C# linq to sql using app.config file

Question

I placed this question a few days ago in the wrong forum. Due to that fact, I thought I would post this question in this forum insteead.

In a C# 2008 application, I used linq to sql to connect to a sql server 2008  database. Now I want to obtain the connection of the database from the
app.config file instead of hardcoding the value into the application. I want to be able to change the connection string to the database since I  will be moving this application from unit testing database to user acceptance database to the the prodction database.

Right now I am getting the error message, ""Object reference not set to an instance of an object." I know this error message is coming from the line of code that I listed below since it is the only line of code I changed.

The following is a snippet of the code from the *.designer.cs file (this is part of the dbml file).


using System.Configuration;

public esampleDataContext() :
base(ConfigurationManager.ConnectionStrings["sample.Properties.Settings.DEVConnectionString"].ConnectionString,mappingSource)

**Note:  base(global::sample.Properties.Settings.Default.DEVConnectionString, mappingSource) is the line of code that I replaced that was generated by the
         .net framework

I want to mention the following:
1. The *.dbml file is associated with a particular project file.
2. The connection string settings for this *.dbml file are the following:
   a. DEVConnectionString (settings)
   b. Connection String = Data Source=dev1;Initial Catalog=DEV_test;Integrated Security=True
   c. application settings=True
   d. settings property name is DEVConnectionString.
3. The project file that has the *.dbml file. has a reference set to the system.configuration.

**Note: This code in similar applications that I wrote, but I do not why it does not work here.

Thus can you tell me, show me in code, and/or point me to a reference that I can use so I can obtain the value in the app.config file for the database connection string I want to use. I do not want to use a hard-coded connection string to the database.