Saturday, 25 February 2012

Unit Testing Workflow Code Activities - Part 1

When I first started looking into Windows Workflow one of the first things that I liked about it was how it separated responsibilities. The workflow was responsible for handling the procedural logic with all it's conditional statements, etc. Whilst individual code activities could be written to handle the business logic processing; created in small easily re-usable components. To try and realise my original perception this series of blog posts will cover the unit testing of bespoke code activities; broken down into:

  • Part One: Unit testing a code activity with a (generic) cast return type (this post)
  • Part Two: Unit testing a code activity that assigns it's (multiple) output to "OutArguments" (Not yet written)

So to make a start consider the following really basic code activity; it expects an InArgument<string> of "Input" and returns a string containing the processed output; in this case a reverse copy of the value held in "Input".

namespace ExampleCode.Workflow
{
using System.Activities;
using System.Linq;
 
public class StringReverse : CodeActivity<string>
{
public InArgument<string> Input { get; set; }
 
protected override string Execute(CodeActivityContext context)
{
var input = this.Input.Get(context);
return string.IsNullOrWhiteSpace(input)
? input
: new string(Enumerable.Range(1, input.Length).Select(i => input[input.Length - i]).ToArray());
}
}
}

This code should have been extremely easy to unit test, apart from two immediate problems.

  1. The protected method "Execute" is not exposed to the calling code; making it impossible to call directly.
  2. I have no idea what is required to set up a "CodeActivityContext" or how to go about doing it - as a concrete implementation it's not possible to mock.
I don't really want to create a public method I can call directly without having to worry about the "context" as this is creating code for testing sake; something that is never a good idea. Just for completeness this could be implemented as follows, but I really wouldn't recommend it!

protected override string Execute(CodeActivityContext context)
{
return this.Process( this.Input.Get(context));
}
 
public string Process(string input)
{
return string.IsNullOrWhiteSpace(input)
? input
: new string(Enumerable.Range(1, input.Length).Select(i => input[input.Length - i]).ToArray());
}

So if we don't want to create new code and expose protected functionality purely for testing, what can we do? the answer lies in the unit test itself. Checking the prototypes for the "Invoke" method of the static class WorkflowInvoker class highlights that it takes either an instance of Activity or an instance of Activity<TResult>; it's important to remember that even a complex XAML workflow is contained within a single Sequence or Flow activity, which both inherit from Activity! Checking the return value of the "Invoke" method further highlights that we should get back the return value of the code activity instance. This means our unit test can simply be:

namespace ExampleCode.Workflow.Tests
{
using System.Activities;
using Microsoft.VisualStudio.TestTools.UnitTesting;
 
[TestClass]
public class StringReverseTests
{
[TestMethod]
public void TestMethod1()
{
var output = WorkflowInvoker.Invoke(new StringReverse { Input = new InArgument<string>("123") });
Assert.AreEqual("321", output);
}
}
}

It could be argued that it's not ideal because we really haven't isolated the code we want to test, but this solution doesn't require any test only changes to the code activity. It's also extremely easy to set up and call - given that I think it's an acceptable risk. No matter what logic is contained within the code activity, the only additional complexity in this instance is the number of input arguments. Things like external dependencies (Inversion of Control) and multiple output arguments will be covered in future posts.

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