public double bigzarb(long u,long v)
{
    double n;
    long x;
    long y;
    long w;
    long z;
    string[] i = textBox7.Text.Split(',');
    long[] nums = new long[i.Length];
    for (int counter = 0; counter < i.Length; counter++)
    {
        nums[counter] = Convert.ToInt32(i[counter]);
    }

    u = nums[0];
    int firstdigits = Convert.ToInt32(Math.Floor(Math.Log10(u) + 1));
    v = nums[1];
    int seconddigits = Convert.ToInt32(Math.Floor(Math.Log10(v) + 1));
    if (firstdigits >= seconddigits)
    {
        n = firstdigits;

    }
    else
    {
        n = seconddigits;        
    }
    if (u == 0 || v == 0)
    {
        MessageBox.Show("the Multiply is 0");
    }

    int intn = Convert.ToInt32(n);
    if (intn <= 3)
    {
        long uv = u * v;
        string struv = uv.ToString();
        MessageBox.Show(struv);
        return uv;
    }
    else
    {
        int m =Convert.ToInt32(Math.Floor(n / 2));

        x = u % Math.Pow(10,m); // here
        y = u / Math.Pow(10,m); // here
        w = v % Math.Pow(10,m); // here
        z = v / Math.Pow(10,m); // here

        long result = bigzarb(x,w) * Math.Pow(10,m) + (bigzarb(x,w) + bigzarb(w,y)) * Math.Pow(10,m) + bigzarb(y,z);///here
        textBox1.Text = result.ToString();
        return result;
    }
}

有什么问题？谢谢！