string s = Convert.ToString(textBox1.Text);
int sum = 0;
for (int i = 0; i < s.Length; i++)
{
   if (Char.IsDigit(s[i]))
   {
      string s2 = Convert.ToString(s[i]);
      sum += int.Parse(s2);
   }
}

textBox2.Text = sum.ToString();

var numbers = s.Split(',').Select(n => Int32.Parse(n)).ToArray();

var sum = 0;
foreach (var number in numbers)
{
    sum += number;
}