Intro to T4 Templates: Generating Text in a Hurry

Editorial Note: I originally wrote this post for the SubMain blog.  You can check out the original here, at their site.  While you’re there, take a look at GhostDoc.  In addition to comment generation capabilities, you can also effortlessly produce help documentation.

Today, I’d like to tackle a subject that inspires ambivalence in me.  Specifically, I mean the subject of automated text generation (including a common, specific flavor: code generation).

If you haven’t encountered this before, consider a common example.  When you file->new->(console) project, Visual Studio generates a Program.cs file.  This file contains standard includes, a program class, and a public static void method called “Main.”  Conceptually, you just triggered text (and code) generation.

Many schemes exist for doing this.  Really, you just need a templating scheme and some kind of processing engine to make it happen.  Think of ASP MVC, for instance.  You write markup sprinkled with interpreted variables (i.e. Razor), and your controller object processes that and spits out pure HTML to return as the response.  PHP and other server side scripting constructs operate this way and so do code/text generators.

However, I’d like to narrow the focus to a specific case: T4 templates.  You can use this powerful construct to generate all manner of text.  But use discretion, because you can also use this powerful construct to make a huge mess.  I wrote a post about the potential perils some years back, but suffice it to say that you should take care not to automate and speed up copy and paste programming.  Make sure your case for use makes sense.

The Very Basics

With the obligatory disclaimer out of the way, let’s get down to brass tacks.  I’ll offer a lightning fast getting started primer.

Open some kind of playpen project in Visual Studio, and add a new item.  You can find the item in question under the “General” heading as “Text Template.”

newtexttemplate

Give it a name.  For instance, I called mine “sample” while writing this post.  Once you do that, you will see it show up in the root directory of your project as Sample.tt.  Here is the text that it contains.

Save this file.  When you do so, Visual Studio will prompt you with a message about potentially harming your computer, so something must be happening behind the scenes, right?  Indeed, something has happened.  You have generated the output of the T4 generation process.  And you can see it by expanding the caret next to your Sample.tt file as shown here.

samplett

If you open the Sample.txt file, however, you will find it empty.  That’s because we haven’t done anything interesting yet.  Add a new line with the text “hello world” to the bottom of the Sample.tt file and then save.  (And feel free to get rid of that message about harming your computer by opting out, if you want).  You will now see a new Sample.txt file containing the words “hello world.”

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