## NUnit Addins

NUnit originally identified tests in the time-honored way used in many xUnit test frameworks. Test classes inherited from the framework's TestCase class. Individual test case methods were identified by having names starting with "test..."

With NUnit 2.0, we introduced the use of attributes to identify both fixtures and test cases. Use of attributes in this way was a natural outcome of their presence in .NET and gave us a way of identifying tests that was completely independent of both inheritance and naming conventions.

However, by moving away from an inheritance-based mechanism we no longer had an easy way for others to extend NUnit's internal behavior. NUnit Addins are intended to fill that gap, providing an mechanism to introduce new or changed behavior without modifying NUnit itself.

### Extension Points, Extensions and Addins

NUnit provides a number of Extension Points, places where it is possible to extend its behavior. Because NUnit works with various hosts and uses separate AppDomains to run tests, Extension Points are categorized into three types: Core, Client and Gui. Each of these types is supported by a different Extension Host.

An NUnit Addin provides enhanced functionality through one or more extensions, which it installs at identified Extension Points. Each Addin is characterized by the types of extensions it supplies, so that an Extension Host knows whether to invoke it.

> Note: In the current release, only Core extensions are actually supported. An Addin may characterize itself as a Client or Gui extension and will be listed as such in the Addins Dialog, but no other action is taken.

### Addin Identification, Loading and Installation

NUnit examines all assemblies in the bin/addins directory, looking for public classes with the NUnitAddinAttribute and implementing the IAddin interface. It loads all those it finds as Addins.

NUnitAddinAttribute supports three optional named parameters: Type, Name and Description. Name and Description are strings representing the name of the extension and a description of what it does. If no name is provided, the name of the class is used. Type may be any one or a combination of the ExtensionType enum members:

    	[Flags]
    	public enum ExtensionType
    	{
    		Core=1,
    		Client=2,
    		Gui=4
    	}

The values may be or'ed together, allowing for future Addins that require extensions at multiple levels of the application. If not provided, Type defaults to ExtensionType.Core.

The IAddin interface, which must be implemented by each Addin, is defined as follows:

    	public interface IAddin
    	{
    		bool Install( IExtensionHost host );
    	}

The Install method is called by each host for which the addin has specified an ExtensionType. The addin should check that the necessary extension points are available and install itself, returning true for success or false for failure to install. The method will be called once for each extension host and - for Core extensions - each time a new test domain is loaded.

The Install method uses the IExtensionHost interface to locate extension points. It is defined as follows:

    	public interface IExtensionHost
    	{
    	 	IExtensionPoint[] ExtensionPoints { get; }
    		IExtensionPoint GetExtensionPoint( string name );
    		ExtensionType ExtensionTypes { get; }
    	}

The ExtensionPoints property returns an array of all extension points for those extensions that need the information. The ExtensionTypes property returns the flags for the type of extension supported by this host, allowing, for example, Gui extensions to only load in a Gui host.

Most addins will only need to use the GetExtensionPoint method to get the interface to a particular extension point. The IExtensionPoint interface is defined as follows:

    	public interface IExtensionPoint
    	{
    		string Name { get; }
    		IExtensionHost Host { get; }
    		void Install( object extension );
    		void Remove( object extension );
    	}

Once again, most addins will only need to use one method - the Install method in this case. This method passes an extension object to the Extension Point where it is installed. Generally, extensions do not need to remove themselves once installed, but the method is provided in any case.

With NUnit 2.5, an additional interface, IExtensionPoint2 is available. It derives from IExtensionPoint and also allows setting the priority order in which the extension will be called in comparison to other extensions on the same extension point. The interface is defined as follows:

    	public interface IExtensionPoint2 : IExtensionPoint
    	{
    		void Install( object extension, int priority );
    	}

Only extension points that use a priority scheme implement this interface. In general, extension points with a priority scheme will use a default value for priority if the Install method without a priority is called.

In the NUnit 2.5 release, only the TestDecorators extension point implements IExtensionPoint2.

### Extension Point Details

Depending on the particular extension point, the object passed will need to implement one or more interfaces. The following ExtensionPoints are implemented in this release of NUnit:

- SuiteBuilders
- TestCaseBuilders
- TestDecorators
- TestCaseProviders
- DataPointProviders
- EventListeners

For examples of implementing each type of extension, see the Extensibility samples provided with NUnit. More complete examples are available in the code of NUnit itself, since NUnit uses the same mechanism internally.

#### See also...

- Tips for Writing Extensions