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CHAPTER 8 DATA PERSISTENCE
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Probably the most common means of serializing a class in .NET is through binary serialization, as it s used for files and remoting. Binary serialization is unique in that the footprint of the object s data in memory is almost identical to the footprint of the data in the other medium. To illustrate how the Serialization pattern is implemented for binary serialization, consider the following class declaration: [Serializable] class SampleClass { private int _value; private string _buffer; public SampleClass(int value, string buffer) { _value = value; _buffer = buffer; } public int Value { get { return _value; } } public string Buffer { get { return _buffer; } } } SampleClass has been marked serializable because of the [Serializable] attribute. This means when doing binary serialization, all data members of SampleClass will be serialized regardless of the scope of the data member. Binary serialization iterates each data member and writes the contents of it to the data stream. For SampleClass, this means that _value and _buffer will be serialized. The Serialization pattern indicates the serializable type implements the methods ReadFrom and Writeto. Because .NET binary serialization uses the metadata of the type to perform serialization, those methods aren t necessary. This modification of the Serialization pattern is acceptable, because part of the reason for the methods is that the pattern assumes a type doesn t have any metadata. For example, the C++ Microsoft Foundation Classes (MFCs) fully implement the Serialization pattern. This is because C++ MFCs don t have metadata information. Getting back to serialization, binary serialization is a bit of challenge because everything is serialized. Maybe some data members shouldn t be serialized because the state can t be carried across media. An example could be delegates or database connections. These data members are transient and only applicable when the object is executing in memory. Applying the attribute [NonSerialized] to a data member causes the data member to be ignored when serialization is executed. Following is some code that illustrates how to serialize the class SampleClass to a file:
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SampleClass obj = new SampleClass(10, "hello"); Stream stream = File.OpenWrite( @"C:\sampleclass.bin"); BinaryFormatter bf = new BinaryFormatter(); bf.Serialize(stream, obj); stream.Close(); The class SampleClass is instantiated and assigned to the variable obj. Then a file stream is opened using the method File.OpenWrite. It s important to realize that a plain vanilla file is being opened. Then BinaryFormatter is instantiated, which represents a serializer assigned to the variable bf. The sample class instance obj is written to the file stream using the bf.Serialize method. After the object has been serialized, the file stream contains a representation of the object instance obj. What is interesting is that the binary serializer isn t part of the file. This makes it possible to perform a binary serialization to a file or to a network connection. The way that the binary serializer does this is by manipulating the stream interface. The resulting binary file would appear similar to the following text: ^@^A^@^@^@ ^A^@^@^@^@^@^@^@^L^B^@^@^@ &test, Version=0.0.0.0,Culture=neutral^E^A^@^@^@^ KSampleClass^B^@^@^@^F_value^G_buffer^@^A^H^B^@^@^@ ^@^@^@^F^C^@^@^@^Ehello^K The funny characters (@ etc.) represent binary data used by the formatter. Ignore these characters and look at the data that is text based. Notice the identifier SampleClass, which represents the class that has been serialized. Then _value and _buffer, which represent the data members of SampleClass that have been serialized. The remaining textual value, hello, represents the value of the data member _buffer. The value of the data member _value exists, but is encoded using a numeric value, and hence hidden in the funny characters. What you should remember is that the stream of characters can only be written and read by the class BinaryFormatter. To re-create the class, the source code used to deserialize SampleClass is as follows: FileStream stream=new FileStream( @"C:\\sampleclass.bin", FileMode.Open); BinaryFormatter bf = new BinaryFormatter(); SampleClass obj = bf.Deserialize(file) as SampleClass; Console.WriteLine( "Object Value (" + obj.Value + ") Buffer (" + obj.Buffer + ")"); file.Close(); To deserialize the class SampleClass, a file stream (FileStream) is opened, and again a binary formatter (BinaryFormatter) is instantiated. This time the binary formatter reads an object from the stream using the method bf.Deserialize. The read object is typecast to the type SampleClass and assigned to the variable obj.
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