Optional properties when deserializing a DataContract/Serializable mish-mash

I have an existing codebase that persists a couple of simple classes to disk via NetDataContractSerializer, but the classes are unfortunately not adorned with [DataContract], but rather with [Serializable]. This works fine, but now I want to add a few new properties to the persisted classes, while still be able to read the files generated by the old version.

Let's say this is the class:

[Serializable]
 public class Persisted
 {
     public int OldProperty {get;set;}
     public int NewProperty {get;set;}
 }
 

Now, when I deserialize the old files, I get an exception because they don't contain NewProperty. This makes sense. So I wanted to have NewProperty ignored, but while there's a [OptionalField] attribute to have the serializer ignore the missing field, it can't be applied to properties - only fields.

So I figured I'll use [DataContract] and [DataMember], which also has an IsRequired property, but this changes the layout of the serialized file, and it can't read the old data files. Moreover, you can't mix [Serializable] and [DataMember] - if the serializer sees the [Serializable] attribute, it ignores the [DataMember] directives.

So, barring the option to do a one-time conversion of the old files (possible, but not my first choice), is there a way to get the NetDataContractSerializer to ignore a field in an existing XML serialized object?

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