Problem with SQL Server connection using web.config.

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C#2.0

C#

ASP.NET

We are getting problem with using connection string in web.config for a webform in ASP.Net application. The connection string is as follows:

Code in web.config:

<configuration>
     <appSettings>
            <add key="connectionString" value="Server=.\\SQLEXPRESS; database=MLM; Trusted_Connection=True"/>
      </appSettings>
        ....

And Code in WebForm is as below:

SqlConnection con;
        SqlCommand cmd;
        SqlDataAdapter adp;
        DataSet ds;
        protected void Page_Load(object sender, EventArgs e)
        {
          string connecStr = System.Configuration.ConfigurationSettings.AppSettings["connectionString"]; 
            con = new SqlConnection(connecStr); 
            cmd = new SqlCommand("Select * from tblNodeTree");
            cmd.Connection = con;
            ds = new DataSet(); 
            adp = new SqlDataAdapter(cmd);
            adp.Fill(ds, "tblNodeTree");	//  This line shows error message "instance failure" 
            GridView1.DataSource = ds.Tables[0].DefaultView;
            GridView1.DataBind();
        }

The Error Message is : Instance failure
But it works fine on replacing first line of code:

string connecStr = System.Configuration.ConfigurationSettings.AppSettings["connectionString"];
With
   string connecStr= "Server=.\\SQLEXPRESS; database=MLM; Trusted_Connection=True";   

What's the problem here with web.config's database connection. And what should be its optimum solution. Kindly advice.