How write stub method with NUnit in C#

Question

I have 2 classes:

FirstDeep.cs
SecondDeep.cs

I did simple code for example:


class FirstDeep
    {
        public FirstDeep() { }

        public string AddA(string str)
        {
            SecondDeep sd = new SecondDeep();
            bool flag = sd.SomethingToDo(str);

            if (flag == true)
                str = string.Concat(str, "AAA");
            else
                str = string.Concat(str, "BBB");

            return str;
        }
    }

and

class SecondDeep
    {
        public bool SomethingToDo(string str)
        {
            bool flag = false;
            if (str.Length < 10)
            {
                //todo something in DB, and after that flag should be TRUE
            }
            return flag;
        }
    }

Then I want to write unit test for method "AddA":

class Tests
    {
        [Test]
        public void AddATest()
        {
            string expected = "ABCAAA";

            FirstDeep fd = new FirstDeep();
            string res = fd.AddA("ABC");

            Assert.AreEqual(expected, res);
        }
    }

And after that I have trouble, I don't know how correct write stub for method SomethingToDo in my Test class. I always have false. I should just return TRUE. But how?

you can still work with your pattern: introduce `bool expected = false; SecondDeep sd = new SecondDeep(); bool actualResult = sd.SomethingToDo("ABC"); Assert.AreEqual(excpected, actualResult);` ...!? if this doesn't meed your needs you sould probably consider to elaborate and improve your question! — , May 08 '12 at 08:56
Have you debugged your code? If it's happening in the database we can't help you as we have no details of what's happening there. — Jon, May 08 '12 at 08:58
Yes, I debugged my code, and where I wrote: "//todo something in DB, and after that flag should be TRUE" I using class MembershipUser from .NET and this method can't connect to DB, that's why I should just return true on this case. — Smit, May 08 '12 at 09:02

Martin Liversage · Accepted Answer · 2012-05-08T09:07:02.557

A good way to allow you to write stubs is to use dependency injection. FirstDeep depends on SecondDeep and in your test you want to replace SecondDeep with a stub.

First change your existing code by extracting an interface for SecondDeep and then inject that into FirstDeep in the constructor:

interface ISecondDeep {

  Boolean SomethingToDo(String str);

}

class SecondDeep : ISecondDeep { ... }

class FirstDeep {

  readonly ISecondDeep secondDeep;

  public FirstDeep(ISecondDeep secondDeep) {
    this.secondDeep = secondDeep;
  }

  public String AddA(String str) {   
    var flag = this.secondDeep.SomethingToDo(str);
    ...
  }

}

Note that FirstDeep no longer creates a SecondDeep instance. Instead an instance is injected in the constructor.

In your test you can create a stub for ISecondDeep where SomethingToDo always returns true:

class SecondDeepStub : ISecondDeep {

  public Boolean SomethingToDo(String str) {
    return true;
  }

}

In the test you use the stub:

var firstDeep = new FirstDeep(new SecondDeepStub());

In production code you use the "real" SecondDeep:

var firstDeep = new FirstDeep(new SecondDeep());

Using a dependency injection container and a stubbing framework can make a lot of this easier to do.

If you don't want to rewrite your code you can use a framework for intercepting calls like Microsoft Moles. In the next version of Visual Studio a similar technology will be available in the Fakes Framework.

Sergey Berezovskiy · Answer 2 · 2012-05-08T09:13:12.840

To make your code testable, do not instantiate dependencies inside your class. Use dependency injection (via constructor, property or parameter). Also use abstract classes or interfaces to allow mocking of dependencies:

class FirstDeep
{
    private ISecondDeep oa;

    public FirstDeep(ISecondDeep oa) 
    { 
        this.oa = oa;
    }

    public string AddA(string str)
    {
       return String.Concat(str, oa.SomethingToDo(str) ? "AAA" : "BBB");
    }
}

Depending on abstractions allows you to test your class in isolation.

interface ISecondDeep
{
   bool SomethingToDo(string str);
}

class SecondDeep : ISecondDeep
{
    public bool SomethingToDo(string str)
    {
       bool flag = false;
       if (str.Length < 10)
       {
           // without abstraction your test will require database
       }
       return flag;
    }
}

Here is test sample (using Moq). It shows you how you can return true from call to your mocked dependency:

[TestFixture]
class Tests
{
    [Test]
    public void AddAAATest()
    {
        // Arrange
        Mock<ISecondDeep> secondDeep = new Mock<ISecondDeep>();
        secondDeep.Setup(x => x.SomethingToDo(It.IsAny<string>())).Returns(true);
        // Act
        FirstDeep fd = new FirstDeep(secondDeep.Object);
        // Assert
        Assert.That(fd.AddA("ABD"), Is.EqualTo("ABCAAA"));
     }
}

How write stub method with NUnit in C#

2 Answers2