//字符转ASCII码：
public static int Asc(string character)
  {
if (character.Length == 1)
   {
    System.Text.ASCIIEncoding asciiEncoding = new System.Text.ASCIIEncoding();
int intAsciiCode = (int)asciiEncoding.GetBytes(character)[0];
return (intAsciiCode);
   }
else
   {
throw new Exception("Character is not valid.");
   }

  }

ASCII码转字符：

public static string Chr(int asciiCode)
  {
if (asciiCode >= 0 && asciiCode <= 255)
   {
    System.Text.ASCIIEncoding asciiEncoding = new System.Text.ASCIIEncoding();
byte[] byteArray = new byte[] { (byte)asciiCode };
string strCharacter = asciiEncoding.GetString(byteArray);
return (strCharacter);
   }
else
   {
throw new Exception("ASCII Code is not valid.");
   }
  }


//另一种写法
string str="abcd";
byte[] bytetest = System.Text.Encoding.Default.GetBytes(str.ToString());