Create Custom T4 Text Template Directive Processors

The text template transformation process takes a text template file as the input and produces a text file as the output. The text template transformation engine controls the process, and the engine interacts with a text template transformation host and one or more text template directive processors to complete the process. For more information, see The Text Template Transformation Process.

To create a custom directive processor, you create a class that inherits from either DirectiveProcessor or RequiresProvidesDirectiveProcessor.

The difference between these two is that DirectiveProcessor implements the minimum interface that is necessary to get parameters from the user and to generate the code that produces the template output file. RequiresProvidesDirectiveProcessor implements the requires/provides design pattern. RequiresProvidesDirectiveProcessor handles two special parameters, requires and provides. For example, a custom directive processor might accept a file name from the user, open and read the file, and then store the text of the file in a variable that is named fileText. A subclass of the RequiresProvidesDirectiveProcessor class might take a file name from the user as the value of the requires parameter, and the name of the variable in which to store the text as the value of the provides parameter. This processor would open and read the file and then store the text of the file in the specified variable.

Before you call a custom directive processor from a text template in Visual Studio, you must register it.

For more information about how to add the registry key, see Deploying a Custom Directive Processor.

Custom Directives

A custom directive looks like this:

<#@ MyDirective Processor="MyDirectiveProcessor" parameter1="value1" ... #>

You can use a custom directive processor when you want to access external data or resources from a text template.

Different text templates can share the functionality that a single directive processor provides, so directive processors provide a way to factor code for reuse. The built-in include directive is similar, because you can use it to factor out code and share it among different text templates. The difference is that any functionality that the include directive provides is fixed and does not accept parameters. If you want to provide common functionality to a text template and allow the template to pass parameters, you must create a custom directive processor.

Some examples of custom directive processors could be:

• A directive processor to return data from a database that accepts a user name and password as parameters.

• A directive processor to open and read a file that accepts the name of the file as a parameter.

Principal parts of a custom directive processor

To develop a directive processor, you must create a class that inherits from either DirectiveProcessor or RequiresProvidesDirectiveProcessor.

The most important DirectiveProcessor methods that you must implement are as follows.

• bool IsDirectiveSupported(string directiveName) - Return true if your directive processor can deal with the named directive.

• void ProcessDirective (string directiveName, IDictionary<string, string> arguments) - The template engine calls this method for each occurrence of a directive in the template. Your processor should save the results.

After all calls to ProcessDirective() the templating engine will call these methods:

• string[] GetReferencesForProcessingRun() - Return the names of assemblies that the template code requires.

• string[] GetImportsForProcessingRun() - Return the namespaces that can be used in the template code.

• string GetClassCodeForProcessingRun() - Return the code of methods, properties, and other declarations that the template code can use. The easiest way to do this is to build a string containing the C# or Visual Basic code. To make your directive processor capable of being called from a template that uses any CLR language, you can construct the statements as a CodeDom tree and then return the result of serializing the tree in the language used by the template.

• For more information, see Walkthrough: Creating a Custom Directive Processor.

Related content

• Deploy a Custom Directive Processor explains how to register a custom directive processor.
• Walkthrough: Create a Custom Directive Processor describes how to create a custom directive processor, how to register and test the directive processor, and how to format the output file as HTML.