1. ConfigurationManager的命名空间：using System.Configuration;

2. To be able to save you have to use a configuration object returned by the OpenExeConfiguration Method

//Create the object
Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);

//make changes
config.AppSettings.Settings["Username"].Value = txtUsername.Text;
config.AppSettings.Settings["Password"].Value = txtPassword.Text;

//save to apply changes
config.Save(ConfigurationSaveMode.Modified);
ConfigurationManager.RefreshSection("appSettings");

3. When you run your application with F5,

• your code is compiled,
• the executable is copied to the bin or bin\Debug subdirectory of your source code directory,
• your app.config is copied as yourexecutable.exe.config into that directory, and
• your executable is started in that directory.

Thus, your application uses the yourexecutable.exe.config in the bin or bin\Debug directory, and it is there that ConfigurationManager saves the changes, not in your source code directory. This won't be an issue after deploying your application, because then, changes will go to yourexecutable.exe.config in the deployment directory, which is what you want.