Read connection string from web.config

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.

For some bizarre reason it's not included by default.

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

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