This chapter is from the book
This chapter is from the book
This chapter is from the book
Serialization
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 objects, such as classes, structs, and arrays, to a byte stream. Deserialization converts the byte stream back into the objects. Serializing and deserializing can be done on different machines as 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 to allow it to understand the memory layout of the object. 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 cached values need not be serialized. All you have to do is mark the class with the serializable attribute, and you then do not have to write any other code to serialize the object's fields.
The Serialization example shows how to apply serialization to the case study's HotelBroker class in the Hotel assembly.4 The Serializable attribute has been applied to the HotelBroker class definition. The Serializable attribute has also been applied to all the classes that get 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 serializable as well. If any of those classes were not marked, a runtime exception would be thrown when the framework tries to serialize an object of that type.
[Serializable]
public __gc class HotelBroker :
public Broker,
public IHotelInfo,
public IHotelAdmin,
public IHotelReservation
{
private:
const int MAXDAY;
const int MAXUNIT;
[NonSerialized] ArrayList *cities;
...
};
[Serializable]
public __gc class Hotel : public Reservable
{
...
};
[Serializable]
public __gc class HotelReservation : public Reservation
{
...
};
[Serializable]
public __gc __abstract class Reservable
{
...
};
[Serializable]
public __gc __abstract class Reservation
{
...
};
[Serializable]
public __gc __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
private:
void AddCity(String *city)
{
if (cities == 0)
{
cities = new ArrayList;
IEnumerator *pEnum = units->GetEnumerator();
while (pEnum->MoveNext())
{
Hotel *h =
dynamic_cast<Hotel *>(pEnum->Current);
AddCity(h->City);
}
}
// check if city already on list, add if not
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 and the storage medium. To specify the format that an object is saved in, you use an instance of an object that supports the IFormatter interface.6
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 CLR. The SoapFormatter uses the industry standard SOAP protocol that is discussed in Chapter 11, "Web Services." Since it is an XML-based, 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 have to talk to a foreign system with its own persistent object 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. Notice the special care taken to ensure that the Load method is able to modify the parameter that points to the HotelBroker. To enable this, the parameter is declared to be a reference to a pointer to a HotelBroker.
static void Save(
HotelBroker *broker, String *formatter)
{
FileStream *s;
if (String::Equals(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();
}
static void Load(
HotelBroker *&broker, /*ref to pointer */
String *formatter)
{
FileStream *s;
if (String::Equals(formatter, "b"))
{
s = new FileStream("hotels.bin", FileMode::Open);
BinaryFormatter *b = new BinaryFormatter;
broker =
dynamic_cast<HotelBroker *>
(b->Deserialize(s));
}
else
{
s = new FileStream("hotels.txt", FileMode::Open);
SoapFormatter *sf = new SoapFormatter;
broker =
dynamic_cast<HotelBroker *>(sf->Deserialize(s));
}
s->Close();
ShowHotelList(broker->GetHotels());
}
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 Marriot 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 saved7 in the first run.
Enter command: cities Atlanta Boston Commands: quit, cities, list, add, fetch, save Enter command: list City Name Rooms Rate Atlanta Dixie 100 115 Atlanta Marriot 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 Marriot 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, as shown in the Serialization project in the ISerializable directory. The ISerializable interface has one member: GetObjectData. This method is used when data is serialized.
The ISerializable example8 demonstrates how this is done. As before, the class has to be marked as Serializable.
[Serializable]
public __gc class HotelBroker :
public Broker,
public IHotelInfo,
public IHotelAdmin,
public IHotelReservation,
public ISerializable
{
...
};
The SerializationInfo class is used to store all the data that needs to be saved in the ISerializable::GetObjectData method. The AddValue method of SerializationInfo is overloaded to handle the saving of various types, including Object*.9 When you save the type, you provide a name so that it can be restored later. 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.
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 when the object is deserialized. It has the same arguments as GetObjectData has. Here you use the various GetXXX 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)
: Broker(366, 10), MAXDAY(366), MAXUNIT(10)
{
long numberHotels =
info->GetInt32("NumberHotels");
units = dynamic_cast<ArrayList *>(
info->GetValue(
"Hotels", units->GetType()));
if (numberHotels == units->Count)
Console::WriteLine("All hotels deserialized.");
else
Console::WriteLine("Error in deserialization.");
cities = new ArrayList;
IEnumerator *pEnum = units->GetEnumerator();
while (pEnum->MoveNext())
{
Hotel *h =
dynamic_cast<Hotel *>(pEnum->Current);
AddCity(h->City);
}
}
Remember, after you make any changes and build the Hotel project, will need to copy the Hotel.dll to the directory of the Serialization.exe client program. This does not get done automatically, as in C#, unless you add a custom build step to the project.
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 therefore look the same.