C# - Get switch value if in default case

Question

Help please, I have this case:

switch(MyFoo()){
    case 0: //...
        break;
    case 1: //...
        break;
    case 2: //...
        break;
    default:
        // <HERE>
        break;
}

As you can see the switch gets the value directly from a method without saving it as a variable.

Is it possible to get which value fires the default case? For example if MyFoo() returns 7, how can I get that value?

I want to avoid to save the method result as a variable, is there a way to get the switch value from inside a case? Something like this:

default:
    this.SwitchValue // <<--
    break;

Thank you for reading, ~Saba

Answer 1

Is there a way to get the switch value from inside a case?

The only (proper) way is actually to store the result of MyFoo() in a variable.

var fooResult = MyFoo();
switch (fooResult)
{
    case 0:
        ...
        break;
    ...
    default:
        handleOthersCase(fooResult);
        break;
}

This code is readable and understandable and have no extra cost (As @SheldonNeilson says: It's on the stack anyway).

Also, the MSDN first example about switch totally look like this. You can also find informations int the language specification.

You also can make your own switch based on a dictionary, but the only advantage I see is that you can use it for complex cases (any kind of object instead of string/int/...). Performance is a drawback.

It may look like this:

public class MySwitch<T> : Dictionary<T, Action<T>>
{
    private Action<T> _defaultAction;

    public void TryInvoke(T value)
    {
        Action<T> action;
        if (TryGetValue(value, out action))
        {
            action(value);
        }
        else
        {
            var defaultAction = _defaultAction;
            if (defaultAction != null)
            {
                defaultAction(value);
            }
        }
    }

    public void SetDefault(Action<T> defaultAction)
    {
        _defaultAction = defaultAction;
    }
}

And be used like this:

var mySwitch = new MySwitch<int>();

mySwitch.Add(1, i => Console.WriteLine("one"));                             // print "one"
mySwitch.Add(2, i => Console.WriteLine("two"));                             // print "two"
mySwitch.SetDefault(i => Console.WriteLine("With the digits: {0}", i));     // print any other value with digits.

mySwitch.TryInvoke(42);                                                     // Output: "With the digits: 42"

Or based on this response, this:

public class MySwitch2<T>
{
    private readonly T _input;

    private bool _done = false;

    private MySwitch2(T input)
    {
        _input = input;
    }

    public MySwitch2<T> On(T input)
    {
        return new MySwitch2<T>(input);
    }

    public MySwitch2<T> Case(T caseValue, Action<T> action)
    {
        if (!_done && Equals(_input, caseValue))
        {
            _done = true;
            action(_input);
        }
        return this;
    }

    public void Default(Action<T> action)
    {
        if (!_done)
        {
            action(_input);
        }
    }
}

Can be used like that:

MySwitch2<int>.On(42)
    .Case(1, i => Console.WriteLine("one"))
    .Case(2, i => Console.WriteLine("two"))
    .Default(i => Console.WriteLine("With the digits: {0}", i));

Answer 2

I can't see a reason as well why to use it like that but may be a work around will be like this:

int x;
switch ( x = MyFoo())
{
    case 0: //...
        break;
    case 1: //...
        break;
    case 2: //...
        break;
    default:
        var s = x; // Access and play with x here
        break;
}