how to binary xor binary in C#?

public static string StringToBinary(string data)
        {
            StringBuilder sb = new StringBuilder();

            foreach (char c in data.ToCharArray())
            {
                sb.Append(Convert.ToString(c, 2).PadLeft(8, '0'));
            }
            return sb.ToString();
        }

private void button12_Click(object sender, EventArgs e)
        {

            string plainText = "a"+textBox3.Text;
            string keyText = textBox4.Text;
            string binaryplain = StringToBinary(plainText);
            string binarykey = StringToBinary(keyText);

            int int1 = Convert.ToInt32(binaryplain, 2); //this trouble : Value was either too large or too small for a UInt32.
            int int2 = Convert.ToInt32(binarykey, 2);
            int int3 = int1 ^ int2;
            string hasil = Convert.ToString(int3,2);
            textBox5.Text = hasil;
        }