Initialize enum with a null value

Question

How do i create an enum with a null value

ex:

public enum MyEnum
{
    [StringValue("X")]
    MyX,
    [StringValue("Y")]
    MyY,      
    None
}

where None value is null or String.Empty

Possible duplicate of [How to set enum to null](http://stackoverflow.com/questions/4337193/how-to-set-enum-to-null) — Michael Freidgeim, Jul 26 '16 at 03:45

score 22 · Accepted Answer · answered Nov 28 '12 at 12:44

22

You can try something like this:-

public MyEnum? value= null;

or you can try this:-

 public MyEnum value= MyEnum.None;

answered Nov 28 '12 at 12:44

Rahul Tripathi

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Although his question asks for that answer it isn't what he was asking. – John Sobolewski Nov 28 '12 at 12:44
1

I think the OP is asking for a member of an enum to have a `null` value. – Oded Nov 28 '12 at 12:45
2

@jsobo what they are asking is impossible; this is fine as an answer IMO – Marc Gravell Nov 28 '12 at 12:50
2

Sorry to be so obtuse. The answer definitely answers what was asked. However I strongly believe the question was actually about how to allow MyEnum.None to have a StringValue of string.empty. – John Sobolewski Nov 28 '12 at 13:24

score 5 · Answer 2 · answered Nov 28 '12 at 12:48

5

use the "?" oeprator for a nullable type !!

public MyEnum? myEnum= null;

answered Nov 28 '12 at 12:48

shaouari

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Blachshma · Answer 3 · 2012-12-08T12:55:37.157

As all the other answers said, you can't have an enum value be null. What you can do is add a Description attribute (like your StringValue) which has a null value.

For instance:

public enum MyEnum
{
    [Description(null)]
    None = 0,
    [Description("X")]
    MyX,
    [Description("Y")]
    MyY
}

You can get the description of the Enum with the following function:

    public static string GetEnumDescription(Enum value)
    {
        FieldInfo fi = value.GetType().GetField(value.ToString());

        DescriptionAttribute[] attributes =
            (DescriptionAttribute[])fi.GetCustomAttributes(
            typeof(DescriptionAttribute),
            false);

        if (attributes != null && attributes.Length > 0)
            return attributes[0].Description;
        else
            return value.ToString();
    }

Usage:

MyEnum e = MyEnum.None;
string s2 = GetEnumDescription((MyEnum)e); // This returns null

Oded · Answer 4 · 2012-11-28T12:52:41.037

4

All enum types are value types and the different members are also derived from member types (the allowed types are byte, sbyte, short, ushort, int, uint, long, or ulong - as documented).

This means that members of an enumeration cannot be null.

One way to deal with a default enum value is to put the None value as the first value and setting it to a negative value (for signed enums) or 0.

public enum MyEnum
{
    [StringValue(null)]
    None = -1,
    [StringValue("X")]
    MyX,
    [StringValue("Y")]
    MyY      
}

You can have a null reference to an enumeration if you declare a nullable instance - myenum?.

edited Nov 28 '12 at 12:52

answered Nov 28 '12 at 12:47

Oded

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The value None should always have a value of 0. – erikkallen Nov 28 '12 at 12:53
1

@erikkallen - Why should it always be 0. It could also be -1, it actually could by any value, that cannot actually be a real value in the given situation. – Security Hound Nov 28 '12 at 13:03
Because the default initializer of an enum will set the enum to `(MyEnum)0`. [FxCop agrees](http://msdn.microsoft.com/en-us/library/ms182149(v=vs.80).aspx). Perhaps None shouldn't be the default always though, eg. if you have an enum of "negative options", perhaps `All` should be the default value. – erikkallen Nov 28 '12 at 20:35

score 2 · Answer 5 · answered Nov 28 '12 at 12:48

There is no such thing as an "enum with a null value". An enum in C# is just a named integer. In your case, None is just an alias for 2 (for completeness, MyX is 0 and MyY is 1). By default, the underlying data-type of an enum is int, but it can also be other integer-based primitive types of different sizes/sign.

If you want a "null" enum, you would have to use MyEnum?, aka Nullable<MyEnum>.

Initialize enum with a null value

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