При выполнении страницы ASPX я получаю следующую ошибку. Можете ли вы, пожалуйста, дайте мне знать, как я могу решить это.

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[]& buf, Int32& 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& completedSynchronously) +75

Спасибо