my connection string is not working

Question

this my code:

 

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Data.SqlClient;

using System.Configuration;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

using System.Windows.Forms;

using System.Collections;

using Sample;

 

 

if (txtcategory.Text != " ")

{

try

{

ConnectionString con = ConfigurationManager.ConnectionStrings["RetailTestConnection"].ToString();

con.Open();

string query = String.Format("Select [Item_Id]='{0}' order by Item_Id", txtcategory.Text);

SqlCommand cmd = new SqlCommand(query, con);

SqlDataReader dr = cmd.ExecuteReader();

string[] s = new string[] { };

while (dr.Read())

{

s = dr["Items"].ToString().Split(',');

}

int Length = s.Length;

for (int i = 0; i < Length - 1; i++)

// for (int i = 0; i <= 1; i++)

{

string fetr = s[i];

for (int j = 0; j <= clbbrand.Items.Count - 1; j++)

{

if (clbbrand.Items[j].ToString() == s[i])

{

clbbrand.SetItemChecked(j, true);

break;

}

}

}

}

catch (Exception ex)

{

MessageBox.Show(ex.Message.ToString());

}

}

else

{

// Message.Show("Please Eneter Your Choice");

}

 

 

 

note error:

 

the type or namespace name 'Connection string'  could not be found