با سلام
string value = textBox1.Text;
textBox1.Text = "";
string letter;
char[] chp = value.ToCharArray();//new char[value.Length];
for (int cont = 0; cont < chp.Length; cont++)
{
letter = System.Text.Encoding.UTF8.GetString(new byte[] { Convert.ToByte(chp[cont]) });
textBox1.Text += letter;
}