C＃单元测试失败，即使AssertAreEqual看起来相等

Question

I have a unit test as follows:

[TestMethod]
public void ShowPreviousGuessesSetsTheModelPropertyToTheListOfGuessObjectsStoredInTheGuessingGameObject()
{
    //Arrange
    //First, set up a game and store the expected result
    var theGame = new GuessingGame();
    List<Guess> expectedResult = theGame.ShowGuessesMade();
    //Next, set up a FakeHttpContext with this game stored in the Session
    var theContext = new FakeHttpContext();
    var theKey = "GameState";
    theContext.Session.Add(theKey, theGame);
    //Now, set up a controller with this context
    var controller = new Exercise09Controller();
    var request = new System.Web.Routing.RequestContext(theContext, new System.Web.Routing.RouteData());
    controller.ControllerContext = new System.Web.Mvc.ControllerContext(request, controller);

    //Act
    var result = controller.ShowPreviousGuesses();

    //Assert
    Assert.AreEqual(expectedResult, result.Model);
}

And from the above code I have made the following class:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Prigmore2013_01.Models
{
    public class GuessingGame
    {

        public List<int> Target { get; set; }
        public List<int> Guesses { get; set; }

        public List<Guess> ShowGuessesMade()
        {
            var listRange = new List<Guess>();

            if (listRange != null)
            {
                return listRange;
            }
                return listRange;
        }
    }
}

I have run the test and I get the error:

Message: AssertAreEqual failed.

Expected:<System.Collections.Generic.List`1[Prigmore2013_01.Models.Guess]>.

Actual:<System.Collections.Generic.List`1[Prigmore2013_01.Models.Guess]>.

Would someone be able to explain to me why this unit test is failing even though the Expected value == to the Actual value? And how would I fix it so that my unit test passes?

Answer 1

Two things you'll need to do, first use CollectionAssert.AreEqual and second, override the Equals and GetHashCode methods in the Guess class.

public class Guess
{
    ...

    public override bool Equals(object obj)
    {
        // determine equality
    }

    public override int GetHashCode()
    {
        // if you have an INT primary key here, it would be good to use that
        // Example: return this.IntProperty.GetHashCode();
    }
}

Answer 2

When printing out the "Expected" and "Actual" values, the test runner calls ToString() on the objects. By default reference types print out the fully qualified type name, in your case: System.Collections.Generic.List`1[Prigmore2013_01.Models.Guess] .

You are confused because this prints out the same string in both cases; this is because the two objects are instances of the same class (colloquially, List<Guess> ), which is why they print out the same identifying string. But as others have pointed out, they are two different objects, stored in two different memory locations. Assert.AreEqual on reference types will perform "reference equality" -- not "is the content of these objects the same" but "are these two variables referencing the same memory location?"

As others have pointed out, the .Net framework has a helpful tool to do what you're looking to do, compare the content of two lists ( CollectionAssert.AreEqual ); I just wanted to provide a little more background and address something that confused me when I was getting started.

Answer 3

Look at what you're comparing in the assertion:

//Assert
Assert.AreEqual(expectedResult, result.Model);

expectedResult is the result of invoking theGame.ShowGuessesMade() , while result is the result of invoking controller.ShowPreviousGuesses()

These are two different lists. Note that even though they both might be empty, or contain the same elements in the same order, that does not make them equal - equivalent , in a semantic sense, maybe - but not equal. Equality in this case (for reference types) normally means "is this object the exact same object as this other object".

As @MichaelPerrenoud said above, you'll want to use CollectionAssert.AreEqual when you're trying to assert equivalency of lists.

Answer 4

Having not set up a constructor as follows:

 public GuessingGame()
        {
            //Set up the GuessingGame object
            this.Guesses = new List<Guess>();
        }

I was returning a different instance of the object. Having created a constructor I have being able to return the correct set of lists and in my controller I can then access the property directly:

public ViewResult ShowPreviousGuesses()
        {
    GuessingGame theGame = this.Session["GameState"] as GuessingGame;

    return View("Index", theGame.Guesses); 
}