You can use your own code:

    protected void Button1_Click(object sender, EventArgs e)
    {
        int[] num = new int[6];
        Random RandomClass = new Random();
        ArrayList nums = new ArrayList();
        for (int i = 1; i < 50; i++)
        {
            nums.Add(i);
        }
        for (int i = 0; i < 6; i++)
        {
            num[i] = (int)nums[RandomClass.Next(1, 49 - i)];
            nums.Remove(num[i]);
        }
        DataList1.DataSource = num;
        DataList1.DataBind();
        txt_b1.Text = Convert.ToString(num[0]);
        txt_b2.Text = Convert.ToString(num[1]);
        txt_b3.Text = Convert.ToString(num[2]);
        txt_b4.Text = Convert.ToString(num[3]);
        txt_b5.Text = Convert.ToString(num[4]);
        txt_b6.Text = Convert.ToString(num[5]);
    }