title: Unable to serialize the session state
body: <p>While executing a .aspx page i am getting the following error. Can you please let me know how can i solve it.</p>
  
  <pre><code>Unable to serialize the session state. In 'StateServer' and 'SQLServer' mode, ASP.NET will serialize the session state objects, and as a result non-serializable objects or MarshalByRef objects are not permitted. The same restriction applies if similar serialization is done by the custom session state store in 'Custom' mode. 
  Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 
  
  Exception Details: System.Web.HttpException: Unable to serialize the session state. In 'StateServer' and 'SQLServer' mode, ASP.NET will serialize the session state objects, and as a result non-serializable objects or MarshalByRef objects are not permitted. The same restriction applies if similar serialization is done by the custom session state store in 'Custom' mode.
  
  Source Error: 
  
  An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.  
  
  Stack Trace: 
  
  
  [SerializationException: Type 'System.Web.UI.WebControls.DropDownList' in Assembly 'System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' is not marked as serializable.]
     System.Runtime.Serialization.FormatterServices.InternalGetSerializableMembers(RuntimeType type) +9472709
     System.Runtime.Serialization.FormatterServices.GetSerializableMembers(Type type, StreamingContext context) +247
     System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitMemberInfo() +160
     System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitSerialize(Object obj, ISurrogateSelector surrogateSelector, StreamingContext context, SerObjectInfoInit serObjectInfoInit, IFormatterConverter converter, ObjectWriter objectWriter, SerializationBinder binder) +491
     System.Runtime.Serialization.Formatters.Binary.ObjectWriter.Write(WriteObjectInfo objectInfo, NameInfo memberNameInfo, NameInfo typeNameInfo) +388
     System.Runtime.Serialization.Formatters.Binary.ObjectWriter.Serialize(Object graph, Header[] inHeaders, __BinaryWriter serWriter, Boolean fCheck) +444
     System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize(Stream serializationStream, Object graph, Header[] headers, Boolean fCheck) +133
     System.Web.Util.AltSerialization.WriteValueToStream(Object value, BinaryWriter writer) +1762
  
  [HttpException (0x80004005): Unable to serialize the session state. In 'StateServer' and 'SQLServer' mode, ASP.NET will serialize the session state objects, and as a result non-serializable objects or MarshalByRef objects are not permitted. The same restriction applies if similar serialization is done by the custom session state store in 'Custom' mode.]
     System.Web.Util.AltSerialization.WriteValueToStream(Object value, BinaryWriter writer) +1847
     System.Web.SessionState.SessionStateItemCollection.WriteValueToStreamWithAssert(Object value, BinaryWriter writer) +34
     System.Web.SessionState.SessionStateItemCollection.Serialize(BinaryWriter writer) +638
     System.Web.SessionState.SessionStateUtility.Serialize(SessionStateStoreData item, Stream stream) +244
     System.Web.SessionState.SessionStateUtility.SerializeStoreData(SessionStateStoreData item, Int32 initialStreamSize, Byte[]&amp; buf, Int32&amp; length, Boolean compressionEnabled) +67
     System.Web.SessionState.SqlSessionStateStore.SetAndReleaseItemExclusive(HttpContext context, String id, SessionStateStoreData item, Object lockId, Boolean newItem) +140
     System.Web.SessionState.SessionStateModule.OnReleaseState(Object source, EventArgs eventArgs) +807
     System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +148
     System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean&amp; completedSynchronously) +75
  </code></pre>
  
  <p>Thanks</p>