Sample code for the post "Property-based BDD Examples with SpecFlow and FsCheck"

Addition.feature

Scenario: Add two numbers
  Given I have entered 1 into the calculator
  And I have entered 2 into the calculator
  When I press add
  Then the result should be 3 on the screen

Constraints.cs

[Binding]
public class Constraints : ConstraintsBase
{
    [StepArgumentTransformation("any number")]
    public int AnyNumber()
    {
        return AsParam("any", Arb.Default.Int32());
        //could be constrainded: AsParam("any", Gen.Choose(0, 100));
    }

    [StepArgumentTransformation("the first number")]
    public int TheFirstNumber()
    {
        return AsFormula(actualParams => (int)actualParams.First());
    }
}

FsCheck.cs

[TestMethod]
public void FsCheckDemo_Addition_Identity()
{
    Func<int, bool> identity = 
        (a) => Addition.Add(a, 0) == a;

    Prop.ForAll(identity).QuickCheckThrowOnFailure();
}

[TestMethod]
public void FsCheckDemo_Addition_Commutativity()
{
    Func<int, int, bool> commitative = 
        (a, b) => Addition.Add(a, b) == Addition.Add(b, a);

    Prop.ForAll(commitative).QuickCheckThrowOnFailure();
}

PropertyBased.feature

@propertyBased
Scenario: Identity property
  Given I have entered any number into the calculator
  And I have entered 0 into the calculator
  When I press add
  Then the result should be the first number on the screen

RealLifeExamples.feature

Scenario: Should be able to choose color
	Given the user is logged in
	And selected a *product with color variations*
	When the user adds the product to the basket
	Then it should be able to choose the color

Scenario: Restricted pages require login
	Given the user has not logged in yet
	When the user tries to access a *restricted page*
	Then the user should be redirected to the login page

Scenario: Do not let the user type while driving
	Given the vehicle is driving with speed *greater than 5 km/h*
	When the driver attempts to type in an address
	Then a warning should be displayed

WrongAddition.cs

public static int Add(int op1, int op2)
{
    // "after 38, every year counts twice"
    return op1 >= 38 ? op1 * 2 + op2 : op1 + op2;
}