Metadata Attributes

A complete discussion of Metadata Attributes is beyond the scope here. Nevertheless we need to understand the concepts behind metadata attributes to understand how XML serialization works. When we talk about attributes in this section, we will always refer to metadata attributes, not XML attributes.

Attributes are a programming concept first introduced to the Microsoft platform with Microsoft Transaction Sever, which later became COM+. Attributes are annotations to an interface or a class definition to specify certain behavior. For example, no explicit coding was necessary to modify the transactional behavior of a class, it was declared by the presence of a transaction attribute. This is why the concept is also referred to as declarative programming.

The .NET platform takes attributes much further and uses them in a variety of places. Assemblies, classes, fields and methods, each can have attributes. Some are used by the compiler, some are used by the runtime, e.g. to identify a method requires a call to a web service, or how to serialize a class to XML. There is very little overhead associated when using attributes.

Attaching attributes to a class is done directly in the source code. The syntax to initialize a metadata attribute and attach it to a class or a method in C# is either:

[Attribute( constructor-parameters-list )]

or:

[Attribute(constructor-parameters-list, property=value, … )]

This chapter uses the second variation where possible because it is more descriptive and easier to understand.

XmlRootAttribute

Now what does all this have to do with XML or serialization? A whole lot! The System.Xml.Serialization namespace introduces a set of attributes to control how classes are mapped to XML. Let’s look at a quick example: One of the attributes used with XML serialization is the XmlRootAttribute to change the name of the root element of a serialization hierarchy. You would add the XmlRootAttribute to a class like this:

using System.Xml.Serialization;

[XmlRootAttribute(Name="Car", IsNullable=false)]

public class Automobile

{

      // class implementation goes here

}

This is as far as we’ll go introducing attributes. You now know enough about attributes to use them for serialization. If you are interested to learn more about using attributes throughout the .NET platform you can find some references to more in depth discussions in the further reading section.