Why Can C# Enums Have Non-Unique Integer Values, and When Should I Avoid This?

Non-Unique Enum Values

Question:

Why does C# allow multiple enum values to be assigned the same underlying integer value? Is it safe to use enums for this purpose, or should structs be used instead?

Answer:

Contrary to popular belief, an enum in C# is not a special type but a struct that derives from System.Enum. When you declare an enum, the compiler generates a struct with named constants for the enum values.

Your enum definition:

public enum Color
{
    Red = 1,
    Blue = 1,
    Green = 1
}

Is translated into:

public struct Color : System.Enum
{
    public const int Red = 1;
    public const int Blue = 1;
    public const int Green = 1;
}

Since structs can have multiple constants with the same value, it is possible to define enums with non-unique values. However, this can lead to inconsistencies when converting to enums:

Color color1 = (Color)1; // Returns Red
Color color2 = (Color)Enum.Parse(typeof(Color), "1"); // Also returns Red

The comparison Color.Red == Color.Green will also evaluate to true, which can be confusing.

While it is legal to use enums with non-unique values, it is recommended to only use them when it makes sense. If you need unique values, consider using a struct or a Dictionary instead.

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