Names and Namespaces

Just as .NET types can have namespaces, so too can XML elements and attributes.

XML namespaces achieve two things. First, rather like namespaces in C#, they help avoid naming collisions. This can become an issue when you merge data from one XML file into another. Second, namespaces assign absolute meaning to a name. The name “nil,” for instance, could mean anything. Within the namespace, however, “nil” means something equivalent to null in C# and comes with specific rules on how it can be applied.

A namespace in XML is defined with the xmlns attribute:

	<customer xmlns="OReilly.Nutshell.CSharp"/>

xmlns is a special reserved attribute. When used in this manner, it performs two functions:

  • It specifies a namespace for the element in question.

  • It specifies a default namespace for all descendant elements.

You can also specify a namespace with a prefix —an alias that you assign to a namespace to avoid repetition. There are two steps—defining the prefix and using the prefix. You can do both together as follows:

	<nut:customer xmlns:nut="OReilly.Nutshell.CSharp"/>

Two distinct things are happening here. On the right, xmlns: nut="…" defines a prefix called nut and makes it available to this element and all its descendants. On the left, nut:customer assigns the newly allocated prefix to the customer element.

A prefixed element does not define a default namespace for descendants. In the following XML, firstname has an empty namespace:


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