Serialization

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Using the File and Stream classes can be quite cumbersome if you have to save a complicated data structure with linked objects. You have to save the individual fields to disk, remembering which field belongs to which object, and which object instance was linked to another object instance. When restoring the data structure you have to reconstitute that arrangement of fields and object references.

The serialization technology provided by the .NET Framework does this for you. Serialization converts managed data structures to a byte stream. Deserialization converts the byte stream back to managed data structures. Serializing and deserializing can be done on different machines so long as they both host the CLR.

Objects can be serialized without writing special code because, as we have seen, the runtime can query the object's metadata and the serialized stream tags each value with the name of the field to which it applies.

To inform the framework that a class can be serialized, mark the class with the System.Serializable attribute. Any field or property that should not be serialized can be marked with the System.NonSerialized attribute. For example, fields that represent calculated values need not be serialized. All you have to do is mark the class with the attribute; you need write no other code to save the class's fields. Only instance fields are saved; the static fields of a class are never saved.

The Serialization example shows how to apply serialization to the case study's HotelBroker class in the Hotel assembly. The Serializable attribute has been applied to the HotelBroker class definition. The Serializable attribute has also been applied to all the classes that are used by HotelBroker or that HotelBroker derives from ” Broker , Hotel , HotelReservation , Reservable , and Reservation ”because in order for HotelBroker to be serializable, those classes must be as well. If any of those classes were not marked, a runtime exception would be thrown when the framework tried to serialize an object of that type.

 [Serializable]  public class HotelBroker : Broker, IHotelInfo,  IHotelAdmin, IHotelReservation   {  private const int MAXDAY = 366;  private const int MAXUNIT = 10;  [NonSerialized] private ArrayList cities;  . . .  [Serializable] public class Hotel : Reservable  . . .  [Serializable] public class HotelReservation : Reservation  . . .  [Serializable] public abstract class Reservable  . . .  [Serializable] public abstract class Reservation  . . .  [Serializable] public abstract class Broker  . . . 

The cities field has been marked as NonSerialized , since the hotel's city is saved with the serialized hotels and therefore can be restored, as the modified AddCity method demonstrates . The cities field would be null if the HotelBroker class had been deserialized, because the cities field was not saved. [5]

[5] Of course we could have serialized the cities field and not have to deal with the case where cities could be null, but we wanted to demonstrate the NonSerialized attribute.

 private void AddCity(string city)  {    if (cities == null)      {      cities = new ArrayList();      foreach(Hotel h in units)        AddCity(h.City);    }    if (!cities.Contains(city))    cities.Add(city);  } 

Serialization Objects

Although the framework knows how to save an object marked with the Serializable attribute, you still have to specify the format in which the object is saved (i.e., store the data as binary, XML, or some custom format), and the storage medium. To specify the format in which an object is saved, you use an instance of an object that supports the IFormatter interface. [6]

[6] How does the runtime know whether a class supports the IFormatter interface? Query the metadata!

The Framework ships with two such classes, System.Runtime. Serialization.Formatters.Binary.BinaryFormatter and System.Runtime. Serialization.Formatters.Soap.SoapFormatter . The BinaryFormatter uses a binary, compact format for serializing and deserializing on platforms that support the Common Language Runtime. The SoapFormatter uses the industry-standard SOAP protocol that is discussed in Chapter 11 on Web Services. Since it is an XML-, and therefore text-based protocol, it can be used to communicate with a non-CLR-based platform. The binary format is faster when serializing and deserializing data.

You can, of course, implement your own formatter classes. You might do this if you had to talk to a system with its own legacy byte format.

The Serialization example has code to demonstrate saving and restoring both binary and SOAP formats using a FileStream . Of course, you could use any Stream -based class representing some data medium.

 private static void Save(HotelBroker broker,    string formatter)  {    FileStream s;    if (formatter == "b")    {      s = new FileStream("hotels.bin", FileMode.Create);      BinaryFormatter b = new BinaryFormatter();      b.Serialize(s, broker);    }    else    {      s = new FileStream("hotels.txt", FileMode.Create);      SoapFormatter sf = new SoapFormatter();      sf.Serialize(s, broker);    }    s.Close();  }  private static void Load(ref HotelBroker broker,    string formatter)  {    FileStream s;    if (formatter == "b")    {      s = new FileStream("hotels.bin", FileMode.Open);      BinaryFormatter b = new BinaryFormatter();      broker = (HotelBroker) b.Deserialize(s);    }    else    {      s = new FileStream("hotels.txt", FileMode.Open);      SoapFormatter sf = new SoapFormatter();      broker = (HotelBroker)sf.Deserialize(s);    }  s.Close(); 

Here is some sample output from the Serialization example. First we add a hotel and save it with the SOAP formatter. We then exit the program.

 Enter command: cities  Atlanta  Boston  Commands: quit, cities, list, add, fetch, save  Enter command: list  City            Name                 Rooms      Rate  Atlanta         Dixie                100        115  Atlanta         Marriott             500        70  Boston          Sheraton             250        95  Commands: quit, cities, list, add, fetch, save  Enter command: add  Hotel City: Philadelphia  Hotel Name: Franklin  Number Rooms: 100  Room Rate: 200  Commands: quit, cities, list, add, fetch, save  Enter command: save  Formatter: b(inary), s(oap)s  Commands: quit, cities, list, add, fetch, save  Enter command: cities  Atlanta  Boston  Philadelphia  Commands: quit, cities, list, add, fetch, save  Enter command: list  City            Name                 Rooms      Rate  Atlanta         Dixie                100        115  Atlanta         Marriott             500        70  Boston          Sheraton             250        95  Philadelphia    Franklin             100        200  Commands: quit, cities, list, add, fetch, save  Enter command: quit 

We then run the program again and restore what we saved [7] in the first run.

[7] If you look at the hotels.txt file you will see a huge file, with a lot of "empty" entries. This stems from the simplistic array data structure we used for reservations , which is a very sparse matrix.

 Enter command: cities  Atlanta  Boston  Commands: quit, cities, list, add, fetch, save   Enter command: list  City            Name                 Rooms      Rate  Atlanta         Dixie                100        115  Atlanta         Marriott             500        70  Boston          Sheraton             250        95  Commands: quit, cities, list, add, fetch, save  Enter command: fetch  Formatter: b(inary), s(oap)s  City            Name                 Rooms      Rate  Atlanta         Dixie                100        115  Atlanta         Marriott             500        70  Boston          Sheraton             250        95  Philadelphia    Franklin             100        200  Commands: quit, cities, list, add, fetch, save  Enter command: cities  Atlanta  Boston  Philadelphia 

ISerializable

Sometimes the serialization provided by the Framework is not satisfactory. You can provide custom serialization for a class by implementing the ISerializable interface and adding a constructor to the class. The ISerializable interface has one member: GetObjectData . This method is used when data is serialized.

The ISerializable example demonstrates how this is done. As before, the class has to be marked as Serializable .

 [Serializable]  public class HotelBroker : Broker, IHotelInfo,  IHotelAdmin, IHotelReservation, ISerializable  {    private const int MAXDAY = 366;    private const int MAXUNIT = 10;    private ArrayList cities;  ... 

The SerializationInfo class is used to store all the data that needs to be saved. This class's AddValue method is overloaded to handle the saving of various types, including object . [8] When you save the type you provide a name that is used to recall the object when deserializing the data. The StreamingContext class gives you information about the stream being used in the serialization. For example, you can find out if the stream being used is a file or is being remoted to another computer.

[8] Some of the AddValue overloads are not CLS compliant when the types being saved are not CLS-compliant types such as unsigned integers. Be careful not to use those types where .NET language interoperability is required. You have to watch for this in other places in the Framework such as the Convert class or the Parse methods of the various CTS types, or any other place where data is formatted, converted, read, or written out (such as the TextWriter classes).

 public void GetObjectData(SerializationInfo info,    StreamingContext context)  {    long numberHotels = units.Count;    info.AddValue("NumberHotels", numberHotels);    info.AddValue("Hotels", units);  } 

You also have to implement a special constructor that is used by the framework to recreate the object when it is deserialized. It has the same arguments as does GetObjectData . Here you use the various Get methods on SerializationInfo to restore the data. Note that since we did not save the cities field, we had to manually restore it. The constructor is private because only the Framework uses it. If you forget to add the constructor, you will get a SerializationException when you try to restore the object.

 private HotelBroker(SerializationInfo info,    StreamingContext context) : base(MAXDAY, MAXUNIT)  {    long numberHotels = info.GetInt32("NumberHotels");    units = (ArrayList)info.GetValue("Hotels",      typeof(ArrayList));    if (numberHotels == units.Count)      Console.WriteLine("All hotels deserialized.");    else      Console.WriteLine("Error in deserialization.");    cities = new ArrayList();    foreach(Hotel h in units)      AddCity(h.City);  } 

In this example we only did custom serialization for the HotelBroker object. For all the other objects we still relied on the Framework's serialization. This example works the same way that the Serialization example did. The sample output would look the same.

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Application Development Using C# and .NET
Application Development Using C# and .NET
ISBN: 013093383X
EAN: 2147483647
Year: 2001
Pages: 158

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