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?

Answer 1

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.