c#, asp.net- using linq in .net 2.0

Hi,
I am trying to code in .net 2.0 when I'm used to .net 3.0
I have linked the LinqBridge.dll but I'm still having troubles with establishing a link between my database and tha aspx code
I have the error:  System.NullReferenceException: Object reference not set to an instance of an object.
 for line:
 conn = new SqlConnection(ConfigurationManager.ConnectionStrings["std"].ConnectionString);

Should I still have the namespace using System.Linq; or something else now that i have the LinqBridge.dll?

Many thanks

public class DBConnect
{
 
    public static SqlConnection conn = null;
 
    public static void OpenConnection()
    {
        DBConnect.CloseConnection();
        if (conn == null)
        {
            conn = new SqlConnection(ConfigurationManager.ConnectionStrings["std"].ConnectionString);
        }
 
        conn.Open();
    }
 
    public static void CloseConnection()
    {
        if (conn != null)
            conn.Close();
    }

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