class Program
{
  static void Main()
  {
    string hexstr = "30";
    string ascii = ((char)int.Parse(hexstr, System.Globalization.NumberStyles.HexNumber)).ToString();  
    System.Console.WriteLine(ascii);
  }
}

------解决方案--------------------
C# code
string str = "30";
int sss=Convert.ToInt32(str,16);

------解决方案--------------------
给你一段完整的代码:
       const string HexChs = "0123456789ABCDEF";


       private static string str2hex(string s)
       {
           StringBuilder sb = new StringBuilder();
           for (int i=0;i<s.Length; i++) {
               if (i>0) sb.Append(' ');
               sb.Append( ((UInt16)s[i]).ToString("X2"));
           }

           return sb.ToString();
       }

       private static string hex2str(string s)
       {
           string str = s.ToUpper();
           StringBuilder sb = new StringBuilder();
           for (int i=0; i<str.Length; i+=3){
               int n1 = HexChs.IndexOf( str[i]);
               if (n1==-1) throw new Exception("格式错");
               if (i+1==str.Length) throw new Exception("格式错");
               int n2 = HexChs.IndexOf( str[i+1]);
               if (n2==-1) throw new Exception("格式错");
               if (i+2< str.Length && str[i+2]!= ' ') throw new Exception("格式错");
               char c = (char)((n1 << 4) + n2);
               sb.Append(c);
           }
           return sb.ToString();
       }


------解决方案--------------------
探讨

反过来怎么算

知道 ASCII 0 怎么转换到 16进制30去？


------解决方案--------------------
Java code

public static String toStringHex(String s) 
    { 
    byte[] baKeyword = new byte[s.length()/2]; 
    for(int i = 0; i < baKeyword.length; i++) 
    { 
    try 
    { 
    baKeyword[i] = (byte)(0xff & Integer.parseInt(s.substring(i*2, i*2+2),16)); 
    } 
    catch(Exception e) 
    { 
    e.printStackTrace(); 
    } 
    } 
    try 
    { 
    s = new String(baKeyword, "utf-8");// UTF-16le:Not
    } 
    catch (Exception e1) 
    { 
    e1.printStackTrace(); 
    } 
    return s; 
    }