Is there a more an efficient way to get the index of an item in a SortedSet?

RedisModel.ZRank.cs

/// <summary>
/// Returns the rank of member in the sorted set stored at key, with the scores ordered from low to high.
/// The rank (or index) is 0-based, which means that the member with the lowest score has rank 0.
/// </summary>
/// <param name="key"></param>
/// <param name="member"></param>
/// <returns>the rank of member or null if the set or member does not exist</returns>
public int? ZRank(string key, string member)
{
    SortedSet<ZSetEntry> set = GetSortedSet(key);
    if (set == null) return null;
    int idx = -1;
    
    foreach (var entry in set)
    {
        idx++;
        if (entry.Value == member) return idx;
    }
    return null;
}

Binary Search?