title: System.Configuration.ConnectionStringSettingsCollection.this[string].get returned null in class
body: <p>In <strong>web.config</strong> file i have :</p>
  
  <pre><code>  &lt;connectionStrings&gt; 
      &lt;add name="connectionString" connectionString="Data Source=Server; Initial Catalog=DB; Persist Security Info=true; User ID=****; Password=****" /&gt;
    &lt;/connectionStrings&gt;
  </code></pre>
  
  <p>I try to reed this connection string in the class :</p>
  
  <pre><code>var connString = ConfigurationManager.ConnectionStrings["connectionString"].ConnectionString;
  </code></pre>
  
  <p>But I get this error :</p>
  
  <blockquote>
    <p>System.Configuration.ConnectionStringSettingsCollection.this[string].get
    returned null</p>
  </blockquote>