public string NarrowToSmall(string inputString)
{ 
　   char[] c = inputString.ToCharArray(); 
      for (int i = 0; i < c.Length; i++)
      { 
            byte[] b = System.Text.Encoding.Unicode.GetBytes(c,i,1); 
             if (b.Length == 2) 
            { 
                  if (b[1] == 255)
                 { 
                        b[0] = (byte)(b[0] + 32); 
                        b[1] = 0; 
                          c[i] = System.Text.Encoding.Unicode.GetChars(b)[0]; 
                    } 
             } 
        } 
        string returnString = new string(c); 
         return returnString;   // 返回半角字符 
}

------解决方案--------------------

C# code
        private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
        {
            if ((int)e.KeyChar >= 65296 && (int)e.KeyChar <= 65305)
            {
                e.KeyChar = (char)((int)e.KeyChar - 65248);
            }
        }