#HhtmlDropdown throw SerializationException Exception - asp.net-mvc-4

Question

I have a form that has a DropDownList. I am using SQLServer State mode to save the session in SQL Server. The form is displayed correctly; however, it throws an exception during the form submission. I've been looking and tried some of the suggested answer that were posted online with no luck. What am I doing wrong here?
//This is the section where I am building the SelectList Item in the controller
var ListDepartment = ManageUsersMembership.GetDepartment(SessionWrapper.studentBookTrade.SelectedSchoolID);
// book.DepartmentList = ListDepartment; //new SelectList(ListDepartment, "DepartmentID", "DepartmentName", SessionWrapper.studentBookTrade.SelectedDepartmentID);
List<SelectListItem> lDepartment = new List<SelectListItem>();
foreach (var item in ListDepartment)
{
if (SessionWrapper.studentBookTrade.SelectedDepartmentID > 0 && item.DepartmentID == SessionWrapper.studentBookTrade.SelectedDepartmentID)
{
lDepartment.Add(new SelectListItem { Text = item.DepartmentName, Value = item.DepartmentID.ToString(), Selected = true });
}
else
{
lDepartment.Add(new SelectListItem { Text = item.DepartmentName, Value = item.DepartmentID.ToString() });
}
}
book.DepartmentList = lDepartment;
//this is my property in the Model:
public virtual IEnumerable<SelectListItem> DepartmentList { get; set; }
//This is the HTML Code in my View:
#Html.DropDownListFor(model => model.DepartmentID, new SelectList(Model.DepartmentList, "Value", "Text"), "Select Department", new {#class="form-control", id="DepartmentID", onchange = "ValueSelected()" })
#Html.ValidationMessageFor(model => model.DepartmentID)
//The exception
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.Mvc.SelectList' in Assembly 'System.Web.Mvc, Version=4.0.0.1, Culture=neutral, PublicKeyToken=31bf3856ad364e35' is not marked as serializable.]
System.Runtime.Serialization.FormatterServices.InternalGetSerializableMembers(RuntimeType type) [phone]
System.Runtime.Serialization.FormatterServices.GetSerializableMembers(Type type, StreamingContext context) +230
System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitMemberInfo() +121
System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitSerialize(Type objectType, ISurrogateSelector surrogateSelector, StreamingContext context, SerObjectInfoInit serObjectInfoInit, IFormatterConverter converter, SerializationBinder binder) +185
System.Runtime.Serialization.Formatters.Binary.ObjectWriter.Write(WriteObjectInfo objectInfo, NameInfo memberNameInfo, NameInfo typeNameInfo) +565
System.Runtime.Serialization.Formatters.Binary.ObjectWriter.Serialize(Object graph, Header[] inHeaders, __BinaryWriter serWriter, Boolean fCheck) +446
System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize(Stream serializationStream, Object graph, Header[] headers, Boolean fCheck) +131
System.Web.Util.AltSerialization.WriteValueToStream(Object value, BinaryWriter writer) +1666
[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) +1754
System.Web.SessionState.SessionStateItemCollection.WriteValueToStreamWithAssert(Object value, BinaryWriter writer) +34
System.Web.SessionState.SessionStateItemCollection.Serialize(BinaryWriter writer) +628
System.Web.SessionState.SessionStateUtility.Serialize(SessionStateStoreData item, Stream stream) +240
System.Web.SessionState.SessionStateUtility.SerializeStoreData(SessionStateStoreData item, Int32 initialStreamSize, Byte[]& buf, Int32& length, Boolean compressionEnabled) +62
System.Web.SessionState.SqlSessionStateStore.SetAndReleaseItemExclusive(HttpContext context, String id, SessionStateStoreData item, Object lockId, Boolean newItem) +135
System.Web.SessionState.SessionStateModule.OnReleaseState(Object source, EventArgs eventArgs) +565
System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +136
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +69

Answer 1

I think you have too much here. Try changing your code this way
public IEnumerable<SelectListItem> DepartmentList { get; set; }
I usually use List but ienumerable should work too
then on your view you don't need to cast the list
#Html.DropDownListFor(model => model.DepartmentID, Model.DepartmentList, "Select Department", new {#class="form-control", id="DepartmentID", onchange = "ValueSelected()" })
the "for" helper will map the list for you. See if this helps