Tag Archives: XUnit

Git Bisect with Automated Tests

Some time ago, I saw a talk at DDD North about git bisect (it may well have been this one). I blogged about it here. I can honestly say that it’s one of, if not the, most useful thing I’ve ever learnt in 10 minutes!

However, the problem with it is that you, essentially, have to tell it what’s good and what’s bad. In this post, I’ll be detailing how you can write automated tests to determine this, and then link them in.

Using existing tests to determine where something broke

In this example, I’ll be using this repository (feel free to do the same). The code in the repository is broken, but it hasn’t always been, and there are some tests within the repository that clearly weren’t run before check-in, and are now broke (I know this, because I purposely broke the code – although this does happen in real life, and often with good intentions – or at least not bad).

We’re using xUnit here; but I’m confident that any test framework would do the same. The trick is the dotnet test command; from the docs on that command:

If all tests are successful, the test runner returns 0 as an exit code; otherwise if any test fails, it returns 1.

As with the previous post, we need to start with a good and bad commit; for the purpose of this post, we’ll assume the current commit is bad, and the first ever commit was good.

git log

Will give a list of commits:

We need the first, which, for this repo, is:

3cbd757dd4e92d8ab2424c6a1e46a73bef23e056

Now we need to go through the process of setting up git bisect; the process is: you tell git that you wish to start a bisect:

git bisect start

Next, you tell git which commit is bad. In our case, that’s the current one:

git bisect bad

Finally, you tell it which was the last known good one – in our case, the first:

git bisect good 3cbd757dd4e92d8ab2424c6a1e46a73bef23e056

Now that we’re in a bisect, you could just tell git each time which is good and which bad (see the previous post on how you might do that), but here you can simply tell it to run the test:

git bisect run dotnet test GitBisectDemo/

This will then iterate through the commits and come back with the breaking commit:

That’s great, but in most cases you didn’t actually have a breaking test – something has stopped working, and you don’t know why or when. In these cases, you can write a new breaking test, and then give that to git bisect for it to tell you the last time that test passed.

Create a new test to determine where something broke

Firstly, the new test must not be checked in to source control, as this works by checking out code from previous releases. Then create your new test; for example:

namespace GitBisectDemo.Tests
{
    public class CalculationTests2
    {
        [Fact]
        public void DoCalculation_ReturnsCorrectValue()
        {
            // Arrange
            var calculationEngine = new CalculationEngine();

            // Act
            float result = calculationEngine.DoCalculation(2, 3);

            // Assert
            Assert.True(result > 4);
        }

    }
}

This is a new class, and it’s not checked into source control.

Executing a specific test from the command line

We now want to execute just one test, and you can do that using dotnet test like so:

dotnet test GitBisectDemo/ --filter "FullyQualifiedName=GitBisectDemo.Tests.CalculationTests2.DoCalculation_ReturnsCorrectValue"

You need to give it the full namespace and class name; we can now incorporate that into our git bisect:

git bisect start
git bisect bad
git bisect good 3cbd757dd4e92d8ab2424c6a1e46a73bef23e056

These are the same as before.

Note: if, at any time, you wish to cancel the bisect, it’s git bisect reset

Now, we feed the filtered test run into git bisect:

git bisect run dotnet test GitBisectDemo/ --filter  "FullyQualifiedName=GitBisectDemo.Tests.CalculationTests2.DoCalculation_ReturnsCorrectValue"

And we get a result when the new test would have broken.

That’s two cases covered. The final case is the situation whereby the thing that has broken cannot be determined by an automated test; say, for example, that an API call isn’t working correctly, or a particular process has slowed down. In this situation, we can have git bisect call out to an external executable.

Custom Console App

The first step here is to return a value (exit code) from the console app. In fact, this is deceptively simple:

static int Main(string[] args)
{
    var calculationEngine = new CalculationEngine();
    float result = calculationEngine.DoCalculation(3, 1);

    return (result == 4) ? 0 : -1;
}

Notice that all we’ve done here is change the Main signature to return an int. This console app could now be calling an external API, running a performance test, or anything that has a verifiable result.

Publish the console app

Because we’re calling this from another location, we’ll need to publish this test as a self-contained console app:

dotnet publish -r win-x64

Run the test

Again, the same set-up:

git bisect start
git bisect bad
git bisect good 3cbd757dd4e92d8ab2424c6a1e46a73bef23e056

Finally, we call the console app to run the test:

git bisect run GitBisectDemo/GitBisectDemo.ConsoleTest/bin/Debug/netcoreapp3.1/win-x64/GitBisectDemo.ConsoleTest.exe

References

https://docs.microsoft.com/en-us/dotnet/core/tools/dotnet-test

https://stackoverflow.com/questions/155610/how-do-i-specify-the-exit-code-of-a-console-application-in-net

My XUnit Tests won’t run in a .Net Standard 2.0 Class Library

Firstly, this isn’t a bug, or something that you might have done wrong; it’s intentional. Essentially, you can’t run a .Net Standard Library, so your tests aren’t runnable.

Okay – so I want to convert to .Net Core 3.0!

Yep – that’s exactly what you want, and it’s this easy; open up the csproj file – it’ll look like this:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
  </PropertyGroup>

And replace it with this:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netcoreapp3.0</TargetFramework>
  </PropertyGroup>

And that’s it – your tests should now run!

Short Walks – XUnit Warning

As with many of these posts – this is more of a “note to self”.

Say you have an assertion that looks something like this in your Xunit test:

Assert.True(myEnumerable.Any(a => a.MyValue == "1234"));

In later versions (not sure exactly which one this was introduced it), you’ll get the following warning:

warning xUnit2012: Do not use Enumerable.Any() to check if a value exists in a collection.

So, Xunit has a nice little feature where you can use the following syntax instead:

Assert.Contains(myEnumerable, a => a.MyValue == "1234");

Using NSubstitute for partial mocks

I have previously written about how to, effectively, subclass using Nsubstitute; in this post, I’ll cover how to partially mock out that class.

Before I get into the solution; what follows is a workaround to allow badly written, or legacy code to be tested without refactoring. If you’re reading this and thinking you need this solution then my suggestion would be to refactor and use some form of dependency injection. However, for various reasons, that’s not always possible (hence this post).

Here’s our class to test:

public class MyFunkyClass
{
    public virtual void MethodOne()
    {        
        throw new Exception("I do some direct DB access");
    }
 
    public virtual int MethodTwo()
    {
        throw new Exception("I do some direct DB access and return a number");

        return new Random().Next(5);
    }
 
    public virtual int MethodThree()
    {
        MethodOne();
        if (MethodTwo() <= 3)
        {
            return 1;
        }
 
        return 2;
    }
}

The problem

Okay, so let’s write our first test:

[Fact]
public void Test1()
{
    // Arrange
    MyFunkyClass myFunkyClass = new MyFunkyClass();
 
    // Act
    int result = myFunkyClass.MethodThree();
 
    // Assert
    Assert.Equal(2, result);
}

So, what’s wrong with that?

Well, we have some (simulated) DB access, so the code will error.

Not the but a solution

The first thing to do here is to mock out MethodOne(), as it has (pseudo) DB access:

[Fact]
public void Test1()
{
    // Arrange
    MyFunkyClass myFunkyClass = Substitute.ForPartsOf<MyFunkyClass>();
    myFunkyClass.When(a => a.MethodOne()).DoNotCallBase();
 
    // Act
    int result = myFunkyClass.MethodThree();
 
    // Assert
    Assert.Equal(2, result);
}

Running this test now will fail with:

Message: System.Exception : I do some direct DB access and return a number

We’re past the first hurdle. We can presumably do the same thing for MethodTwo:

[Fact]
public void Test1()
{
    // Arrange
    MyFunkyClass myFunkyClass = Substitute.ForPartsOf<MyFunkyClass>();
    myFunkyClass.When(a => a.MethodOne()).DoNotCallBase();
    myFunkyClass.When(a => a.MethodTwo()).DoNotCallBase();
 
    // Act
    int result = myFunkyClass.MethodThree();
 
    // Assert
    Assert.Equal(2, result);
}

Now when we run the code, the test still fails, but it no longer accesses the DB:

Message: Assert.Equal() Failure
Expected: 2
Actual: 1

The problem here is that, even though we don’t want MethodTwo to execute, we do want it to return a predefined result. Once we’ve told it not to call the base method, you can then tell it to return whatever we choose (there are separate events – see the bottom of this post for a more detailed explanation of why); for example:

[Fact]
public void Test1()
{
    // Arrange
    MyFunkyClass myFunkyClass = Substitute.ForPartsOf<MyFunkyClass>();
    myFunkyClass.When(a => a.MethodOne()).DoNotCallBase();
    myFunkyClass.When(a => a.MethodTwo()).DoNotCallBase();
    myFunkyClass.MethodTwo().Returns(5);
 
    // Act
    int result = myFunkyClass.MethodThree();
 
    // Assert
    Assert.Equal(2, result);
}

And now the test passes.

TLDR – What is this actually doing?

To understand this better; we could do this entire process manually. Only when you’ve felt the pain of a manual mock, can you really see what mocking frameworks such as NSubtitute are doing for us.

Let’s assume that we don’t have a mocking framework at all, but that we still want to test MethodThree() above. One approach that we could take is to subclass MyFunkyClass, and then test that subclass:

Here’s what that might look like:

class MyFunkyClassTest : MyFunkyClass
{
    public override void MethodOne()
    {
        //base.MethodOne();
    }
 
    public override int MethodTwo()
    {
        //return base.MethodTwo();
        return 5;
    }
}

As you can see, now that we’ve subclassed MyFunkyClass, we can override the behaviour of the relevant virtual methods.

In the case of MethodOne, we’ve effectively issued a DoNotCallBase(), (by not calling base!).

For MethodTwo, we’ve issued a DoNotCallBase, and then a Returns statement.

Let’s add a new test to use this new, manual method:

[Fact]
public void Test2()
{
    // Arrange 
    MyFunkyClassTest myFunkyClassTest = new MyFunkyClassTest();
 
    // Act
    int result = myFunkyClassTest.MethodThree();
 
    // Assert
    Assert.Equal(2, result);
}

That’s much cleaner – why not always use manual mocks?

It is much cleaner if you always want MethodThree to return 5. Once you need it to return 2 then you have two choices, either you create a new mock class, or you start putting logic into your mock. The latter, if done wrongly can end up with code that is unreadable and difficult to maintain; and if done correctly will end up in a mini version of NSubstitute.

Finally, however well you write the mocks, as soon as you have more than one for a single class then every change to the class (for example, changing a method’s parameters or return type) results in a change to more than one test class.

It’s also worth mentioning again that this problem is one that has already been solved, cleanly, by dependency injection.

Short Walks – XUnit Tests Not Appearing in Test Explorer

On occasion, there may be a case where you go into Test Explorer, knowing that you have XUnit tests within the solution; the Xunit tests are in a public class, they are public, and they are decorated correctly (for example, [Fact]). However, they do not appear in the Text Explorer.

If you have MS Test tests, you may find that they do appear in the Test Explorer – only the XUnit tests do not.

Why?

To run Xunit tests from the command line, you’ll need this package.

To run Xunit tests from Visual Studio, you’ll need this package.

References

https://xunit.github.io/docs/nuget-packages.html