.net windows application

If you need help on storing connection strings in app.config, see here[^].

In VS, look at the Solution Explorer.
Under your poject, open the "Properties" branch.
Double click "Settings.settings"
Add a user string called "ConnectionString"
Close and save the settings.

In your code:

string conStr = Properties.Settings.Default.ConnectionString;
using (SqlConnection con = new SqlConnection(strCon))
   {
   using (SqlCommand com = new SqlCommand("SELECT * from MyTable", con))
      {
      SqlDataReader r = com.ExecuteQuery();
      while (r.Read())
         {
         ....
         }
      }
   }

When you change it, and want to save:

Properties.Settings.Default.ConnectionString = conStr;
Properties.Settings.Default..Save();