[Proposal]: Allow pattern matching to assign to new variable in more cases

[OJacot-Descombes]

Given the declaration

class C
{
    public string? P { get; set; }
}

we can write

C c = new();

if (c is { P: { } s }) { }         // OK

This tests the property P for being not null and assigns its value to the new variable s if the result is true. This is not very readable. It would be more readable if we could write:

if (c is { P: not null s }) { }    // Does not compile

There are also other cases where assigning a new variable would be handy:

if (c is { P.Length: > 0 i }) { }  // Does not compile
if (c is { P: "x" or "y" s }) { }  // Does not compile

Proposal: allow pattern matching to test an expression and conditionally declare a new variable to the result of that expression in more cases.

[333fred]

Note that in all of these cases, you can just append and var s to declare a variable (the or case could wrap that part in parentheses). I'm generally unconvinced that removing the need for an and pattern results in code that is easier to understand.

[OJacot-Descombes]

Good point. not null and var s is more explicit than { } s but looks very convoluted. I would still prefer the latter in this case.

[Unknown6656]

@OJacot-Descombes or, you could simply do:

if (c is { P: string s })
{
    ...
}

https://sharplab.io/#v2:C4LgTgrgdgPgAgJgIwFgBQ70GEAEBjHAXhygFMB3ACiggBtaBKAbnQHpX8iSLKAiYUgGdgvZjnY4APAFppOaHgD2AW2WkowHADNFYHAE8AhvoCEOEA0wY0ASy05KBG4JwBvHAAUQOOEgAMOC4AvpZoOOE+SACclAAkvEb65gxugkGiLGiktIKk6BGRMfFQigAOpOZ8zFboYKRKYAAmOFiUvn4A/J7MQA

[OJacot-Descombes]

Sure, you can write c is { P: string s } or c is { P: object s }, but this is a type test and it does not clearly express the intent of testing for not null.

[TahirAhmadov]

There are also other cases where assigning a new variable would be handy:

These examples look a bit strange to me, it's not as easy to read.

if (c is { P: not null s }) { } // Does not compile

This would be great though.

[OJacot-Descombes]

Does if (c is { P: { } s }) { } look less strange and easier to read to you? (We are not talking about the "strangeness" of pattern matching in general. We are comparing different ways of expressing the same pattern matching test.)