Updating reference to a member variable in use

internal sealed class SektionCache : CacheBase
{
    public static readonly SektionCache Instance = new SektionCache();
    private static readonly object lockObject = new object();
    private static bool isUpdating;

    private IEnumerable<Sektion> sektioner;

    static SektionCache()
    {
        UpdateCache();
    }

    public IEnumerable<Sektion> Sektioner
    {
        get { return sektioner; }
    }

    public static void UpdateCache()
    {
    // SNIP - getting data, locking etc.
    Instance.sektioner = newSektioner;
    // SNIP
    }
}

internal sealed class SektionCache : CacheBase
{
    //public static readonly SektionCache Instance = new SektionCache();

    // this template is better ( safer ) than the previous one, for thread-safe singleton patter >>>
    private static SektionCache defaultInstance;
    private static object readonly lockObject = new object();
    public static SektionCach Default {
        get {
            SektionCach result = defaultInstance;
            if ( null == result ) {
                lock( lockObject ) {
                    if ( null == result ) {
                        defaultInstance = result = new SektionCache();
                    }
                }
            }

            return result;
        }
    }
    // <<< this template is better ( safer ) than the previous one

    //private static readonly object lockObject = new object();
    //private static bool isUpdating;
    //private IEnumerable<Sektion> sektioner;

    // this declaration is enough
    private volatile IEnumerable<Sektion> sektioner;

    // no static constructor is required >>>
    //static SektionCache()
    //{
    //    UpdateCache();
    //}
    // <<< no static constructor is required

    // I think, you can use getter and setter for reading & changing a collection
    public IEnumerable<Sektion> Sektioner {
        get {
            IEnumerable<Sektion> result = this.sektioner;
            // i don't know, if you need this functionality >>>
            // if ( null == result ) { result = new Sektion[0]; }
            // <<< i don't know, if you need this functionality
            return result;
        }
        set { this.sektion = value; }
    }

    //public static void UpdateCache()
    //{
    //// SNIP - getting data, locking etc.
    //Instance.sektioner = newSektioner;
    //// SNIP
    //}
}