custom serialization - GetObjectData() not being invoked - framework using default serialization

I'm using VS2013 and am trying to experiment with custom serialization in C# code (C# 5, .NET 4.5, Win7 Pro x64).  Here's what I have so far.

First, a test class which implements custom serialization.  Of course, for this class the custom serialization isn't any benefit but I'm trying to demonstrate the calls are being invoked as expected.

    [Serializable]
    public class Foo : ISerializable
    {
        public string Name { get; set; }
        public int TheAnswer { get; set; }

        public Foo()
        {
            Name = "John Doe";
            TheAnswer = 42;
        }

        // SERIALIZE (SAVE)
        public virtual void GetObjectData(SerializationInfo info, StreamingContext context)
        {
            Trace.WriteLine(">>> custom serialize, SAVE");
            info.AddValue("Name",Name);
            info.AddValue("TheAnswer",TheAnswer);
        }

        // DESERIALIZE (LOAD)
        protected Foo(SerializationInfo info, StreamingContext context)
        {
            Trace.WriteLine(">>> custom serialize, LOAD");
            Name = info.GetString("Name");
            TheAnswer = info.GetInt32("TheAnswer");
        }
    }

I then have the below code which is triggered on a button click to demonstrate serializing the above to a file using the binary formatter.

        private const string _testFileName = @"c:\users\dave\testapp.dat";

        private void SaveBtn_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                var foo = new Foo();

                BinaryFormatter binFormat = new BinaryFormatter();
                using (Stream fStream = File.Create(_testFileName))
                {
                    binFormat.Serialize(fStream, foo);
                }
            }

            catch (Exception ex)
            {
                MessageBox.Show("EXCEPTION: " + ex.ToString());
            }
        }

When I run the above code, the serialization doesn't make use of my custom code.  Specifically, GetObjectData() is not called.  I verified this by placing a breakpoint in that code as well as with the Trace line.  Furthermore, I examined the created file and can see its writing out the fields, just using the default binary serialization.

I'm thinking there's something obvious I'm missing here.  I thought I was following the pattern correctly for custom serialization, but apparently not :)

Any idea what I'm missing?