Multiple statements in a switch expression: C# 8 [duplicate]

Question

Switch expressions were introduced in C# 8. There's plenty of places in codebases, which may be rewritten in this new style.

For example, I have some code, which is used for parsing packets from a stream of bytes:

switch (command)
{
    case Command.C1:
        return new P1();
    case Command.C2:
        return new P2();
    default:
        stream.Position++;
        return null;
}

The problem is - it can't be converted to a switch expression like

return command switch
{
    Command.C1 => new P1(),
    Command.C3 => new P2(),
    _ => { stream.Position++; return null; }
};

The first thing that got in my mind was to use a Func<>, which compiles:

return command switch
{
    Command.C1 => new P1(),
    Command.C3 => new P2(),
    _ => new Func<AbstractPacket>(() => { stream.Position++; return null; })()
};

F# already allows code with multiple statements in each branch:

match command with
| Command.C1 -> Some(P1() :> AbstractPacket)
| Command.C2 -> Some(P2() :> AbstractPacket)
| _ ->
    stream.Position <- stream.Position + 1
    None

Now I'm stuck using switch-statements, but is there any option to write this as a switch-expression without any weird hacks?

Personally, I would convert to a switch statement in such cases. That said, C# LDM is discussing some options to allow statements inside switch expressions and possibly more generally ("expression blocks"). Here's the proposal we most recently reviewed: github.com/dotnet/csharplang/issues/3086 — Julien Couvreur
– Julien Couvreur, Commented Jan 24, 2020 at 5:13
The switch expression is not meant to replace the switch statement. It serves a different purpose. It will not handle multiple statements, and it shouldn't, because you're just switching on a value to get the right single expression evaluated. So you're entirely right, getting the switch expression to handle multiple statements is a pain, in pretty much the same way as using a hammer to split a plank in two is. — Lasse V. Karlsen
– Lasse V. Karlsen, Commented Jan 24, 2020 at 8:57
@JL0PD you can use local functions to combine multiple statements into one function that can be used in any expression, including switch expressions — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Jan 24, 2020 at 10:14

John Lord · Accepted Answer · 2022-09-14 14:18:52Z

37

Your only supported choice is the func like you did. See [this article][1] for more information. His example:

var result = operation switch
{
"+" => ((Func<int>)(() => {
    Log("addition");
    return a + b;
}))(),
"-" => ((Func<int>)(() => {
    Log("subtraction");
    return a - b;
}))(),
"/" => ((Func<int>)(() => {
    Log("division");
    return a / b;
}))(),
_ => throw new NotSupportedException()
};

Just because switch expressions are new doesn't mean they are the best for all use cases. They are not designed to contain multiple commands.

Edit: I suppose you could also simply call external functions instead of making anonymous ones. [1]: https://alexatnet.com/cs8-switch-statement/

edited Sep 14, 2022 at 14:18

answered Jan 24, 2020 at 4:42

John Lord

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3 Comments

Jeroen van Langen Over a year ago

"It's not designed to contain multiple commands." and is certainly not more readable.

John Lord Over a year ago

@Jeroen van Langen that's the understatement of the year.

John Lord Over a year ago

this is actually very similar to javascript event functions. They are designed for a single call but you can embed a function in them.

Sebastian Kühn · Accepted Answer · 2020-09-17 21:36:50Z

6

With:

TRes Call<TRes>(Func<TRes> f) => f();

its looks like:

return command switch {
  Command.C1 => new P1(),
  Command.C3 => new P2(),
  _ => Call(() => { stream.Position++; return null; }),
};

or:

var result = operation switch {
  "+" => Call(() => {
    Log("addition");
    return a + b;
  }),
  "-" => Call(() => {
    Log("subtraction");
    return a - b;
  }),
  "/" => Call(() => {
    Log("division");
    return a / b;
  }),
  _ => throw new NotSupportedException(),
};

answered Sep 17, 2020 at 21:36

Sebastian Kühn

771 silver badge1 bronze badge

2 Comments

DCCoder Over a year ago

Generally, answers are much more helpful if they include an explanation of what the code is intended to do, and why that solves the problem without introducing others.

John Lord Over a year ago

it's also basically ripping off my answer

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