C# – preferred method for modifying connection string settings in DAL class library when deploying asp.net web app

I've never had a problem with the Data Access Layer (DAL) being able to use the connection strings from my web.config file. Usually I just copy the connection strings section from the DAL and paste it into the web.config. I'm using the DBML designer to create the data context.

If this won't work for you, you can specify the connection string in the data context constructor. In your web project have a static class that loads your settings, including your connection strings, and when you create your DAL object (or data context, if creating it directly) just pass it in to the constructor.

public static class GlobalSettings
{
    private static string dalConnectionString;
    public static string DALConnectionString
    {
       get
       {
           if (dalConnectionString == null)
           {
              dalConnectionString = WebConfigurationManager
                                      .ConnectionStrings["DALConnectionString"]
                                        .ConnectionString;
           }
           return dalConnectionString;
       }
    }
}
...

using (var context = new DALDataContext(GlobalSettings.DALConnectionString))
{
   ...
}

When you deploy an Windows Forms application you could have an app.config file.

yourproject.exe gives you a yourproject.exe.config file, which is the app.config file.