesn't matter. What matters is what you tell the compiler to do. If you tell it to compile your code in Debug mode, it produces less optimized (and easier debuggable) IL, which in this case includes the unnecessary flag check.

And the Debug/Release option in SharpLab has the same effect as the Debug/Release option in VS.

[Logerfo]

@svick thank you very much