Read connection string from web.config

You need to add a reference to System.Configuration and then use:

System.Configuration.ConfigurationManager.
    ConnectionStrings["connectionStringName"].ConnectionString;

C#

// Add a using directive at the top of your code file    
using System.Configuration;

// Within the code body set your variable    
string cs = ConfigurationManager.ConnectionStrings["connectionStringName"].ConnectionString;

VB

' Add an Imports statement at the top of your code file    
Imports System.Configuration

' Within the code body set your variable    
Dim cs as String = ConfigurationManager.ConnectionStrings("connectionStringName").ConnectionString

Add System.Configuration as a reference then:

 using System.Configuration;

 ...

 string conn = 
    ConfigurationManager.ConnectionStrings["ConnectionName"].ConnectionString;

I guess you need to add a reference to the System.Configuration assembly if that have not already been added.

Also, you may need to insert the following line at the top of your code file:

using System.Configuration;

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data.SqlClient;
using System.Drawing;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.DataVisualization.Charting;
using System.Web.UI.WebControls;  

C#

string constring = ConfigurationManager.ConnectionStrings["ABCD"].ConnectionString;
                using (SqlConnection con = new SqlConnection(constring))

BELOW WEB.CONFIG FILE CODE

<connectionStrings>
    <add name="ABCD" connectionString="Data Source=DESKTOP-SU3NKUU\MSSQLSERVER2016;Initial Catalog=TESTKISWRMIP;Integrated Security=True" providerName="System.Data.SqlClient"/>
  </connectionStrings>

In the above Code ABCD is the Connection Name

In VB : This should work

ConfigurationManager.ConnectionStrings("SQLServer").ConnectionString

In C# it would be (as per comment of Ala)

ConfigurationManager.ConnectionStrings["SQLServer"].ConnectionString

You have to invoke this class on the top of your page or class :

using System.Configuration;

Then you can use this Method that returns the connection string to be ready to passed to the sqlconnection object to continue your work as follows:

    private string ReturnConnectionString()
    {
       // Put the name the Sqlconnection from WebConfig..
        return ConfigurationManager.ConnectionStrings["DBWebConfigString"].ConnectionString;
    }

Just to make a clear clarification this is the value in the web Config:

  <add name="DBWebConfigString" connectionString="....." />   </connectionStrings>

using System.Configuration;


string conn = ConfigurationManager.ConnectionStrings["ConStringName"].ToString();

using System.Configuration;


string connString = ConfigurationManager.ConnectionStrings["ConStringName"].ToString();

Remember don't Use ConnectionStrings[index] because you might of Global machine Config and Portability

First add this:

using System.Configuration;

Everybody seems to be suggesting that adding

using System.Configuration;

which is true.

But might I suggest that you think about installing ReSharper's Visual Studio extension?

With it installed, instead of seeing an error that a class isn't defined, you'll see a prompt that tells you which assembly it is in, asking you if you want it to add the needed using statement.

Add Namespace System.Configuration as a reference then

using System.Configuration;

 string conn =  ConfigurationManager.ConnectionStrings["ConnectionName"].ConnectionString;