gistfile1.cs

public static int HighestBitPosition(ulong value)
{
    if (value == 0) return -1;

    var r = 0;

    foreach (var e in new [] { 32, 16, 8, 4, 2, 1 })
        if (value >= 1ul << e) { value >>= e; r += e; }

    return r;
}