C＃中的switch语句“使用未分配的局部变量”编译器错误？

Question

I have the following C# code:

AnimalTypeEnum animal;
string s = Console.ReadLine();
switch (s.ToLower())
{
case "dog":
    animal = AnimalTypeEnum.DOG;
    break;
case "cat":
    animal = AnimalTypeEnum.CAT;
    break;
case "rabbit":
    animal = AnimalTypeEnum.RABBIT;
    break;
}

Console.WriteLine(animal); #compiler error here

I get this error on the last line: Use of unassigned local variable 'animal' . I know that it's because animal may not have a value depending on the user input, so how do I fix that?

Ideally I'd like to show an error message if an unknown animal type was entered and make the user input the value again.

Thanks.

Answer 1

Here's one way to fix it, using recursive calls instead of needing to catch and throw exceptions, or use a loop (loops in a case like this obfuscate the meaning in my opinion; too much about how you're doing it instead of what you're doing):

private static AnimalTypeEnum GetAnimalFromInput()
{
    AnimalTypeEnum animal;
    string s = Console.ReadLine();
    switch (s.ToLower())
    {
        case "dog":
            animal = AnimalTypeEnum.DOG;
            break;
        case "cat":
            animal = AnimalTypeEnum.CAT;
            break;
        case "rabbit":
            animal = AnimalTypeEnum.RABBIT;
            break;
        default:
            Console.WriteLine(s + " is not valid, please try again");
            animal = GetAnimalFromInput();
            break;
    }
    return animal;
}
static void Main(string[] args)
{
    AnimalTypeEnum animal = GetAnimalFromInput();

    Console.WriteLine(animal);
}

I'll also note that it's good practice to refactor your switch into an if/else chain, using if (s.Equals("dog", StringComparison.CurrentCultureIgnoreCase)) (or the appropriate case-insensitive comparison) to keep it working in other cultures. Of course, this may not apply to your scenario (eg test/homework app, or something that will only possibly be used in your culture).

---

Update: Thanks to Mennan Kara for the idea, if your values (eg "dog" ) will always match the enum's values (eg DOG ), then you can use Enum.TryParse to improve your code:

private static AnimalTypeEnum GetAnimalFromInput()
{
    AnimalTypeEnum animal;
    string s = Console.ReadLine();
    if (Enum.TryParse(s, true, out animal))
        return animal;
    else
    {
        Console.WriteLine(s + " is not valid, please try again");
        return GetAnimalFromInput();
    }
}

If you need the flexibility of having them separate, then keep your existing switch.

Answer 2

In case s.ToLower() is something else that dog , cat or rabbit , animal has no value.

You should add default in your switch for that case:

switch (s.ToLower())
{
case "dog":
    animal = AnimalTypeEnum.DOG;
    break;
case "cat":
    animal = AnimalTypeEnum.CAT;
    break;
case "rabbit":
    animal = AnimalTypeEnum.RABBIT;
    break;
default:
    animal = ...
    break;
}

Answer 3

AnimalTypeEnum animal;
var s = Console.ReadLine();
Console.WriteLine(!Enum.TryParse(s, true, out animal) ? "Not a valid animal" : animal.ToString());

Answer 4

You should have a default ENUM for any animal unknown to your code . You could even make your code to learn new animals. For instance.

switch (s.ToLower())
{
default:
    animal = AnimalType.Unkown;
    break;
}

or

default:
    animal = new MakeEnum(s.ToLower());
    myEnumList.Add(animal);
    break;

Your MakeEnum basically just needs to check length of current number of enums, and make a new enum using the number or some other parameter.