Allow comparison patterns to declare a variable

[YairHalberstadt]

eg.

public class C
{
    public void CheckLength(string str) {
        if (str.Length is > 100 length)
            throw new Exception($"{length} is greater than maxlength of 100");
    }
}

[YairHalberstadt]

Current workaround:

public class C
{
    public void CheckLength(string str) {
        if (str.Length is var length and > 100)
            throw new Exception($"{length} is greater than maxlength of 100");
    }
}

[jnm2]

I've been writing as str.Length is > 100 and var length because it's closer. I'd also like str.IndexOf(c) is not -1 index.

[333fred]

You can use and patterns:
https://sharplab.io/#v2:EYLgtghgzgLgpgJwDQxASwDZICYgNQA+AAgEwCMAsAFDVEDMABKQwMLUDe1D3TjRALKwAWcAMYBrADJwAdgHMYQgBREyABgawEASgacqPQwzQAzBkq0A6afMXGoDAHwN1GiDOwMAbhAQMMsgpC2lxGYYoIAPYA7gwycLEAogAeonAADjBokTJKACQAROwBtkIAvvYMcghwEPB+iu4MkMklQQyRZq4F2gDcodxl1GVAA=

I completely missed Yair's workaround comment.

[spydacarnage]

This might just be a bad example for the requested feature, because why would you not just use:

public class C
{
    public void CheckLength(string str) {
        if (str.Length > 100)
            throw new Exception($"{str} is greater than maxlength of 100");
    }
}

[YairHalberstadt]

It was purely a made up on the spot example. Let's say that I don't write very long exception messages :-)

[spydacarnage]

Fair enough :-)

…

On Tue, 21 Jul 2020 at 20:24, Yair Halberstadt ***@***.***> wrote: It was purely a made up on the spot example. Let's say that I don't won't very long exception messages :-) — You are receiving this because you commented. Reply to this email directly, view it on GitHub <https://github.com/dotnet/csharplang/issues/3709#issuecomment-662059662>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ADIEDQOO6LAXLOCU4Z762WLR4XTO5ANCNFSM4PD3EGPQ> .

[TonyValenti]

eg.

public class C
{
    public void CheckLength(string str) {
        if (str.Length is > 100 length)
            throw new Exception($"{length} is greater than maxlength of 100");
    }
}

I am a huge fan of this.

[TonyValenti]

I would use it for declaring variables in my IF statements that are the result of a computation. Basically like the following:

public class C
{
    public void Test1(int a, int b) {
        if(a + b is > 100 c && c <200){
                throw new Exception($@"{c} is not in range");
        }

    }
}

[DavidArno]

@TonyValenti,

Your example highlights my concern with this proposal. Your example can already be written as:

if (a + b is var c and > 100 and < 200)
{
        throw new Exception($@"{c} is not in range");
}

which takes the "pure pattern" approach rather than mixing patterns and boolean expressions as your example does. So this proposal risks making patterns more complex just to save having to type and var as per @YairHalberstadt /@jnm2's examples. I see their examples as highlighting how to use patterns rather than in any way being a "work-around".

[canton7]

Agreed. We consistently emphasise that readability trumps character count.

str.Length is > 100 length reads to me as "(str.Length) (is greater than) (100 length)", and leaves me wondering what on earth "100 length" is. The > character, being punctuation, naturally stands out as separating the two text bits on each side, which is how precedence in conditions normally works. I know you intend it to be parsed as "((str.Length) (is greater than) (100)) (length)", but it takes me a couple of scans through it to get to that point.

At least str.Length is var length and > 100, although still hard to parse, separates the "variable declaring" bit and the condition bit, and the > character still has high precedence.

That said, I'd still write this as:

if (str.Length is var length && length > 100) { }

(which lets the && punctuation break up the condition a bit), or even;

int length = str.Length;
if (length > 100) { }

---

I do wonder whether something Go-like might be clearer:

if (int length = str.Length; length > 100) { }

Or something swift-like:

if (int length = str.Length, length > 100) {} 

Using is var always feels dirty: is means you're asking a question. This makes sense with e.g. x is string s, you're asking whether x is a string. x is var s is a question which only has one answer, being abused to assign a variable.

[canton7]

These are also just weird:

if (str.Length is > 50 length and < 100)
if (str.Length is > 50 and < 100 length)

[Flutterish]

Id much rather just write

if ( ( var length = str.Length ) > 100 )

Which turns out is not valid syntax, which surprised me bc this was the very first thing i thought of when i saw the proposal