"The Clean Code Talks -- Unit Testing"

By: GoogleTechTalks

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Uploaded on 11/01/2008

Google Tech Talks
October, 30 2008


Clean Code Talks - Unit Testing

Speaker: Misko Hevery

Comments (4):

By anonymous    2017-09-20

I think this is what you want, I already tested this code and works

The tools used are: (all these tools can be downloaded as Nuget packages)





var fixture = new Fixture().Customize(new AutoMoqCustomization());
var myInterface = fixture.Freeze<Mock<IFileConnection>>();

var sut = fixture.CreateAnonymous<Transfer>();

myInterface.Setup(x => x.Get(It.IsAny<string>(), It.IsAny<string>()))

sut.Invoking(x => 


Let me explain:

When you write a test, you must know exactly what you want to test, this is called: "subject under test (SUT)", if my understanding is correctly, in this case your SUT is: Transfer

So with this in mind, you should not mock your SUT, if you substitute your SUT, then you wouldn't be actually testing the real code

When your SUT has external dependencies (very common) then you need to substitute them in order to test in isolation your SUT. When I say substitute I'm referring to use a mock, dummy, mock, etc depending on your needs

In this case your external dependency is IFileConnection so you need to create mock for this dependency and configure it to throw the exception, then just call your SUT real method and assert your method handles the exception as expected

  • var fixture = new Fixture().Customize(new AutoMoqCustomization());: This linie initializes a new Fixture object (Autofixture library), this object is used to create SUT's without having to explicitly have to worry about the constructor parameters, since they are created automatically or mocked, in this case using Moq

  • var myInterface = fixture.Freeze<Mock<IFileConnection>>();: This freezes the IFileConnection dependency. Freeze means that Autofixture will use always this dependency when asked, like a singleton for simplicity. But the interesting part is that we are creating a Mock of this dependency, you can use all the Moq methods, since this is a simple Moq object

  • var sut = fixture.CreateAnonymous<Transfer>();: Here AutoFixture is creating the SUT for us

  • myInterface.Setup(x => x.Get(It.IsAny<string>(), It.IsAny<string>())).Throws<System.IO.IOException>(); Here you are configuring the dependency to throw an exception whenever the Get method is called, the rest of the methods from this interface are not being configured, therefore if you try to access them you will get an unexpected exception

  • sut.Invoking(x => x.TransferFiles(myInterface.Object, It.IsAny<string>(), It.IsAny<string>())).ShouldThrow<System.IO.IOException>();: And finally, the time to test your SUT, this line uses the FluenAssertions library, and it just calls the TransferFiles real method from the SUT and as parameters it receives the mocked IFileConnection so whenever you call the IFileConnection.Get in the normal flow of your SUT TransferFiles method, the mocked object will be invoking throwing the configured exception and this is the time to assert that your SUT is handling correctly the exception, in this case, I am just assuring that the exception was thrown by using the ShouldThrow<System.IO.IOException>() (from the FluentAssertions library)

References recommended:






Original Thread

By anonymous    2017-09-20

Since you request a way to mock a DataContext I assume that you really want to do some unit tests and not integration tests.

Well, I will tell you how to accomplish this, but first I would like to encourage you to read the following links, they are all about writing clean testable code.

And check the links from this response:

Watch the clean code talks from Misko Hevery (given to the Google people)

One thing that I used to repeat to myself and to my fellows at work, is that anyone can write a unit test, because they are silly easy to write. So a simple test is essentially all about making some comparisons and throw exceptions if the results fails, anyone can do that. Of course, there are hundreds of frameworks to help us write those tests in an elegant way. But the real deal, and the real effort shroud be put on learn how to write clean testable code

Even if you hire Misko Hevery to help you write tests, he will have a real hard time writing them if your code is not test-friendly.

Now the way to mock a DataContext objects is: do not do it

Instead wrap the calls using a custom interface instead:

public interface IMyDataContextCalls
    void Save();
    IEnumerable<Product> GetOrders();
// this will be your DataContext wrapper
// this wll act as your domain repository
public class MyDataContextCalls : IMyDataContextCalls
    public MyDataContextCalls(DataClasses1DataContext context)
        this.Context = context;

    public void Save()

    public IEnumerable<Product> GetOrders()
        // place here your query logic
        return this.Context.Products.AsEnumerable();

    private DataClasses1DataContext Context { get; set; }


// this will be your domain object
// this object will call your repository wrapping the DataContext
public class MyCommand
    private IMyDataContextCalls myDataContext;
    public MyCommand(IMyDataContextCalls myDataContext)
        this.myDataContext = myDataContext;

    public bool myDomainRule = true;

    // assume this will be the SUT (Subject Under Test)
    public void Save()
        // some business logic
        // this logic will be tested
        if (this.myDomainRule == true)
            // handle your domain validation  errors
            throw new InvalidOperationException();

public class MyTestClass
    public void MyTestMethod()
        // in this test your mission is to test the logic inside the 
        // MyCommand.Save method
        // create the mock, you could use a framework to auto mock it
        // or create one manually
        // manual example:
        var m = new MyCommand(new MyFakeDataContextFake());

        m.Invoking(x => x.Save())
            //add here more asserts, maybe asserting that the internal
            // state of your domain object was changed
            // your focus is to test the logic of the domain object

        //auto mock example:
        var fix = new Fixture().Customize(new AutoMoqCustomization());
        var sut = fix.CreateAnonymous<MyCommand>();
        sut.myDomainRule = false;

        sut.Invoking(x => x.Save())

    public class MyFakeDataContextFake : IMyDataContextCalls
        public void Save()
            // do nothing, since you do not care in the logic of this method,
            // remember your goal is to test the domain object logic

        public IEnumerable<Product> GetOrders()
            // we do not care on this right now because we are testing only the save method

            throw new NotImplementedException();


  • When you declare your IMyDataContextCalls interface you are actually abstracting the use of a DataContext, therefore this interface should contain only POCO objects (most of the time), if you follow this approach your interfaces will be decoupled from any undesired dependency.

  • In the specific MyDataContextCalls implementation, you are explicitly using a DataClasses1DataContext context, but you are free to change the implementation at any time and that won't affect your external code, and that's because you are always working with the IMyDataContextCalls interface instead. So at any time you could change for example this implementation for another one using the wonderful NHibernate =) or the poor ef or a mock one

  • At last, but not least. please double check my code, and you will notice that there are no new operators in the domain objects. This is a rule of dumb when writing test friendly code: decouple the responsibility of creating objects outside of your domain objects

I personally use three frameworks on every project and on every test I write, I really recommend them:

For example, in the code above, I showed you how to write a manual fake for your repository, but that clearly is something we do not want to do in a real project, imagine the number of objects you would have to code in order to write your tests.

Instead use the power of AutoFixture combined with Moq:

This line: var m = new MyCommand(new MyFakeDataContextFake());

Will become:

        var fixture = new Fixture().Customize(new AutoMoqCustomization());
        var sut = fixture.CreateAnonymous<MyCommand>();

And that's it, this code will automatically create mocks for all the objects needed in the constructor of MyCommand.

Original Thread

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