BDD-Style Testing with MSTest

A way to make BDD-style “specs” using the MSTest framework in Visual Studio.


BDD tests are a nice, human-understandable way to write tests.
The idea is to write the test code in a way that it can be read and understood
like a functional spec. The benefit of this approach is that you end up with
a set of tests that are self-descriptive and act as a living spec that
can both validate and describe the code that they test.

There are many BDD test frameworks available, but the ones I looked at
lacked good Visual Studio integration. One option would be to write
adapters that allow other frameworks – like nspec
to run with the MSTest runner. The biggest problem with that is that
without significant work, all you end up with is a single pass/fail
for the hosted framework result. Also, debugging through multiple test
harnesses is a pain. So I ended up writing BDD-style in MSTest directly.


There are a few core concepts to grok first.

  • Tests should describe expected behavior not implementation details.
    You don’t want the test to fail because some expected API wasn’t called.
    It’s more important that the expected outcome be tested.
  • Tests should only test one behavior at a time. When a test fails, it should
    obvious to the developer what aspect of the behavior is broken. Mondo tests
    with tons of setup and assertions make debugging failures a problem.
  • Tests (and their context) must accurately describe a behavior. If you
    cannot describe the behavior you’re trying to test in a few words, you’re
    trying to test too many things at once. It’s better to break the test
    into multiple tests or add a new context.
  • Test should be written first. With many BDD frameworks it’s simple to write
    a set of basic tests that assert the behavior the code should have. Once in
    place, it’s very useful to have the tests there as a feedback mechanism that
    the code is behaving as expected.

Check out for more concepts and
best practices. It’s targeted toward Ruby, but the concepts are the same.

Specs, Contexts, and Tests

The basic layout of a test looks something like this:

  • describe_MyClass (a spec)
    • when_some_condition (a context)
      • it_should_behave_like_this (an assertion)

A “spec” describes some piece of code. Typically this will be a container
object in the test framework – a class in C#. The spec contains a set of
“contexts”. Contexts describe some state surrounding a test. This can be
both literal state (of fields, etc.) or it can be a description of an
action that is being taken.

For example “when running” would be an example of a state context and
“when concatenating with a string” would be an example of an action.

Contexts will then have “tests/assertions”. These are the behaviors
that the code is expected to have given the context they are in. Usually
a context will have multiple assertions which both test and describe the
behavior of the code for a given context.

For example in the context of “when running”: “IsRunning should return true”
and “calling start should throw InvalidOperation” would be tests that
explain and test the expected behavior.

BDD in MSTest

With a basic understanding of BDD concepts, lets look at how we can write
BDD-style tests using the MSTest framework built into Visual Studio.

To get started, you’ll need to add a new UnitTest project to your solution.


There are a few ways to organize your test code so that it’s easy to
determine what product code it’s meant to test.

One way is to make a parallel namespace structure:

  • ProductName.TheNamespace
  • ProductName.TheNamespace.Utils
  • ProductName.Test.TheNamespace
  • ProductName.Test.TheNamespace.Utils

The problem with this approach is that you end up with namespace name
collisions which leads to confusing code and usually ends up with
name and type aliasing. It also really confuses Intellisense which makes
authoring the tests a chore.

Another way, and the way I’m recommending, is to have flattened test namespaces.

  • ProductName.TheNamespace
  • ProductName.TheNamespace.Utils
  • ProductName.Test.TestTheNamespace
  • ProductName.Test.TestUtils

This makes it clear that you’re testing things in TheNamespace and also
prevents the annoying namespace collisions. One thing to note is that
because the namespaces are flattened, you should avoid the having namespaces
with the same name in your product code. This isn’t a requirement, but
it makes things saner and is generally a better practice.


Specs in MSTest are mapped 1:1 with classes. Spec classes should be named
like describe_<ClassName>. The top-level test class should NOT be
marked with the [TestClass] attribute because it itself should not contain
any test code*.

*One exception may be if you want a [TestInitialize] method that
would apply to all of the tests in your spec.

For example let’s write a spec for a class called Base64Utilities. I’ll
build on this example throughout the rest of the doc.

public class describe_Base64Utilities {
  // ... test code ...

The only code that should be in the top-level class is member fields
possibly some helper code that is applicable to all of the tests in the spec.


We need to have a way to define some test contexts. This could be state, actions,
or in this case, a function. To express the context, use an inner class.

For the example, I’m going to describe the static helper method EnsureValidLength.
To add this context I would define an inner class that inherits from the top-level class.
This class will be marked with [TestClass].

Inheritance gives us access to the helpers and state defined in the top-level class.
Also, inner classes show up in the test explorer as TopLevel.InnerClass which makes
the test explorer and test results more readable.

public class describe_Base64Utilities
    public class EnsureValidLength : describe_Base64Utilities
      // ... test code ...

In the inner class, you can also define a method and mark it as [TestInitialize]
and MSTest will ensure that the code will be run before any test methods.


Once your context has been written, you can write some tests. The general naming
convention I prefer is something like “given_blah_it_should_return_foobar” or
“it_should_run_the_initializer”. This style is more readable than the same
names in camel or Pascal case. “ForExampleThisIsTheAlternativeAndItIsNotAsNice”.

The test method itself should do very little work itself and it should only
assert the things that it’s explicitly trying to validate. This is true even
if there are other things in the test that you might want to assert; resist the
temptation and create two tests instead. It will make test failures more useful and
will make your spec more comprehensive.

Here are a few example tests.

public class describe_Base64Utilities
    public class EnsureValidLength : describe_Base64Utilities
          public void given_null_or_empty_it_string_should_return_null()

          public void given_string_less_than_4_chars_it_should_return_null()

          // ... more tests ...

Test Explorer and Test Results

What do you get from writing your tests this way?

First, in the Visual Studio Test Explorer, if you sort by type
the tests are nicely grouped by the inner class names.

  • describe_Base64Utilities.EnsureValidLength
    • given_null_or_empty_it_string_should_return_null
    • given_string_less_than_4_chars_it_should_return_null
    • … other tests
  • describe_Base64Utilities.DecodePartialBase64String
    • given_invalid_base64_it_should_return_null
    • … other tests

When you run the tests the results are more meaningful because it’s
easy to understand what behavior is working and what is not.



  • If you inherit from a class “context” that has TestMethods in it
    the derived class will also run the test methods defined in its parents.

    • To deal with that you should either not have test methods defined in parent scopes or you must hide them in the derived class by redefining them with ‘new’.
  • Nested classes need to have different names for their before methods.
    • If they are hidden with new, the base implementations will not be run.

Testing MSTest

Included in case you want to play around with the MSTest behavior yourself.

public class describe_NestedContexts
    public string theState = String.Empty;

    public void before_all()
        theState += "Before Parent";

    public class when_running_tests_in_inner_classes : describe_NestedContexts
        public void before()
            theState += ", Before Child";    // before names must be unique or they will collide

        public void it_should_run_all_test_initializers()
            Assert.AreEqual("Before Parent, Before Child", theState);

        public class when_going_deeper : when_running_tests_in_inner_classes
            public void before_deeper()   // before names must be unique or they will collide
                theState += ", Before Other Child";

            // have to hide the parent's test methods
            public new void it_should_run_all_test_initializers() { }
            public new void it_should_not_be_affected_by_child_classes() { }

            public void it_should_run_all_test_initializers_again()
                Assert.AreEqual("Before Parent, Before Child, Before Other Child", theState);

        public void it_should_not_be_affected_by_child_classes()
            Assert.AreEqual("Before Parent, Before Child", theState);