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copied!<p>Right; looking at them you have indeed completely changed the contract. I know of no contract-based serializer that is going to love you for that, and protobuf-net is no different. If you already had a root node, you <em>could</em> do something like (in pseudo-code):</p> <pre><code>[Contract] class Wrapper { [Member] public A A {get;set;} [Member] public B B {get;set;} [Member] public C C {get;set;} } </code></pre> <p>and just pick whichever of A/B/C is non-null, perhaps adding some conversion operators between them. However, if you just have a naked A in the old data, this gets hard. There are two approaches I can think of:</p> <ul> <li>add <em>lots</em> of shim properties for compatibility; <strong><em>not</em></strong> very maintainable, and I don't recommend it</li> <li>sniff the <code>Version</code> as an initial step, and tell the serializer what to expect.</li> </ul> <p>For example, you could do:</p> <pre><code>int version = -1; using(var reader = new ProtoReader(inputStream)) { while(reader.ReadFieldHeader() > 0) { const int VERSION_FIELD_NUMBER = /* todo */; if(reader.FieldNumber == VERSION_FIELD_NUMBER) { version = reader.ReadInt32(); // optional short-circuit; we're not expecting 2 Version numbers break; } else { reader.SkipField(); } } } inputStream.Position = 0; // rewind before deserializing </code></pre> <p>Now you can use the serializer, telling it what <code>version</code> it was serialized as; either via the generic <code>Serializer.Deserialize<T></code> API, or via a <code>Type</code> instance from the two non-generic APIs (<code>Serializer.NonGeneric.Deserialize</code> or <code>RuntimeTypeModel.Default.Deserialize</code> - either way, you get to the same place; it is really a case of whether generic or non-generic is most convenient).</p> <p>Then you would need some conversion code between <code>A</code> / <code>B</code> / <code>C</code> - either via your own custom operators / methods, or by something like auto-mapper.</p> <p>If you don't want any <code>ProtoReader</code> code kicking around, you could also do:</p> <pre><code>[DataContract] class VersionStub { [DataMember(Order=VERSION_FIELD_NUMBER)] public int Version {get;set;} } </code></pre> <p>and run <code>Deserialize<VersionStub></code>, which will give you access to the <code>Version</code>, which you can then use to do the type-specific deserialize; the main difference here is that the <code>ProtoReader</code> code allows you to short-circuit as soon as you have a version-number.</p>

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