Question

The base class will only contain methods.

Answer 1

If you don’t change the version number of containing assembly then adding base class shouldn’t be a problem for binary serialization and for XML it doesn’t matter at all since it is very loosly coupled with actual type.

Just make sure that the base is also marked [Serializable] for binary serialization.
I’ve just tested and confirmed this.

Serialized the following class with Binary Formatter

[Serializable]
    public class Test
    {
        public string Name { get; set; }
    }

Got the following base64 encoded output:

AAEAAAD/////AQAAAAAAAAAMAgAAAEpDb25zb2xlQXBwbGljYXRpb24xLCBWZXJzaW9uPTEuMC4wLjAs
IEN1bHR1cmU9bmV1dHJhbCwgUHVibGljS2V5VG9rZW49bnVsbAUBAAAAIENvbnNvbGVBcHBsaWNhdGlv
bjEuUHJvZ3JhbStUZXN0AQAAABU8TmFtZT5rX19CYWNraW5nRmllbGQBAgAAAAYDAAAABWhhc2FuCw==

Then added following base class for Test:

[Serializable]
public class Base
{
   public void Dhuz() { }
}

And de serialized the base64 encoded thing again successfully without problems.