Ferris Beuller asked me in response to my previous post:

Why can the serializer not serialize types that implement IDictionary? SortedList in types cannot be serialized, you get an InvalidOperationException with the innerexception of {"The type System.Collections.SortedList is not supported because it implements IDictionary."}

The serializer should be able to serialize any type as long as it is marked serializable. SortedList is marked serializable. So, you should not be getting this error. I was able to run the following test without getting any errors.

using System;
using System.Collections;
using System.Runtime.Serialization.Formatters.Binary;
using System.IO;

namespace SerializeDict
{
    public class Test
    {
        public static void Main()
        {
            MemoryStream stream = new MemoryStream();
            BinaryFormatter bf = new BinaryFormatter();
            SortedList list = new SortedList();
            list.Add(10, 100);
            list.Add(100, 1000);
            bf.Serialize(stream, list);
            stream.Position = 0;
            bf = new BinaryFormatter();
            list = (SortedList)bf.Deserialize(stream);
            Console.WriteLine(list[100]);
        }
    }
}

Can you share a reproducible sample with me where you get the error?