据该的 MSDN：ConfigurationManager.GetSection方法，

的 ConfigurationManager.GetSection 方法访问运行时配置信息 它不能改变的。要更改配置，您可以使用 Configuration.GetSection 您使用以下开放式方法之一获取配置文件的方式：

The ConfigurationManager.GetSection method accesses run-time configuration information that it cannot change. To change the configuration, you use the Configuration.GetSection method on the configuration file that you obtain by using one of the following Open methods:

• OpenExeConfiguration

的 OpenMachineConfiguration

的 OpenMappedExeConfiguration

不过，如果你要更新app.config文件，我会读它作为XML文档并操纵它作为一个正常的XML文档。

However, if you want to update app.config file, I would read it as an xml document and manipulate it as a normal xml document.

请参见下面的例子：
注：此示例只是为了证明概念。不应该在生产中使用，因为它是。

Please see the following example: Note: this sample is just for proof-of-concept. Should not be used in production as it is.

using System;
using System.Linq;
using System.Xml.Linq;

namespace ChangeAppConfig
{
    class Program
    {
        static void Main(string[] args)
        {
            MyConfigSetting.CustomerName = "MyCustomer";
            MyConfigSetting.EmailAddress = "MyCustomer@Company.com";
            MyConfigSetting.TimeStamp = DateTime.Now;
            MyConfigSetting.Save();
        }
    }

    //Note: This is a proof-of-concept sample and 
    //should not be used in production as it is.  
    // For example, this is not thread-safe. 
    public class MyConfigSetting
    {
        private static string _CustomerName;
        public static string CustomerName
        {
            get { return _CustomerName; }
            set
            {
                _CustomerName = value;
            }
        }

        private static string _EmailAddress;
        public static string EmailAddress
        {
            get { return _EmailAddress; }
            set
            {
                _EmailAddress = value;
            }
        }

        private static DateTime _TimeStamp;
        public static DateTime TimeStamp
        {
            get { return _TimeStamp; }
            set
            {
                _TimeStamp = value;
            }
        }

        public static void Save()
        {
            XElement myAppConfigFile = XElement.Load(Utility.GetConfigFileName());
            var mySetting = (from p in myAppConfigFile.Elements("MySettings")
                            select p).FirstOrDefault();
            mySetting.Attribute("CustomerName").Value = CustomerName;
            mySetting.Attribute("EmailAddress").Value = EmailAddress;
            mySetting.Attribute("TimeStamp").Value = TimeStamp.ToString();

            myAppConfigFile.Save(Utility.GetConfigFileName());

        }
    }

    class Utility
    {        
        //Note: This is a proof-of-concept and very naive code. 
        //Shouldn't be used in production as it is. 
        //For example, no null reference checking, no file existence checking and etc. 
        public static string GetConfigFileName()
        {            
            const string STR_Vshostexe = ".vshost.exe";
            string appName = Environment.GetCommandLineArgs()[0];

            //In case this is running under debugger. 
            if (appName.EndsWith(STR_Vshostexe))
            {
                appName = appName.Remove(appName.LastIndexOf(STR_Vshostexe), STR_Vshostexe.Length) + ".exe";
            }

            return appName + ".config";
        }
    }
}

我还添加了时间戳 。属性MySettings在app.config中轻松检查结果

I also added "TimeStamp" attribute to MySettings in app.config to check the result easily.

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <configSections>
    <section name="MySettings" type="TestApp.MySettings, TestApp"/>
  </configSections>

  <MySettings CustomerName="" EmailAddress="" TimeStamp=""/>
</configuration>