Can't Refactor Or Save this Class File (Lazily Loaded Singleton)

using System;

public sealed class SessionCache {
    private static readonly Lazy<SessionCache> lazy =
        new Lazy<SessionCache>(() => new SessionCache());

    public static SessionCache Instance { get { return lazy.Value; } }
    public UserData userData { get; set; }

    private SessionCache() {
    }
}

using System;

public sealed class SessionCache {
    private static readonly Lazy<SessionCache> lazy =
        new Lazy<SessionCache>(() => new SessionCache());

    public static SessionCache Instance { get { return lazy.Value; } }
    public UserData userData { get; set; }

    private SessionCache() {
    }
}