Attributes in C#.NET

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Figure 172: BinaryFormatter Does Not Encrypt Data
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Providing Custom Serialization One way to add encryption is to provide custom serialization Ignoring the complexities of encrypting and decrypting, this requires implementing the ISerializable interface in addition to using SerializableAttribute The interface requires only the GetObjectData() method to be implemented However, this is sufficient only for serialization In order to also support deserialization, it is necessary to provide a constructor that takes parameters of type SystemRuntimeSerializationSerializationInfo and SystemRuntimeSerializationStreamingContext (see Listing 1725)
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Listing 1725: Implementing SystemRuntimeSerializationISerializable
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using System; using SystemRuntimeSerialization; [Serializable] class EncryptableDocument : ISerializable { public EncryptableDocument(){ } enum Field { Title, Data } public string Title; public string Data; public static string Encrypt(string data) { string encryptedData = data; // Key-based encryption
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17: Reflection, Attributes, and Dynamic Programming
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return encryptedData; } public static string Decrypt(string encryptedData) { string data = encryptedData; // Key-based decryption return data; } #region ISerializable Members public void GetObjectData( SerializationInfo info, StreamingContext context) { infoAddValue( FieldTitleToString(), Title); infoAddValue( FieldDataToString(), Encrypt(Data)); } public EncryptableDocument( SerializationInfo info, StreamingContext context) { Title = infoGetString( FieldTitleToString()); Data = Decrypt(infoGetString( FieldDataToString())); } #endregion }
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Essentially, the SystemRuntimeSerializationSerializationInfo object is a collection of name/value pairs When serializing, the GetObject() implementation calls AddValue() To reverse the process, you call one of the Get*() members In this case, you encrypt and decrypt prior to serialization and deserialization, respectively Versioning the Serialization One more serialization point deserves mentioning: versioning Objects such as documents may be serialized using one version of an assembly and deserialized using a newer version, sometimes the reverse Without paying attention, however, version incompatibilities can easily be introduced, sometimes unexpectedly Consider the scenario shown in Table 171 Surprisingly, even though all you did was to add a new field, deserializing the original file throws a SystemRuntimeSerializationSerialization
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TABLE 171: Deserialization of a New Version Throws an Exception
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[Serializable] class Document { public string Title; public string Data; }
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Define a class decorated with SystemSerializableAttribute Add a field or two (public or private) of any serializable type Serialize the object to a file called *v1bin
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Stream stream; Document documentBefore = new Document(); documentBeforeTitle = "A cacophony of ramblings from my potpourri of notes"; Document documentAfter; using (stream = FileOpen( documentBeforeTitle + "bin", FileModeCreate)) { BinaryFormatter formatter = new BinaryFormatter(); formatterSerialize( stream, documentBefore); }
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Add an additional field to the serializable class
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[Serializable] class Document { public string Title; public string Author; public string Data; } using (stream = FileOpen( documentBeforeTitle + "bin", FileModeOpen)) { BinaryFormatter formatter = new BinaryFormatter(); documentAfter = (Document)formatterDeserialize( stream); }
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Deserialize the *v1bin file into the new object (Document) version
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17: Reflection, Attributes, and Dynamic Programming Exception This is because the formatter looks for data corresponding to the
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new field within the stream Failure to locate such data throws an exception To avoid this, the 20 framework and later includes a SystemRuntimeSerializationOptionalFieldAttribute When you require backward compatibility, you must decorate serialized fields even private ones with OptionalFieldAttribute (unless, of course, a latter version begins to require it) Unfortunately, SystemRuntimeSerializationOptionalFieldAttribute is not supported in the earlier framework version Instead, it is necessary to implement ISerializable, just as you did for encryption, saving and retrieving only the fields that are available Assuming the addition of the Author field, for example, the implementation shown in Listing 1726 is required for backward-compatibility support prior to the 20 framework
Listing 1726: Backward Compatibility Prior to the 20 Framework
[Serializable] public class VersionableDocument : ISerializable { enum Field { Title, Author, Data, } public VersionableDocument() { } public string Title; public string Author; public string Data; #region ISerializable Members public void GetObjectData( SerializationInfo info, StreamingContext context) { infoAddValue(FieldTitleToString(), Title); infoAddValue(FieldAuthorToString(), Author); infoAddValue(FieldDataToString(), Data); } public VersionableDocument(
Attributes
SerializationInfo info, StreamingContext context) { foreach(SerializationEntry entry in info) { switch ((Field)EnumParse(typeof(Field), entryName)) { case FieldTitle: Title = infoGetString( FieldTitleToString()); break; case FieldAuthor: Author = infoGetString( FieldAuthorToString()); break; case FieldData: Data = infoGetString( FieldDataToString()); break; } } } #endregion }
Serializing in GetObjectData() simply involves serializing all fields (assume here that version 1 does not need to open documents from version 2) On deserialization, however, you can t simply call GetString("Author") because if no such entry exists, it will throw an exception Instead, iterate through all the entries that are in info and retrieve them individually
ADVANCED TOPIC SystemSerializableAttribute and the CIL In many ways, the serialize attributes behave just like custom attributes At runtime, the formatter class searches for these attributes, and if the attributes exist, the classes are formatted appropriately One of the characteristics that make SystemSerializableAttribute not just a custom attribute, however, is the fact that the CIL has a special header notation for serializable classes Listing 1727 shows the class header for the Person class in the CIL