private static void UpdateAppConfig(string newKey, string newValue)
{
    bool isModified = false;
    foreach (string key in ConfigurationManager.AppSettings)
    {
        if (key == newKey)
        {
            isModified = true;
        }
    }
    Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
    
 
if (isModified)
    {
        config.AppSettings.Settings.Remove(newKey);
    }
    config.AppSettings.Settings.Add(newKey, newValue);
    config.Save(ConfigurationSaveMode.Modified);
    ConfigurationManager.RefreshSection("appSettings");
}

之後直接用ConfigurationManager.AppSettings[key]讀取就行了。

C# 不重啟程序修改並保存配置文件（appSettings節點）