Why do "Not all code paths return a value" with a switch statement and an enum?

I have the following code:

public int Method(MyEnum myEnum)
{
    switch (myEnum)
    {
        case MyEnum开发者_JAVA技巧.Value1: return 1;
        case MyEnum.Value2: return 2;
        case MyEnum.Value3: return 3;
    }
}

public enum MyEnum
{
    Value1,
    Value2,
    Value3
}

And I get the error: "Not all code paths return a value". I do not understand how that switch statement could ever not jump to one of the specified cases.

Can an enum somehow be null?

There's nothing to say that the value of myEnum will be one of those values.

Don't mistake enums for being a restrictive set of values. It's really just a named set of values. For example, I could call your method with:

int x = Method((MyEnum) 127);

What would you want that to do? If you want it to throw an exception you can do that in a default case:

switch (myEnum)
{
    case MyEnum.Value1: return 1;
    case MyEnum.Value2: return 2;
    case MyEnum.Value3: return 3;
    default: throw new ArgumentOutOfRangeException();
}

Alternatively you could use Enum.IsDefined upfront, if you want to do some other work before the switch statement. That has the disadvantage of boxing... there are some ways round that, but they're generally more work...

Sample:

public int Method(MyEnum myEnum)
{
    if (!IsDefined(typeof(MyEnum), myEnum)
    {
        throw new ArgumentOutOfRangeException(...);
    }
    // Adjust as necessary, e.g. by adding 1 or whatever
    return (int) myEnum; 
}

This assumes there's an obvious relationship between the underlying values in MyEnum and the value you want to return.

Enums are not limited to values they represent. You can assign this:

MyEnum v = (MyEnum)1000;

And there would be no problem at all. Add a default to your switch and you'll handle all possible situations.

If you change the values in your enum (adding a fourth) your code will break. You should add a default: case to your switch statement.

of course, the other way to achieve this would be to define the integer values in your enum...

public enum MyEnum
{
    Value1 = 1,
    Value2 = 2,
    Value3 = 3
}

and then cast your enum as an int in code. Instead of int myInt = Method(myEnumValue); you can use int myInt = (int)myEnum

MyEnum blah = 0;

Default is always 0 and is implicitly convertible, even if you do not have one with a 0 value.

It has to be either:

public int Method(MyEnum myEnum)
{
    switch (myEnum)
    {
        case MyEnum.Value1: return 1;
        case MyEnum.Value2: return 2;
        case MyEnum.Value3: return 3;
        default: return 0;
    }
}

or:

public int Method(MyEnum myEnum)
{
    switch (myEnum)
    {
        case MyEnum.Value1: return 1;
        case MyEnum.Value2: return 2;
        case MyEnum.Value3: return 3;
    }

    return 0;
}