[Proposal] not specifying all cases in a switch statement generates a compilation error (like in Kotlin)

[MihailsKuzmins]

First of all, I apologise if a similar discussion has already been opened (something like #782).

The current behaviour must not be changed, so maybe it is possible to introduce a new keyword (like required) to force a compilation error if not all cases are specified.

public enum Test
{
    Test1,
    Test2,
    Test3,
    Test4,
}

The code snippet below generates a warning, but compiles: The 'switch' expression does not handle all possible inputs (it is not exhaustive). For example, the pattern 'Test.Test3' is not covered.

var result = Test.Test1 switch
{
    Test.Test1 => "ok",
    Test.Test2 => "ok?",
};

It would be nice to have something that gives errors (obvious feedback) when a new enum is added.

var result = Test.Test1 switch required
{
    Test.Test1 => "ok",
    Test.Test2 => "ok?",
};

[TahirAhmadov]

You can make that warning be treated as an error in your project file.

[MihailsKuzmins]

Even though <WarningsAsErrors>CS8509</WarningsAsErrors> works, it is necessary to keep a note of the correct warning code, so I thought it would be much easier if such a feature is available on the language level

[spydacarnage]

This is the feature being available at language level. The language/compiler produces a warning which you can then either suppress, ignore, or turn into a warning as you see fit.

Imagine the following code:

int choice = Random.Shared.Next(3);
var result = choice switch 
{
    0 => OptionA(),
    1 => OptionB(),
    2 => OptionC()
};

As a developer, I know that is already exhaustive, but to the compiler, there are millions of options I haven't dealt with. If it produced an error for that, it would be inconvenient. The same applies to enums - just because you've only specified 4 values, that enum is backed by an int, so your switch statement is not as exhaustive as you think it is.

There are other discussions about closed enums that would fit your bill, but until then, you have either the compiler switch to treat as warnings, or you have the option of writing an analyzer to perform absolute exhaustiveness checking.