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(), 
        "PropertyTest.tt");

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

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

    Console.Write(output);
    Console.ReadKey();
}

References

• Understanding T4: <#@ property #> directive
• Guidance Automation Extensions Download