SurrogateSelector seems to work only on the root object

Question

I try to use SurrogatSelector to customize the deserialisation of a stream. It works fine for the root object of the object graph but not for contained objects. See the following code:

        Stream stream = File.Open("C:\\Temp\\test.bin", FileMode.Create);
        BinaryFormatter formatter = new BinaryFormatter();
        TestToSerialize tts = new TestToSerialize();
        formatter.Serialize(stream, tts);
        stream.Close();

        stream = File.Open("C:\\Temp\\test.bin", FileMode.Open);
        formatter = new BinaryFormatter();
        SurrogateSelector ss = new SurrogateSelector();
        ss.AddSurrogate(typeof(string), new StreamingContext(StreamingContextStates.All), new StringSerializationSurrogate());
        formatter.SurrogateSelector = ss;
        tts = (TestToSerialize)formatter.Deserialize(stream);
        stream.Close();

The StringSerializationSurrogate gets called (method SetObjectData) when a string is deserialized, but not when an object containing a string (as a serilizable member) is deserialized. The object to be serialized/ deserialized looks like this:

   [Serializable]
   class TestToSerialize
   {
       public string s1;
       public TestToSerialize()
       {
           s1 = "some test";
       }
   }

Is there a way to have the surrogate get called on non root objects? For completeness the Surrogate looks like this (testcode only to set a breakpoint):

sealed class StringSerializationSurrogate : ISerializationSurrogate
{

    public void GetObjectData(Object obj, SerializationInfo info, StreamingContext context)
    {
    }

    public object SetObjectData(object obj, SerializationInfo info, StreamingContext context, ISurrogateSelector selector)
    {
        string s = (String)obj;
        return obj;
    }
}

Answer 1

A surrogate will definitely be called on non-root objects - I regularly use it for exactly that in my own code.

I don't think String is a good Type to be using for your test.