Switch Expression (on conditions)

[TonyValenti]

It would be great if I could do the following:

...
var Value = switch {
   Value1 == Value2 => "Hello",
   Value2 > Value3 => "World",
   Value4 > Value5 && Value6.Foo() => "Complex",
  _ => "Default",
};

[HaloFour]

#812

[333fred]

I don't think that's quite it, as I'm assuming Tony is looking for non-constant patterns as well? Relational operators are coming in 9.

[kyoyama-kazusa]

You can use true switch with discards.

var Value = true switch {
    _ when Value1 == Value2 => "Hello",
    _ when Value2 > Value3 => "World",
    _ when Value4 > Value5 && Value6.Foo() => "Complex",
    _ => "Default",
};

Although true switch is really used rarely...
See this link.

[TonyValenti]

@Sunnie-Shine Wow! That's sweet! I'm glad that exists. It would be really nice if I could just emit the true and _ when.

[bramborman]

And what about the ternary operator? The syntax is a bit different but even less verbose :)

...
var Value =
   Value1 == Value2 ? "Hello"
   : Value2 > Value3 ? "World"
   : Value4 > Value5 && Value6.Foo() ? "Complex"
   : "Default"
;

[surfsky]

The current Pattern Matching switch expression is unreadable, un-c-like syntax. And it brings 'and' 'or' keywords that will obscure with '&&' '||' symbols.

The tranditional c/cpp/c# are syntax-strict, and use () {} || && to make codes readable and maintainable. Too free and oral syntax will make language unmaintainable, the pattern matching switch expression is a dangerous start.

Why not change the switch code to c-like syntax:

    int age = 10;
    var stage = switch(age)
    {
        (age < 0) =>  LiftStage.Prenatal,
        (age < 2) =>  LifeStage.Infant,
        (age < 4) =>  LifeStage.Toddler,
        _         =>  LifeStage.LateAdult,
    };

or

    int longvariant = 10;
    var stage = switch(longvariant as t)
    {
        (t < 0) =>  LiftStage.Prenatal,
        (t < 2) =>  LifeStage.Infant,
        (t < 4) =>  LifeStage.Toddler,
        _       =>  LifeStage.LateAdult,
    };

or use other keyword "map" to avoid syntax collision?

    int t = 10;
    var stage = map(t)
    {
        (t < 0) =>  LiftStage.Prenatal,
        (t < 2) =>  LifeStage.Infant,
        (t < 4) =>  LifeStage.Toddler,
        _       =>  LifeStage.LateAdult,
    };

[HaloFour]

@surfsky

The switch expression syntax has already shipped as a part of C# 8.0.

And for the and/or patterns, they were chosen specifically to differ from && and || because these patterns can be used in if statements where it would be impossible for the compiler to differentiate between them as patterns or as operators.

Specifically:

if (x is true || false) and if (x is true or false) mean two very different things, and the former is already legal C# syntax.

[surfsky]

I maybe know the reason, but this syntax is so ..... oral and unreadable and make csharp much more complex and chaos, It's maybe not a good start.

[belav]

I find myself using ternarys like this

var Value =
   Value1 == Value2 ? "Hello"
   : Value2 > Value3 ? "World"
   : Value4 > Value5 && Value6.Foo() ? "Complex"
   : "Default";

And saw the alternative

var Value = true switch {
    _ when Value1 == Value2 => "Hello",
    _ when Value2 > Value3 => "World",
    _ when Value4 > Value5 && Value6.Foo() => "Complex",
    _ => "Default",
};

That said, it would be nice to have a "cleaner" version of the switch expression. Something like

var Value = switch {
    Value1 == Value2 => "Hello",
    Value2 > Value3 => "World",
    Value4 > Value5 && Value6.Foo() => "Complex",
    _ => "Default",
};

But I don't know that it comes up often enough to spend time adding it to the language.

[TonyValenti]

I would definitely use that all the time.