How to serialize a IntPtr (in c#) pointing to a object that comes from a c++ dll

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C++

C#

DLL

unmanaged

serialization

I've got a c++ dll that i use in my c# windows forms app.

[DllImport("dllname.dll", EntryPoint = "?dllMethod@@YA_NPBDAAPAEAAH@Z", CallingConvention = CallingConvention.Cdecl)]
private static extern bool dllMethod([MarshalAs(UnmanagedType.LPStr)]string fname, out IntPtr descriptor, out int length);

In my c# app, i just want to serialize a hashtable with the IntPtr data that's returned by that Dll... So i'm doing this:

[Serializable]
public class TopSurfDescriptor
{
  public IntPtr descriptor;
  public int length;
}
[Serializable]
public class Descriptors
{        
   private Hashtable descriptors=null;

   public void AddDescriptor(string key,TopSurfDescriptor descriptor)
   {
            descriptors.Add(key, descriptor);
   }
   public Descriptors()
   { 
      if (File.Exists(@"Descriptors.dat"))
      {
           FileStream fs = new FileStream(@"Descriptors.dat", FileMode.Open);
           try
           {
               BinaryFormatter formatter = new BinaryFormatter();
               descriptors = (Hashtable)formatter.Deserialize(fs);
           }
           catch (SerializationException){ throw; }

           finally {fs.Close(); }
       }
       else
           descriptors = new Hashtable();
   }

//AND
   public void Save()
   {
       FileStream fs = new FileStream(@"Descriptors.dat", FileMode.Create);

       BinaryFormatter formatter = new BinaryFormatter();
       try
       {
           formatter.Serialize(fs, descriptors);
       }
       catch (SerializationException e){ throw; }
       finally { fs.Close(); }
   }

if i dont close the app the serialization and deserialization works fine, but if i close it and run it again (from visual studio) in debug mode i can see the process of deserialization working good, (i see the hastable with all the data and the IntPtr pointing to the correct TopSurfDescriptor type, but if i pass this pointer to other method from the DLL i got the exception that the memory can be corrupt or dont have access...

when I analyze the serialized file (on disk) i seems that the data is not there (because the file is to small), but only the IntPtr. So, i think the problem is the fact that the object (memory) that is returned from the external dll is not beeing serializable.