Switch expression returns null or the default value (0) for nullable Enum.

[djs2014]

Switch expression returns a null or Enum default value for nullable Enum.

OS: windows 10 and Linux Mint 20 Cinnamon - kernel 5.4.0-48-generic
SDK: .NET Core SDK (3.1.402)

General

In example 1 the value of orientation is Orientation.North

// example 1
            var direction = Directions.Up;
            Console.Write($"Map view direction is {direction}");

            Orientation? orientation = direction switch
            {
                Directions.Up => default,
                Directions.Right => Orientation.East,
                Directions.Down => Orientation.South,
                Directions.Left => Orientation.West,
            };

In example 2 the value of orientation is null

// example 2
            var direction = Directions.Right;
            Console.Write($"Map view direction is {direction}");

            Orientation? orientation = direction switch
            {
                Directions.Right => default,
            };

public enum Directions
    {
        Up,
        Down,
        Right,
        Left
    }

    public enum Orientation
    {
        North,
        South,
        East,
        West
    }

Unit test:
TestSwitch.zip

[svick]

I believe this is the expected behavior. The switch expression is target-typed, but only when there isn't a best common type for the right-hand side expressions.

The best common type of Orientation.East and default is Orientation, so default becomes default(Orientation).

The best common type of default doesn't exist, so target-typing is used and default becomes default(Orientation?).

In these cases, where it's not clear what default means, I'd suggest being more explicit and writing something like Orientation.North or 0, or null.

[333fred]

Yes, this expected behavior for a switch expression. I would also agree that using default in a context where it could be either a value type or a reference type is confusing, and you should be explicit.

[djs2014]

Thank you for the answer, the clearest way will then be:

// example 1
            var direction = Directions.Up;
            Console.Write($"Map view direction is {direction}");

            Orientation? orientation = direction switch
            {
                Directions.Up => default(Orientation?),
                Directions.Right => Orientation.East,
                Directions.Down => Orientation.South,
                Directions.Left => Orientation.West,
            };

orientation will then be null.

[jnm2]

I think null is clearer than default in every case.

[danmoseley]

@jaredpar can you pleae either close, or move to csharp-lang? I don't have rights to do so.

[alrz]

dotnet/roslyn#38270