title: configurationManager does not exist in the namespace System.Configuration
body: <p>I have used the following namespace to connect my project to the sql server:</p>
  
  <pre><code>using System.Configuration;
  </code></pre>
  
  <p>and also used </p>
  
  <pre><code>string str=System.Configuration.ConfigurationSettings.AppSettings["myconnection"];
  SqlConnection oconnection = new SqlConnection(str);
  oconnection.Open();
  </code></pre>
  
  <p>when I run the program ,an error has been occurred and show the message </p>
  
  <blockquote>
    <p>'System.Configuration.ConfigurationSettings.AppSettings' is
    obsolete.This method is obsolete,it has been replaced by
    'System.Configuration!
    System.Configuration.ConfigurationManager.AppSettings '</p>
  </blockquote>
  
  <p>but I have no found ConfigurationManager in that namespace and for <code>oconnection.Open();</code> the message is </p>
  
  <blockquote>
    <p>InvalidOperationException</p>
  </blockquote>
  
  <p>was unhandled.</p>
  
  <p>What can I do?</p>