Kash Farooq's software development blog

.NET Developer

BDD with SpecFlow and Coypu (Part 1)

Posted by Kash Farooq on August 19, 2011

I’ve been doing TDD for years and I thought it was about time I got into BDD too.

I’m going to use BDD to add a UI in front of the various implementations of Pi calculation algorithms I’ve been working on.

This post will describe what you need to install and how to get a SpecFlow feature file running via NUnit.


Headless browser:

Firing up a real browser from a BDD test can be slow. So, I’m using the Selenium headless Java browser.

Download selenium-server-standalone-2.4.0.jar (and, of course, you need Java)

To start it up:

java -jar selenium-server-standalone-2.3.0.jar


Install SpecFlow with the full installer and it will integrate with Dev Studio.

You can keep your feature files in your solution and SpecFlow will automatically generate NUnit based code-behind files.

I also recommend you watch the SpecFlow Screen Cast – an excellent introduction to quickly get you up and running.


To get my SpecFlow BDD tests to interact with the browser, I’m using Coypu.

Coypu is:

A more intuitive DSL for interacting with the browser in the way a human being would, inspired by the ruby framework Capybara.

Other software

You need NUnit (as SpecFlow creates NUnit tests).

Adding the first feature file

I’ve already got a few implementations of algorithms to calculate Pi. I just want a simple UI that allows you to select an algorithm, type how many decimal places you want Pi calculated to, and then hit “Calculate”. I want the “Calculate” link/button to call back to my application via AJAX (I want to see how the headless browser copes with Javascript).

Step 1: Add a feature file

A feature file is just a text file that describes the functionality you want to implement. It has a simple Given-When-Then structure that should be understandable by non-developers. Hence, they could be created by Business Analysts, etc.

I created a new class library called Spec and added a feature file using the “Add New Item” Dev Studio menu. You’ll see the option “SpecFlow Feature File”.

Feature: Calculate Pi using various algorithms
	I want to be able to view Pi to varying decimal places

Scenario: Display Pi using Taylor series and Bucknall's BigNumber to many decimal places
	Given I visit the Pi.NET website
	And I have selected the 'Bucknall Big Number' algorithm
	And I have entered 500 decimal places
	When I press Go
	Then Pi should be displayed to the correct number of decimal places
	And Calculation statistics should be displayed

You’ll also need to add DLL references to TechTalk.SpecFlow.dll and nunit.framework.dll

Once you’ve done this, compile and run all the unit tests in your Spec class library. You’ll see output like this:

Given I visit the Pi.NET website
-> No matching step definition found for the step. Use the following code to create one:
public class StepDefinitions {
   [Given(@"I visit the Pi\.NET website")]
   public void GivenIVisitThePi_NETWebsite()
etc, etc.

SpecFlow has told you exactly what you need to do.

So, let’s copy and paste the code into a new C# file called PiNetSteps:

public class PiNetSteps {
    [Given(@"I visit the Pi\.NET website")]
    public void GivenIVisitThePi_NETWebsite() {

    [Given(@"I have selected the 'Bucknall Big Number' algorithm")]
    public void GivenIHaveSelectedTheBucknallBigNumberAlgorithm() {

    [Given(@"I have entered 500 decimal places")]
    public void GivenIHaveEntered500DecimalPlaces() {

    [When(@"I press Go")]
    public void WhenIPressGo() {

    [Then(@"Pi should be displayed to the correct number of decimal places")]
    public void ThenPiShouldBeDisplayedToTheCorrectNumberOfDecimalPlaces() {

    [Then(@"Calculation statistics should be displayed")]
    public void ThenCalculationStatisticsShouldBeDisplayed() {

Now when you run the unit tests you get the output:

Ignored: One or more step definitions are not implemented yet.

We can improve the steps code by parameterizing it with Regex. For example, rather than hard coding “500 decimal places”, let’s make this a parameter. And the algorithm name can also be a parameter.

Making these changes gives:

[Given(@"I have selected the '(.+)' algorithm")]
public void GivenIHaveSelectedAnAlgorithm(string algorithm) {

[Given(@"I have entered '(.+)' decimal places")]
public void GivenIHaveEnteredTheRequiredNumberOfDecimalPlaces(int numberOfDecimalPlaces) {
    this.numberOfDecimalPlaces = numberOfDecimalPlaces; //store for a later assert

We’re now at a stage that opens up lots of possibilities. You’ve gone from a feature file to a C# class and you now have options on how deep you want your tests to go. If you system under test is not a web application, you are ready to start implementing it straight away. If your system has lots of external dependencies that are not under your control, you could easily stub them out with a container. Or perhaps you want your tests to go through all the application layers and hit a database or webservice. You could easily introduce test database and webservices if you wish.

In the next post I’ll get Coypu up and running and use it to get my SpecFlow steps class to hit the Selenium headless browser.

Next: BDD with SpecFlow and Coypu (Part 2)

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