Structs not being initialized

Today, I figured out it might be interesting to have some sort of enumeration in C# similar to the class enums in Java (which has been done numerous times) but as immutable structs. I came up with something like this:

public struct TestStruct {

    public static readonly TestStruct Value1 = 1;
    public static readonly TestStruct Value2 = 2;
    public static readonly TestStruct Value3 = 3;

    private int _value;

    private TestStruct(int value) {
        _value = value;
    }

    public static implicit operator int(TestStruct instance) {
        return instance._value;
    }

}

The code compiles fine, however when I try to access TestStruct.Value2, a NullReferenceException occurs.

This was just a test, but now I'm wondering.. what happens to the static readonly TestStructs? Why can I assign an integer to them, although they are not being initialized?

Update

This one compiles.

public struct LogLevel
{
    // log levels
    public static readonly LogLevel Information = 0x0000;
    public static readonly LogLevel Warning = 0x0001;
    public static readonly LogLevel Error = 0x0002;
    public static readonly LogLevel Verbose = 0x0004;
    public static readonly LogLevel Debug = 0x0008;
    public static readonly LogLevel Trace = 0x0016;
    public static readonly LogLevel Critical = 0x0032;
    public static readonly LogLevel Fatal = 0x0064;


    private static readonly Dictionary<int, LogLevel> Levels
        = new Dictionary<int, LogLevel>();

    private readonly int _value;

    private LogLevel(int value)
    {
        if (Levels.ContainsKey(value))
            throw new ArgumentException("Level already defined.");

        _value = value;
        Levels.Add(value, this);
    }

    public static implicit operator LogLevel(int value)
    {
        LogLevel level;
        if (!Levels.TryGetValue(value, out level))
            throw new ArgumentOutOfRangeException("value");
        return level;
    }

    public static implicit operator int(LogLevel level)
    {
        return level._value;
    }
}

Update 2

    // extension method for log levels
    public static void Debug(this ILogger logger, string message)
    {
        logger.Write(message, LogLevel.Debug);
    }

    // method on ILogger, implementation is Log4NetLogger (wrapper)
    void Write(string message, LogLevel severity);

    // call (simple)
    Logger.Debug("Test");