ISerializable and backward compatibility

问题:

I have to work an an old application that used binaryFormatter to serialize application data into filestream (say in a file named "data.oldformat") without any optimizazion the main class has been marked with attribute

<serializable()>public MainClass
....... 
end class

and the serialization code

dim b as new binaryformatter
b.serialize(mystream,mymainclass)

In an attempt to optimize the serialization/deserialization process I simply made the class implement the ISerializable interface and wrote some optimized serialization routines

<serializable()>public MainClass
       implements ISerializable
....... 
end class

The optimization works really well but I MUST find a way to reatrive the data inside the old files for backward compatibility.

How can I do that??

Pierluigi

回答1:

stmax has an excellent answer, however I would implement it like this, which uses SerializationEntry.GetEnumerator() instead of try/catch. This way is cleaner and significantly faster.

public MainClass(SerializationInfo info, StreamingContext context) {
    int version = 0;
    foreach (SerializationEntry s in info)
    {
        if (s.Name == "version") 
        {
            version = (int)s.Value;
            break;
        }
    }

    switch (version) {
      case 0:
        // deserialize "old format"
        break;
      case 1:
        // deserialize "new format, version 1"
        break;
      default:
        throw new NotSupportedException("version " + version + " is not supported.");
    }
}

I would prefer a LINQ version using .FirstOrDefault(), but SerializationInfo does not implement IEnumerable - in face, weirdly enough, it doesn't even implement the old IEnumerable interface.

回答2:

since you've already implemented the ISerializable interface, you've probably also already added the required constructor:

public MainClass(SerializationInfo info, StreamingContext context) {}

you can use the info-object passed to the constructor to retrieve data from the serialized file. by default (i.e. when no ISerializable is implemented), the fields names are used as identifiers during serialization. so if your old class had a field "int x" you can deserialize this using:

this.x = info.GetInt32("x");

for newer versions i normally add a "version" entry during serialization, like this:

public void GetObjectData(SerializationInfo info, StreamingContext context) {
  info.AddValue("version", 1);
  info.AddValue("othervalues", ...);
}

during deserialization you can check this version entry and deserialize accordingly:

public MainClass(SerializationInfo info, StreamingContext context) {
    int version;
    try {
       version = info.GetInt32("version");
    }
    catch {
       version = 0;
    }

    switch (version) {
      case 0:
        // deserialize "old format"
        break;
      case 1:
        // deserialize "new format, version 1"
        break;
      default:
        throw new NotSupportedException("version " + version + " is not supported.");
    }
}

i haven't compiled that code, might contain typos.

hope that helps.

回答3:

Just try the same thing you've been doing so far

BinaryFormatter b = new BinaryFormatter();
MainClass a = b.DeSerialize(mystream) as MainClass;

Implementing ISerializable didn't change your original class, basically you've just added some methods

回答4:

When serializing your objects add an additional Version field (this shouldn't add too much overhead). Then in your GetObjectData method, try to retrieve the version field first and based on whether this exists or not (by catching the SerializationException) deserialize the old way or the new way. The old way will have just serialized all data so you should just be able to call Get... for all fields.

回答5:

Your previous code should work. Do you get an exception? Try to use new constructor:

 Protected Sub New(ByVal info As SerializationInfo, ByVal context As StreamingContext)