How to check whether an enum variable is assigned with a single enum value?

Question

I would like to restrict the user to use any of the enumvalue.if the user tries to assign more than two values,i have to throw an error.

public enum Fruits
{
    Apple,
    Orange,
    Grapes
}

public Fruits Fruits
{
    get
    {
        return m_fruits;
    }
    set
    {
        m_fruits=value;
    }
}

Fruits = Fruits.Apple & Fruits.Grapes; //I need to avoid such type of assignment

Can anyone say how to check this type of validation.

Thanks, Lokesh

Answer 1

You can use Enum.IsDefined to see whether an integral value is actually defined for an enum. In order to be able to detect that no logical operation such as Fruits.Apple | Fruits.Grapes has been used make sure that your enum values are flags.

public enum Fruits
{
    Apple = 1,
    Orange = 2,
    Grapes = 4
}

public Fruits Fruit
{
    get
    {
        return m_fruits;
    }
    set
    {
        if (!Enum.IsDefined(typeof(Fruits), value))
        {
            throw new ArgumentException();
        }
        m_fruits = value;
    }
}

Update:

A faster method without reflection would be to check all enum values yourself:

public Fruits Fruit
{
    get
    {
        return m_fruits;
    }
    set
    {
        switch (value)
        {
            case Fruits.Apple:
            case Fruits.Grapes:
            case Fruits.Orange:
                m_fruits = value;
                break;
            default:
                throw new ArgumentException();
        }
    }
}

Answer 2

You can use the Enum.IsDefined() method to check whether a value is a valid enum value:

Fruits f1 = Fruits.Orange;
Fruits f2 = (Fruits)77;

var b1 = Enum.IsDefined(typeof(Fruits), f1); // => true
var b2 = Enum.IsDefined(typeof(Fruits), f2); // => false

---

BTW: Your example seems incorrect: var f = Fruits.Apple&Fruits.Grapes will assign Fruits.Apple to f. This is because a bitwise AND of Apple (==0) and Grapes (==2) will result in 0 (which still is a valid enum value -> Apple).

If you meant something like var f = Fruits.Orange|Fruits.Grapes, then f will now be 3 (bitwise OR of 1 and 2) and Enum.IsDefined will now return false.