How to Override an Attribute With XML

Utilizing the "XmlAttributeOverrides" class and its methods is helpful when you want to override an attribute while serializing a "XmlSerializer" object. This lets you serialize objects in multiple ways from a single set of serializable classes. It's also possible to extend upon a serializable class from a DLL that you don't have access to. A practical application of this is to serialize members of a class instance as XML attributes instead of XML elements.

Instructions

1. Open your code's source file in an editor such as Microsoft Visual Studio Express.

2. Add the "System," "System::IO" and "System::Xml::Serialization" libraries at the top of your file. This lets you access the XML "serializer" functions and objects. In C++ this is done by adding the code:

   using namespace System;

   using namespace System::IO;

   using namespace System::Xml::Serialization;

3. Define three public classes to use with the XML functions. In C++ this is done by adding the following code at the top of your file before your functions:

   public ref class Fruit

   {

   public:

   String^ ripe_fruit;

   };

   public ref class Apple: public Fruit

   {

   public:

   int amount_apples;

   };

   public ref class Basket

   {

   public:

   array<Fruit^>^Fruits;

   };

4. Create new "XmlAttributes" and "XmlElementAttribute" objects in your function, initialize the "XmlElementAttribute" object to use your replacement class and then add the new element to the collection of elements. In C++ this is done by adding the code:

   XmlAttributes^ my_attrs = gcnew XmlAttributes;

   XmlElementAttribute^ my_attr = gcnew XmlElementAttribute;

   my_attr->ElementName = "Apple";

   my_attr->Type = Apple::typeid;

   my_attrs->XmlElements->Add(my_attr);

5. Create a new "XmlAttributeOverrides" object and call its "Add" method to add the overriding class's type and the "XmlAttributes" object to override it with. In C++ this is done by adding the code:

   XmlAttributeOverrides^ my_attr_override = gcnew XmlAttributeOverrides;

   my_attr_override->Add( Basket::typeid, "Fruits", attrs );

6. Create a new "XmlSerializer" object and initialize it with the "XmlAttributeOverrides" object. In C++ this is done by adding the code:

   XmlSerializer^ my_serializer = gcnew XmlSerializer(Basket::typeid, my_attr_override);

7. Create an object to serialize. In C++ this is done by adding the code:

   Basket^ my_basket = gcnew Basket

8. Create a new object using the derived class. In C++ this is done by adding the code:

   Apple^ a = gcnew Apple;

   a->ripe_fruit = "Slightly Ripe";

   a->amount_apples = 2;

   array<Fruit^>^ my_Fruits = {a};

   my_basket->Fruits = my_Fruits;

9. Create a "TextWriter" object and then serialize your object. In C++ this is done by adding the code:

   TextWriter^ tw = gcnew StreamWriter("file.txt");

   my_serializer->Serialize(tw, my_basket);

   tw->Close();

   The code has overridden the "Fruit" class, allowing the field to accept the "Apple" class, which it inherits from the "Fruit" class.

10. Save the file, compile and run the program to override the attributes.

References

• Microsoft: XmlAttributeOverrides Class