C# 8 switch expression with multiple cases with same result

Question

How can a switch expression be written to support multiple cases returning the same result?

With C# prior to version 8, a switch may be written like so:

var switchValue = 3;
var resultText = string.Empty;
switch (switchValue)
{
    case 1:
    case 2:
    case 3:
        resultText = "one to three";
        break;
    case 4:
        resultText = "four";
        break;
    case 5:
        resultText = "five";
        break;
    default:
        resultText = "unkown";
        break;
}

When I am using the C# version 8, with the expression syntax, it's like so:

var switchValue = 3;
var resultText = switchValue switch
{
    1 => "one to three",
    2 => "one to three",
    3 => "one to three",
    4 => "four",
    5 => "five",
    _ => "unknown",
};

So my question is: How to turn the cases 1, 2 and 3 to just one switch-case-arm so the value doesn't need to be repeated?

Update per suggestion from "Rufus L":

For my given example, this works.

var switchValue = 3;
var resultText = switchValue switch
{
    var x when (x >= 1 && x <= 3) => "one to three",
    4 => "four",
    5 => "five",
    _ => "unknown",
};

But it's not exactly what I want to accomplish. This is still only one case (with a filter condition), not multiple cases yielding to the same right-hand result.

Answer 1

C# 9 supports the following:

var switchValue = 3;
var resultText = switchValue switch
{
    1 or 2 or 3 => "one, two, or three",
    4 => "four",
    5 => "five",
    _ => "unknown",
};

Alternatively:

var switchValue = 3;
var resultText = switchValue switch
{
    >= 1 and <= 3 => "one, two, or three",
    4 => "four",
    5 => "five",
    _ => "unknown",
};

Source

---

For older versions of C#, I use the following extension method:

public static bool In<T>(this T val, params T[] vals) => vals.Contains(val);

like this:

var switchValue = 3;
var resultText = switchValue switch
{
    var x when x.In(1, 2, 3) => "one, two, or three",
    4 => "four",
    5 => "five",
    _ => "unknown",
};

It's a little more concise than when x == 1 || x == 2 || x == 3 and has a more natural ordering than when new [] {1, 2, 3}.Contains(x).

Answer 2

I got around to installing it, but I have not found a way to specify multiple, separate case labels for a single switch section with the new syntax.

However, you can create a new variable that captures the value and then use a condition to represent the cases that should have the same result:

var resultText = switchValue switch
{
    var x when
        x == 1 ||
        x == 2 ||
        x == 3 => "one to three",
    4 => "four",
    5 => "five",
    _ => "unknown",
};

This is actually more concise if you have many cases to test, because you can test a range of values in one line:

var resultText = switchValue switch
{
    var x when x > 0 && x < 4 => "one to three",
    4 => "four",
    5 => "five",
    _ => "unknown",
};

Answer 3

Sadly this appears to be a shortcoming in the switch-expression syntax in C# 8, relative to the switch-statement syntax. As other posters have suggested, if you are stuck with C# 8 then the rather clumsy var syntax is your only real option.

So you might have been hoping you could write:

switchValue switch {
    Type1 t1:
    Type2 t2:
    Type3 t3 => ResultA, // where the ResultX variables are placeholders for expressions.
    Type4 t4 => ResultB,
    Type5 t5 => ResultC
};

Instead you will need to write the rather awkward code below, with typename sprayed about:

switchValue switch {
    var x when x is Type1 || x is Type2 || x is Type 3 => ResultA,
    Type4 t4 => ResultB,
    Type5 t5 => ResultC
};

In such a simple example, you can probably live with this awkwardness. But more complicated example are much less liveable with. In fact my examples are actually a simplification of an example drawn from our own code base, where I was hoping to convert a switch-statement, with roughly six outcomes but over a dozen type-cases, into a switch-expression. And the result was clearly less readable than the switch-statement.

My view is that if the switch-expression needs shared outcomes and is more than a few lines long, then you are better off sticking to a switch-statement. Boo! It's more verbose but probably a kindness to your teammates.

ResultType tmp;
switch (switchValue) {
    case Type1 t1:
    case Type2 t2:
    case Type3 t3:
        tmp = ResultA;
        break;
    case Type4 t4:
        tmp = ResultB;
        break;
    case Type5 t5:
        tmp = ResultC;
        break;
};
return tmp;

UPDATE: However, as pointed out by others, if you can move to C# 9 or later, then you can make use of the or pattern syntax. Then you could write:

return switchValue switch {
    Type1 t1 or Type t2 or Type t3 => ResultA,
    Type4 t4 => ResultB,
    Type5 t5 => ResultC
    _ => default
};

Answer 4

If you're still using C# 8 and so can't use the C# 9 1 or 2 or 3 => method as in the above answer, you can also write it in a way that avoids creating the extra variable x with the same value as switchValue as in the accepted answer (which is completely correct of course, I'm just suggesting an alternative as I don't like that extra x).

var list = new List<int> { 3, 4, 5 };

var resultText = switchValue switch
{
    1 => "one",
    2 => "two",
    _ when list.Contains(switchValue) => "three to five",
    _ => "unknown",
};

Obviously you can replace the condition after the _ when with whatever, like switchValue == 3 || switchValue == 4 || switchValue == 5; the point of the answer is to show the _ when bit.

I just think of _ when => meaning "else when" and _ => meaning "else".

Answer 5

I have done in this way :

public string GetValue(string name)
            {
                return name switch
                {
                    var x when name is "test1" || name is "test2" => "finch",
                    "test2" => somevalue,
                    _ => name
                };
            }

Answer 6

If your switch type is a flag enum

[System.Flags]
public enum Values 
{
    One = 1, 
    Two = 2, 
    Three = 4,
    Four = 8,
    OneToThree = One | Two | Three
}

var resultText = switchValue switch
{
    var x when Values.OneToThree.HasFlag(x) => "one to three",
    Values.Four => "4",
    _ => "unknown",
};