Another problem while retrieving data from Sql server 2005 database and display it into textbox in c# ASP.net

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SQL-Server-2005

C#

ASP.NET

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Below mentioned code is working fine if i display data in one textbox but if if i add multiple texboxes it gives error==== "invalid operation exception was unhandled by user code"

Pls Pls Somebody help me....

thanks
Kshitija

SqlCommand cmd = default(SqlCommand);
        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["Callregister_connectionstring"].ConnectionString);
        con.Open();
        cmd = new SqlCommand("Select * from Call_Reg where Complaint_no='" + ddlupCompl.SelectedItem.ToString() + "'", con);
        SqlDataReader reader = cmd.ExecuteReader();
        while (reader.Read())

            txtFname.Text = Convert.ToString(reader["Cust_FName"]);
            txtLname.Text = Convert.ToString(reader["Cust_LName"]);
            txtPh.Text = Convert.ToString(reader["ph_no"]);
            txtMob.Text = Convert.ToString(reader["mob_no"]);
        con.Close();