Developing Automated Tests Using NUnit and VB.NET


This article describes how to write unit tests in Visual Basic .NET using the market standard automated unit testing tool NUnit 2.0.

What is NUnit?

NUnit is a unit testing framework that has been ported from JUnit. It is a free tool, with source code freely available, and is written in C#.NET. It supports unit tests written in any .NET language. For more information see .

Why write Automated Tests?

Writing automated tests provides many benefits:

  • It’s quicker to develop: You write high quality code quicker. This is because you continuously regression test your code and spend less time manual testing.
  • Refactoring: Refactoring is a term used when you change the code base without changing the functionality of the code. Refactoring is essential to ensure code does not rot, particularly in an iterative type phased development environment. If you have a suite of tests you can refactor as much as you like, provided all the tests still pass. Refactoring without automated tests is suicide.
  • Exhaustiveness: Every time you change the code, it is fully tested. Developers normally do not test all their code. It is boring. They just don’t. With a full suite of tests they can ensure that everything works again and again. They can then get on with writing more code, rather than debugging and fixing previously written code.
  • Repeatability: All tests get run every time. None get missed out.
  • Documented: The test code itself is the documentation.
  • Standard: Because the tests use a standard anyone can run them. We do not loose knowledge when people leave.

Using NUnit 2.0 – An Example

In this example we will do the following:
1. Create a solution in VS.NET for your code project and the test project.
2. Set up the Test class.
3. Write a basic test.
4. Write an exception type test.
5. Run the tests using the NUnit UI.

Preliminaries – The Account Class

Throughout this example I will use our old friend the Account class for the example:

The rules of the Account class are as follows:
1. Overdraft limit must be positive.
2. Amount withdrawn or deposited must be greater than zero.
3. Cannot withdraw over the overdraft limit.

[Note that these rules should normally be written as a set of pre and post conditions together with the class invariants. This is called Design By Contract and is not within the scope of this article. To find out more about DBC go here .]

Preliminaries – Download and install NUnit

NUnit can be downloaded from

Download the MSI and run it. NUnit should be installed into the C:/Program Files directory. If you are working within a team environment, I would suggest everyone installs it in the same directory.

Step 1: Creating an NUnit VB.NET set of tests.

1. Create a blank solution in VS.NET. This will contain the code you are testing and a set of tests.
2. Add the project containing the code which you are going to test.
3. Open VS.NET and create a new VB.NET class library.
4. Name the library {project name to be tested}NUnitTests.
5. Add a project reference to the project you are going to test.

6. Add a reference to the NUnit framework, in the location C:\Program Files\NUnit V2.0\bin
7. Add a new class to your test project called {class to test}Tests.

The solution explorer in your IDE should look something like this:

Step 2: Set up the Test class.

At the top of the class you need references to the assembly you are testing, and also to the NUnit framework. Thus, you need the following Imports statements:

Imports System
Imports Accounting
Imports NUnit.Framework

[I have an Imports System statement here as I prefer to remove all the automatic ‘imports’ that VB does for you in the project properties. I’d rather have everything visible at the top of my classes than have to search around. This is also more compatible with C# syntax. It’s a personal choice.]

You class now needs an attribute of TestFixture. This tells the NUnit framework that this class is a set of tests that can be run by NUnit. NUnit uses Attributes to pick out what it needs:

<TestFixture()> _
Public Class AccountTests

End Class

Before each test is run there will be a standard set of things you might want to set up and then tear down after each test. In my Account example, I want an account set up with an overdraft limit set. NUnit caters for this using the SetUp and TearDown attributes. E.g.

<TestFixture()> _
Public Class AccountTests  

    Private mAccount As Account

    <SetUp()> _
    Public Sub SetUp() 
        mAccount = New Account(1000)
    End Sub

    <TearDown()> _
    Public Sub TearDown()
        'nothing to go here
    End Sub

End Class

Now it is time to write the tests.

Step 3: Write a basic test.

Each test is written as a Sub in VB.NET. We tell NUnit it is a Test by using the Test attribute, like this:

    <Test()> _
    Public Sub AccountWithdraw()

    End Sub

Now we write a test inside the Sub. For this test, I wish to test that I can withdraw an amount from my account provided it is within the overdraft limit. Thus,

    Public Sub AccountWithdraw()

        Dim BalancePre As Double

        BalancePre = mAccount.Balance


        Assertion.Assert(mAccount.Balance = BalancePre - 200)

    End Sub

I’m using the NUnit Assertion object here to make an assertion about what should be correct after I’ve run some action on my target code. The Assertion object has many useful operations. I’ve leave you to find out what they are.

The next topic to consider is how to write a test where you wish to ensure that your code is raising exceptions correctly.

Step 4: Write an exception type test.

To test exceptions we use the ExpectedException attribute that comes with the NUnit Framework as follows:

<Test(), ExpectedException(GetType(ArgumentOutOfRangeException))> _    Public Sub TestWithdrawNegativeAmount()


End Sub

Step 5: Run the tests using the NUnit UI.

To run the tests, open up the Project Options for the Test project and set the following options in the Configuration Properties\Debugging section:

Start External Program: C:\Program Files\NUnit V2.0\bin\nunit-gui.exe

Command Line Arguments: /assembly: {Full path of assembly} [E.g. /assembly:C:\Data\NUnit\Account\Account.NUnitTests\bin\AccountingNUnitTests.dll]

Now set the Test project as the Start Up Project, by right clicking on the project.

Now build and run the project. You should see the following UI appear:


Click the Run button to run your tests. If they pass then you will see green lights. If they fail you will see a red light. If no assertions have been run you will see an amber light.

When you get red lights you can set breakpoints and step into your code and debug away.

What Should You Write Tests For?

Here’s a common sense set of guidelines:

  • Write tests for things which you think are likely to be changed, or have a chance of failing.
  • If the test takes no time to write (a few minutes) then there is no harm in writing it.
  • If a test takes a day or so to write, then think about whether you really need to write the test.
  • If a test takes a few hours to do manually, and you know you will have to do it more than once, then write a test for it, even if it takes a day to do so.
  • If you get a bug reported, then write a test for it and make sure it fails. Then you carry on coding until it passes and everything else still passes. Give the test a name that refers to the bug. This way you can ensure you get no repeat bugs. Repeat bugs in your code are shooting offense!

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