Unable to serialize the session state - asp.net

Question

While executing a .aspx page i am getting the following error. Can you please let me know how can i solve it.
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
Thanks

Answer 1

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.
This line is pretty telling. You need to mark objects with [Serializable] to serialize the object (the MSDN link contains more details about serializing the object graph).

Answer 2

In my case, I was trying to serialize the non-serialized object that is HttpResponse. So this couldn't help me.
Check this if this couldn't solve your problem: Asp.net serialization error for session state
This post save my time and solved my problem.

Answer 3

Add the following attribute to the offending class:
[Serializable()]
If that doesn't work then:
Consider using mode="InProc". I ran into this issue by switching to "SQLServer" and rolling back to "InProc". This might fix the issue because InProc doesn't require objects to be Serializable.
Which one is better, InProc or SQL Server, for Session State mode in asp.net?

Answer 4

[Serializable()]
public partial class CustomersMaintLog
{