Question

I’m trying to write a custom NAnt task that does some file-based operations. One of the things that would be incredibly useful would be the ability to run the <expandproperties> filter on one of the input files.

In the interest of keeping the task suitably generic, I simply want to enable support for the <filterchain> element (similar to how the <copy> task works).

I’ve been using the source for the <copy> task to guide me, however I keep on running into methods that are internal when it comes to writing the tasks. I know I can use reflection to break encapsulation, but I’m reluctant to do this.

Does anyone know of any useful articles, or have any experience with this?

Answer 1

I started going down the road of creating a private Filter subclass that took a TextReader (basically re-creating the PhysicalTextReader inside the NAnt source). However I realised that, actually, there was a much easier way of reading a file through a filter chain:

[TaskName("mytask")]
public class MyTask : Task
{
    /// <summary>
    /// Chain of filters used to alter the input file's content as it is read.
    /// </summary>
    [BuildElement("filterchain")]
    public FilterChain Filters { get; set; }

    /// <summary>
    /// The input file.
    /// </summary>
    [TaskAttribute("input")]
    public FileInfo InputFile { get; set; }

    protected override void ExecuteTask()
    {
        Log(FileUtils.ReadFile(InputFile.FullName, Filters, null));
    }
}

Then you can use this exactly as you would expect:

<mytask input="foo.txt">
    <filterchain>
        <expandproperties />
    </filterchain>
</mytask>