Rob beat me to it. Blogging about T4 (the Text Template Transformation Toolkit) had been on my list literally for a year. He and I were singing its praises last night. Hopefully I can add something small to the conversation.
What‘s the story? Well, T4 is a code generator built right into Visual Studio. To be clear, you HAVE THIS NOW on your system…go play. Now‘s the time to introduce code generation to your company. If you‘re doing something twice or more, manually, in your company, generate it.
However, it‘s not deep-deep built in, because there‘s no item templates in File | New Item and there‘s no intellisense or syntax highlighting.
You don‘t need this, but if you want really get the most out of T4, first, head over to Clarius Consulting and get their "T4 Editor Community Edition." That‘ll get you some basic coloring. They have a pay version that gets you more if you want.
Now, go into Visual Studio and make a Console App (or any app) and add a Text File, but name it something with a .tt extension. You‘ll get a warning since it‘s a generator, that someone could generate evil. Click OK if you are cool with potential evil. ;)
Now, look in Solution Explorer at the .tt file. If you‘re using C#, you‘ll have a sub .cs file, or if you‘re using VB, a sub .vb file. That‘s the file that will hold the result of the generation. This is the same visual metaphor used to the express the template/generated file relationship with .designer files you‘ve seen elsewhere in Visual Studio. 
If you look in the Properties for the .tt file, you‘ll see it‘s using a CustomTool listed as the "TextTemplatingFileGenerator." This custom tool extensibility point is how things like XSD and WSDL code generators hook in to generate their artifacts.
T4 can generate any kind of text file, not just code. Whatever you like. You can use it in your projects, as above, or you can call T4 from the command-line.
Aside from Kzu and the folks at Clarius, Rob notes that Oleg Sych has a great collection of T4 resources. He‘s got some great Tutorials:
Here‘s some of the links from Oleg‘s blog.
Oleg also has a CodePlex project called T4 Toolbox that is a library of T4 templates that get added to File | New Item.
Also, check out Damien Guard‘s T4 templates that are a wholesale replacement of code that LINQ to SQL generates. Here‘s an example, where I use Damien‘s T4 templates against the sample Chinook Database.
Notice that I‘ve named the .tt file the same as the .dbml, so Damian‘s code can find it. I also continue to let original LINQ to SQL generate it‘s .designer.cs file, but make that file‘s Build Action "None" so it‘s not ever compiled. That effectively puts Damian‘s code in charge.
Here‘s a screenshot showing a bit of Damian‘s T4 template using the syntax highlighting from the Clairus T4 Visual Studio free download. If I‘d pay them, I‘d get intellisense and syntax highlighting inside the code blocks also. It looks like a lot like ASP.NET Web Forms, or Velocity, or any templating language really. The code blocks are where your logic is and outside the codeblocks is the template for whatever you want to generate. Notice how Damien gets input and sets output. You have full control, you can read files off the file system, from with your project, etc. He sets the output extension also. I like to use .g.cs or .g.vb, myself. In this example his generated file is Chinook.generated.cs.
I particularly like Damien‘s example because he‘s swapping out parts of LINQ to SQL that he didn‘t like (the generated code) while keeping the part he did (the general mode, the designer, the dbml file.) If you don‘t like something, fix it.
Plus, it all works in Visual Studio without installing anything.
If you‘re doing Code Gen, or thinking about it, check out T4 as it‘s a great place to start. Also, search my blog for "Code Generation" as I was livign and breathing it with CodeSmith for the many years I worked at Corillian. Have fun!
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