Serialization problem

[C# Programming]

I'm having a hard time deserializing old versions of a class after adding a new member. I've written a toy program that's the simplest possible example of this problem. Maybe someone else might notice what I'm doing wrong.

I have a class with one member: A System.Drawing.Color:
[Serializable]
public class SerialObj
{
public System.Drawing.Color color;
}

The following code serializes/deserializes it with no problem:
SoapFormatter formatter = new SoapFormatter(); // Serialize:
formatter.AssemblyFormat = System.Runtime.Serialization.Formatters.FormatterAssemblyStyle.Simple;
formatter.Serialize(myStream, so);
SoapFormatter formatter = new SoapFormatter(); // Deserialize:
formatter.AssemblyFormat = System.Runtime.Serialization.Formatters.FormatterAssemblyStyle.Simple;
so = (SerialObj)formatter.Deserialize(myStream);

I added an int member, and I'm trying to use the ISerializable interface to handle the different versions. This interface requires a constructor with two special parameters, and a GetObjectData method:
[Serializable]
public class SerialObj : ISerializable
{
public System.Drawing.Color color;
public int number; // The new member.
public SerialObj()
{
System.Windows.Forms.MessageBox.Show("In empty ctor.");
}


public SerialObj(SerializationInfo info, StreamingContext cntxt)
{
System.Windows.Forms.MessageBox.Show("In full ctor.");
color = (System.Drawing.Color)info.GetValue("color", typeof(System.Drawing.Color));

try {
number = info.GetInt32("number");
}
catch (Exception ex)
{
System.Windows.Forms.MessageBox.Show(ex.Message);
}
}


void ISerializable.GetObjectData(SerializationInfo inf, StreamingContext cxt)
{
System.Windows.Forms.MessageBox.Show("In GetObjectData.");
inf.AddValue("color", color);
inf.AddValue("number", number);
}
}


It works for serialized new versions with both members. But when I deserialize an old version (without the int), it crashes on the Deserialize call, with the exception: "Top Object cannot be instantiated for element 'color'." It never even calls either constructor.

Can anyone see what I'm doing wrong, or suggest an alternative approach?

Thanks!
Alan