C# winforms, How to persist Changes in my app.config?

I have only been successful in making changes to whats in memory as i debug my project.

my work around was to save values to text file in the project directory and just get those values and set them as my global verables so i can change the app.config in memory everytime i load the project.

///Declare my string builder

StringBuilder s = new StringBuilder();

 

///Make my Connection String from my global verables.

 s.Append("metadata=res://*/Model1.csdl|res://*/Model1.ssdl|res://*/Model1.msl;provider=System.Data.SqlClient;provider connection string=\";"
+ "data source=" + Program.g_s_Server + ";"
+ "initial catalog=" + Program.g_s_Datasource + ";"
+ "integrated security=True;"
+ "User ID =" + Program.g_s_Uid + ";"
+ "Password=" + Program.g_s_Pwd + ";"
+ "App=EntityFramework\";");

 

///Change my app.config

 XmlDocument XmlDoc = new XmlDocument();
XmlDoc.Load(AppDomain.CurrentDomain.SetupInformation.ConfigurationFile);

foreach (XmlElement xElement in XmlDoc.DocumentElement)
{
   if (xElement.Name == "connectionStrings")
   {

      foreach (XmlNode xNode in xElement.ChildNodes)
      {
         if (xNode.Attributes[0].Value == "MyEntities")
         {
         xNode.Attributes[1].Value = s.ToString();
         }
      }
   }
}
XmlDoc.Save(AppDomain.CurrentDomain.SetupInformation.ConfigurationFile);
ConfigurationManager.RefreshSection("connectionStrings");

 

 //This is just to see the change when i debug
var connection =
System.Configuration.ConfigurationManager.ConnectionStrings["MyEntities"];

 

 

 

 

So my Question is firstly most important now, how to persist the changes. Secondly should i use entity framework in my winform project as a datalayer or is there better?