<"1.0" encoding="utf-8" "testkey" value="0"></add>
 </appSettings>
</configuration>

using System.Configuration;
namespace pcauto
{
 public static class ConfigHelper
 { 
  ///<summary>  
  ///返回*.exe.config文件中appSettings配置节的value项  
  ///</summary>  
  ///<param name="strKey"></param>  
  ///<returns></returns> 
  public static string GetAppConfig(string strKey)
  {
   string file = System.Windows.Forms.Application.ExecutablePath;
   Configuration config = ConfigurationManager.OpenExeConfiguration(file); 
   foreach (string key in config.AppSettings.Settings.AllKeys) { 
    if (key == strKey) { 
     return config.AppSettings.Settings[strKey].Value.ToString(); 
    } 
   }
   return null;
  }
  ///<summary>  
  ///在*.exe.config文件中appSettings配置节增加一对键值对  
  ///</summary> 
  ///<param name="newKey"></param> 
  ///<param name="newValue"></param> 
  public static void UpdateAppConfig(string newKey, string newValue) { 
   string file = System.Windows.Forms.Application.ExecutablePath;
   Configuration config = ConfigurationManager.OpenExeConfiguration(file); 
   bool exist = false; 
   foreach (string key in config.AppSettings.Settings.AllKeys) { 
    if (key == newKey) { exist = true; } 
   } 
   if (exist) { config.AppSettings.Settings.Remove(newKey); }
   config.AppSettings.Settings.Add(newKey, newValue); 
   config.Save(ConfigurationSaveMode.Modified);
   ConfigurationManager.RefreshSection("appSettings");
  }  
 }
}

ConfigHelper.GetAppConfig("testkey")

ConfigHelper.UpdateAppConfig("testkey", "abc");