Thursday, June 23, 2011

The GAX Property Directive

One of the things that really bother me about T4 templates is the lack of direct parameter support (at least in the Visual Studio 2008/.NET 3 timeframe).

In an earlier post, I passed parameter by writing the parameter data to a file outside of the template and reading the data from within. I used a common assembly linked to both sides of the divide. This works for me, but it is really hacky.

GAX Template Host

During my preparation for my Portland Code Camp presentation, I came across Guidance Automation Toolkit for Visual Studio 2008. It has its own TemplateHost that includes a Dictionary of parameters.

Parameters are exposed to the Template through a custom directive. On the template, the parameter is declared using a directive like this:

<#@ property processor="PropertyProcessor" name="MyName" type="String" #>
The directive contains the following parts:
processor Should be "PropertyProcessor" (there may be a way to write your own processor, I haven’t gone down that rat hole yet).
name The Variable name that can appear in your code. It works just like a variable, however, tangable T4 Editor doesn’t provide Intenesence (I wouldn’t have expected it).
type Any .NET Type that the template knows about. You should use the .NET type name (as opposed to C# or VB.NET type).
In C# the parameters are created like this:
var arguments = new Dictionary();
arguments.Add("MyProperty", new PropertyData("String", typeof(string)));
To pass parameters from your code you need to create a Dictionary<string, PropertyData>, add your parameters and pass them as the second argument into the TemplateHost constructor (the first can be any string as far as I can tell).

Simple Sample

The Template

<#@ template language="C#" debug="True" #>
<#@ output extension="txt" #>
<#@ assembly name="System.dll" #>
<#@ import namespace="System" #>
<#@ property processor="PropertyProcessor" name="MyProperty" type="Nullable<Int32>"#>
<#@ property processor="PropertyProcessor" name="MyName" type="String"#>
My Property Test
<# if (MyProperty.HasValue) {#>
Property Value: <#= MyProperty.Value #>!
Name: <#= MyName #>

C# code

static void Main(string[] args)
    // Prepare template parameters
    var arguments = new Dictionary<string, PropertyData>();
    arguments.Add("MyProperty", new PropertyData(42, typeof(int?)));
    arguments.Add("MyName", new PropertyData("Jack Stephens", typeof(string)));

    // Initialize GAX template host
    // The Template Host is from GAX, not the default host
    var host = new TemplateHost("Random String", arguments);
    host.TemplateFile = Path.Combine(Directory.GetCurrentDirectory(), 

    // Transform template
    string template = File.ReadAllText(host.TemplateFile);

    ITextTemplatingEngine engine = new Engine();
    string output = engine.ProcessTemplate(template, host);



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