Dec 8, 2019 in Dot Net

Q: Switch expressions in C#

1 Answer

Dec 8, 2019

Often, a SWTICH statement produces a value in each of its case blocks. Switch expressions enable you to use more concise expression syntax. There are fewer repetitive case and break keywords, and fewer curly braces. As an example, consider the following enum that lists the colors of the rainbow

public enum Rainbow

If your application defined an RGBColor type that is constructed from the R, G and B components, you could convert a Rainbow value to its RGB values using the following method containing a switch expression:

public static RGBColor FromRainbow(Rainbow colorBand) =>
    colorBand switch
        Rainbow.Red    => new RGBColor(0xFF, 0x00, 0x00),
        Rainbow.Orange => new RGBColor(0xFF, 0x7F, 0x00),
        Rainbow.Yellow => new RGBColor(0xFF, 0xFF, 0x00),
        Rainbow.Green  => new RGBColor(0x00, 0xFF, 0x00),
        Rainbow.Blue   => new RGBColor(0x00, 0x00, 0xFF),
        Rainbow.Indigo => new RGBColor(0x4B, 0x00, 0x82),
        Rainbow.Violet => new RGBColor(0x94, 0x00, 0xD3),
        _              => throw new ArgumentException(message: "invalid enum value", paramName: nameof(colorBand)),

There are several syntax improvements here:

  • The variable comes before the switch keyword. The different order makes it visually easy to distinguish the switch expression from the switch statement.
  • The case and : elements are replaced with =>. It's more concise and intuitive.
  • The default case is replaced with a _ discard.
  • The bodies are expressions, not statements.

Contrast that with the equivalent code using the classic switch statement:

public static RGBColor FromRainbowClassic(Rainbow colorBand)
    switch (colorBand)
        case Rainbow.Red:
            return new RGBColor(0xFF, 0x00, 0x00);
        case Rainbow.Orange:
            return new RGBColor(0xFF, 0x7F, 0x00);
        case Rainbow.Yellow:
            return new RGBColor(0xFF, 0xFF, 0x00);
        case Rainbow.Green:
            return new RGBColor(0x00, 0xFF, 0x00);
        case Rainbow.Blue:
            return new RGBColor(0x00, 0x00, 0xFF);
        case Rainbow.Indigo:
            return new RGBColor(0x4B, 0x00, 0x82);
        case Rainbow.Violet:
            return new RGBColor(0x94, 0x00, 0xD3);
            throw new ArgumentException(message: "invalid enum value", paramName: nameof(colorBand));
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