How to tell if an enum property has been set? C#

You can use one of two methods: default enum value or a nullable enum.

Default enum value

Since an enum is backed by an integer, and int defaults to zero, the enum will always initialize by default to the value equivalent to zero. Unless you explicitly assign enum values, the first value will always be zero, second will be one, and so on.

public enum Color
{
  Undefined,
  Red,
  Green
}

// ...

Assert.IsTrue(Color.Undefined == 0);  // success!

Nullable enum

The other way to handle unassigned enum is to use a nullable field.

public class Foo
{
   public Color? Color { get; set; }
}

// ...

var foo = new Foo();
Assert.IsNull(foo.Color);     // success!

You can make it so that the underlying private field is nullable, but the property is not.

E.g.

class SomeClass
{
    private Color? _color; // defaults to null

    public Color Color
    {
        get { return _color ?? Color.Black; }
        set { _color = value; }
    }

    public bool ColorChanged
    {
        get { return _color != null; }
    }
}

That way if color == null you know it hasn't been set yet, and you are also stopping the user from setting it to null (or undefined as other answers specify). If color is null you have 100% certainty that the user has not set it ever.

Only catch is the default value returned by the get but you could always throw an excepting if it better matches your program.

You can also take it one step further by making it so that the set only sets the field if the given value is not equal to the default value (depending on your use case):

public Color Color
{
    get { return _color ?? Color.Black; }
    set
    {
        if (value != Color)
        {
            _color = value;
        }
    }
}

Enums are Value Types, which means they are not references to an object stored somewhere else, and hence they cannot be null. They always have a default value just like int which will default to zero upon creation. I suggest two approaches:

1. Add another enum entry called e.g., None with value equal to zero. This way your enum value will default to None upon creation. Then you can check if(foo.ColorType != Color.None).

2. Make your Color property a nullable one like: public Color? ColorType { get; set; }. Now it will default to null and can be assigned the value null. Read more about nullable types here: MSDN - Nullable Types (C#).