I know what this error is, how to fix it, etc.

My question is that I don't know why my current page I am developing is throwing this error when I am not using the foo class directly in any way, nor am I setting anything to the viewstate. I am using postbacks alot, but like I said, I am not storing anything in the viewstate etc one integer.

I am using nhibernate if that is relevant.

Any idea why I need to mark this classes as serializable that arent being used? Where should I start investigating?

[SerializationException: Type 'FlexiCommerce.Persistence.NH.ContentPersister' in Assembly 'FlexiCommerce.Persistence.NH, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' is not marked as serializable.]
   System.Runtime.Serialization.FormatterServices.InternalGetSerializableMembers(RuntimeType type) +9434541
   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) +218
   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.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize(Stream serializationStream, Object graph) +13
   System.Web.UI.ObjectStateFormatter.SerializeValue(SerializerBinaryWriter writer, Object value) +2937

[ArgumentException: Error serializing value 'Music#2' of type 'FlexiCommerce.Components.Category.']
   System.Web.UI.ObjectStateFormatter.SerializeValue(SerializerBinaryWriter writer, Object value) +3252
   System.Web.UI.ObjectStateFormatter.SerializeValue(SerializerBinaryWriter writer, Object value) +2276

[ArgumentException: Error serializing value 'System.Object[]' of type 'System.Object[].']
   System.Web.UI.ObjectStateFormatter.SerializeValue(SerializerBinaryWriter writer, Object value) +3252
   System.Web.UI.ObjectStateFormatter.Serialize(Stream outputStream, Object stateGraph) +116
   System.Web.UI.ObjectStateFormatter.Serialize(Object stateGraph) +57
   System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Serialize(Object state) +4
   System.Web.UI.Util.SerializeWithAssert(IStateFormatter formatter, Object stateGraph) +37
   System.Web.UI.HiddenFieldPageStatePersister.Save() +79
   System.Web.UI.Page.SavePageStateToPersistenceMedium(Object state) +108
   System.Web.UI.Page.SaveAllState() +315
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2492