unable to serialize a composite activity either into xoml or binary stream?

Question

hi experts,

i have a custom activity, as below:

public class BuildMachine : SequenceActivity

{

}

the BuildMachine activity will use several other custom activities to complete the logic.

i need to serialize this activity into either xoml file , or into binary stream:
-------------------------------
[1] serialize to xoml markup
-------------------------------
Activity activity = new BuildMachine();

var stream = new MemoryStream();
System.Xml.XmlWriter xmlWriter = XmlWriter.Create(stream);

ServiceContainer container = new ServiceContainer();
TypeProvider provider = new TypeProvider(container);
foreach (var a in this._referencedAssemblies)
{
  provider.AddAssembly(a);
}
container.AddService(typeof(ITypeProvider), provider);
DesignerSerializationManager dsm = new DesignerSerializationManager(container);
using (dsm.CreateSession())
{
 // need to check the type of activity
 if (!(activity is SequentialWorkflowActivity) && (!(activity is StateMachineWorkflowActivity)))
  {
     if (activity is CompositeActivity)
     {
       CompositeActivity compositeActivityactivity = (CompositeActivity)activity;
       CompositeActivityMarkupSerializer compositeActivityMarkupSerializer = new CompositeActivityMarkupSerializer();
       compositeActivityMarkupSerializer.Serialize(xmlWriter, compositeActivityactivity);
     }
     else
     {
       // it is just a  activity, not a workflow
       ActivityMarkupSerializer activityMarkupSerializer = new ActivityMarkupSerializer();
       activityMarkupSerializer.Serialize(xmlWriter, activity);
     }

  }
  else
  {
      // it is workflow
      var workflowMarkupSerializer = new WorkflowMarkupSerializer();
      workflowMarkupSerializer.Serialize(xmlWriter, activity);
  }
}

xmlWriter.Flush();
var file = new FileStream(activity.QualifiedName, FileMode.Create);
file.Write(stream.GetBuffer(), 0, stream.GetBuffer().Length);
file.Close();

-------------------------------
[2] serialize to binary stream
-------------------------------
Activity activity = new BuildMachine();
System.Runtime.Serialization.Formatters.Binary.BinaryFormatter formatter = new BinaryFormatter();
System.IO.MemoryStream stream = new MemoryStream();
formatter.Serialize(stream, activity);
byte[] data = new byte[stream.Length];
Array.Copy(data, stream.GetBuffer(), stream.Length);
stream.Close();

but, either way not succeed. the [1] way give a incomplete xoml file, the [2] way throw out exception at .Serialize() call, saying "BuildMachine is not marked as serializable", (i tried to add [System.Serializable] to class BuildMachine, but gives this error:

System.Workflow.Activities.SequenceActivity' in Assembly 'System.Workflow.Activities, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' is not marked as serializable.

what i have to do ?

Thanks in advance.