Microsoft's Text Template Transformation Toolkit (usually referred to as "T4") is a template based text generation framework included with Visual Studio. T4 source files are usually denoted by the file extension ".tt".
Contents
T4 is used by developers as part of an application or tool framework to automate the creation of text files with a variety of parameters. These text files can ultimately be any text format, such as code (for example C#), XML, HTML or XAML.
T4 uses a custom template format which can contain .NET code and string literals in it, this is parsed by the T4 command line tool into .NET code, compiled and executed. The output of the executed code is the text file generated by the template. T4 can also be completely run within the .NET applications via the use of the TextTransformation class which eliminates the need for the end user to have Visual Studio installed.
T4 is used within Microsoft in ASP.NET MVC for the creation of the views and controllers, ADO.NET Entity Framework for entity generation, and ASP.NET Dynamic Data. It is also used outside of Microsoft in projects such as SubSonic
T4 templating is also supported in MonoDevelop.
Criticism
Microsoft has often been criticized for the lack of tooling support for T4 within the Visual Studio IDE, and relies on third parties namely Tangible Engineering and Clarius Consulting, however later revisions have included better tooling support.