Choosing the default value of an Enum type without having to change values

An enum's default is whatever enumeration equates to zero. I don't believe this is changeable by attribute or other means.

(MSDN says: "The default value of an enum E is the value produced by the expression (E)0.")

The default value of any enum is zero. So if you want to set one enumerator to be the default value, then set that one to zero and all other enumerators to non-zero (the first enumerator to have the value zero will be the default value for that enum if there are several enumerators with the value zero).

enum Orientation
{
    None = 0, //default value since it has the value '0'
    North = 1,
    East = 2,
    South = 3,
    West = 4
}

Orientation o; // initialized to 'None'

If your enumerators don't need explicit values, then just make sure the first enumerator is the one you want to be the default enumerator since "By default, the first enumerator has the value 0, and the value of each successive enumerator is increased by 1." (C# reference)

enum Orientation
{
    None, //default value since it is the first enumerator
    North,
    East,
    South,
    West
}

Orientation o; // initialized to 'None'

If zero doesn't work as the proper default value, you can use the component model to define a workaround for the enum:

[DefaultValue(None)]
public enum Orientation
{
     None = -1,
     North = 0,
     East = 1,
     South = 2,
     West = 3
 }

public static class Utilities
{
    public static TEnum GetDefaultValue<TEnum>() where TEnum : struct
    {
        Type t = typeof(TEnum);
        DefaultValueAttribute[] attributes = (DefaultValueAttribute[])t.GetCustomAttributes(typeof(DefaultValueAttribute), false);
        if (attributes != null &&
            attributes.Length > 0)
        {
            return (TEnum)attributes[0].Value;
        }
        else
        {
            return default(TEnum);
        }
    }
}

and then you can call:

Orientation o = Utilities.GetDefaultValue<Orientation>();
System.Diagnostics.Debug.Print(o.ToString());

Note: you will need to include the following line at the top of the file:

using System.ComponentModel;

This does not change the actual C# language default value of the enum, but gives a way to indicate (and get) the desired default value.

The default for an enum (in fact, any value type) is 0 -- even if that is not a valid value for that enum. It cannot be changed.