title: 'System.Data.SqlClient.SqlConnection' in Assembly is not marked as serializable?
body: <p>This error occurred in asp.net &amp; C# program:</p>
  
  <pre><code>System.Data.SqlClient.SqlConnection' in Assembly 'System.Data, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' is not marked as serializable
  </code></pre>
  
  <p>what is the reason for this type of error occurred?</p>