public static class ArrayHelper
{

    private static void Main()
    {
        var i = new []{1,1,1,2,2,3,4,5,5,5,6};
         System.Console.WriteLine(i.BinarySeacrh(1));
         System.Console.WriteLine(i.BinarySeacrh(2));
         System.Console.WriteLine(i.BinarySeacrh(3));
         System.Console.WriteLine(i.BinarySeacrh(4));
         System.Console.WriteLine(i.BinarySeacrh(5));
         System.Console.WriteLine(i.BinarySeacrh(6));
         System.Console.WriteLine(i.BinarySeacrh(-1));
        i = new []{1};
        System.Console.WriteLine(i.BinarySeacrh(1));
        System.Console.WriteLine(i.BinarySeacrh(2));
        System.Console.WriteLine(i.BinarySeacrh(-1));
        i = new []{1, 5};
        System.Console.WriteLine(i.BinarySeacrh(-1));
        System.Console.WriteLine(i.BinarySeacrh(1));
        System.Console.WriteLine(i.BinarySeacrh(2));
        System.Console.WriteLine(i.BinarySeacrh(5));
        System.Console.WriteLine(i.BinarySeacrh(10));
        i = new []{1,2,3,4};
        System.Console.WriteLine(i.BinarySeacrh(1));
        System.Console.WriteLine(i.BinarySeacrh(2));
        System.Console.WriteLine(i.BinarySeacrh(3));
        System.Console.WriteLine(i.BinarySeacrh(4));
        System.Console.WriteLine(i.BinarySeacrh(5));
        System.Console.WriteLine(i.BinarySeacrh(-1));
        i = new int[0];
        System.Console.WriteLine(i.BinarySeacrh(1));
        System.Console.WriteLine(i.BinarySeacrh(-1));

    }

    public static int BinarySeacrh(this int[] a, int key)
    {
        if (a == null)
        {
            throw new System.ArgumentNullException();
        }
        int left = 0;
        int rigth = a.Length - 1;
        int m;
        while (left <= rigth)
        {
            m = left + (rigth - left) / 2;
            if (key == a[m])
            {
                return m;
            }
            if (key > a[m])
            {
                left = m + 1;
            }
            else
            {
                if (key < a[m])
                {
                    rigth = m - 1;
                }
                else
                {
                    return -1;
                }
            }
        }
        return -1;
    }
}