Test generation

Last Update: 4/5/2017

Visual Studio 2017 | Visual Studio 2015

In traditional unit testing, it requires several ingredients to put together a test:

void MyTest() {
    // data
    ArrayList a = new ArrayList();

    // method sequence

    // assertions
    Assert.AreEqual(a[0], 5);

The test consists of different aspects:

IntelliTest can often automatically determine relevant argument values for more general Parameterized Unit Tests, which provide the sequence of method calls and assertions.

Test generators

IntelliTest generates test cases by selecting a sequence of methods of the implementation under test to execute, and then generating inputs for the methods while checking assertions over the derived data.

A parameterized unit test directly states a sequence of method calls in its body.

When IntelliTest needs to construct objects, calls to constructors and factory methods will be added automatically to the sequence as required.

Parameterized unit testing

Parameterized Unit Tests (PUTs) are tests that take parameters. Unlike traditional unit tests, which are usually closed methods, PUTs take any set of parameters. Is it that simple? Yes - from there, IntelliTest will try to generate the (minimal) set of inputs that fully cover the code reachable from the test.

PUTs are defined using the PexMethod custom attribute in a similar fashion to MSTest (or NUnit, xUnit). PUTs are instance methods logically grouped in classes tagged with PexClass. The following example shows a simple PUT stored in the MyPexTest class:

void ReplaceFirstChar(string target, char c) {

    string result = StringHelper.ReplaceFirstChar(target, c);

    Assert.AreEqual(result[0], c);

where ReplaceFirstChar is a method that replaces the first character of a string:

class StringHelper {
    static string ReplaceFirstChar(string target, char c) {
        if (target == null) throw new ArgumentNullException();
        if (target.Length == 0) throw new ArgumentOutOfRangeException();
        return c + target.Substring(1);

From this test, IntelliTest can automatically generate inputs for a PUT that covers many execution paths of the tested code. Each input that covers a different execution path gets "serialized" as a unit test:

[TestMethod, ExpectedException(typeof(ArgumentNullException))]
void ReplaceFirstChar0() {
    this.ReplaceFirstChar(null, 0);
void ReplaceFirstChar10() {
    this.ReplaceFirstChar("a", 'c');

Generic parameterized unit testing

Parameterized unit tests can be generic methods. In this case, the user must specify the types used to instantiate the method by using PexGenericArguments.

public partial class ListTest {
    public void AddItem<T>(List<T> list, T value)
    { ... }

Allowing exceptions

IntelliTest provides numerous validation attributes to help triage exceptions into expected exceptions and unexpected exceptions.

Expected exceptions generate negative test cases with the appropriate annotation such as ExpectedException(typeof(xxx)), while unexpected exceptions generate failing test cases.

[PexMethod, PexAllowedException(typeof(ArgumentNullException))]
void SomeTest() {...}

The validators are:

Testing internal types

IntelliTest can "test" internal types, as long as it can see them. For IntelliTest to see the types, the following attribute is added to your product or test project by the Visual Studio IntelliTest wizards:

[assembly: InternalsVisibleTo("Microsoft.Pex, PublicKey=002400000480000094000000060200000024000052534131000400000100010007d1fa57c4aed9f0a32e84aa0faefd0de9e8fd6aec8f87fb03766c834c99921eb23be79ad9d5dcc1dd9ad236132102900b723cf980957fc4e177108fc607774f29e8320e92ea05ece4e821c0a5efe8f1645c4c0c93c1ab99285d622caa652c1dfad63d745d6f2de5f17e5eaf0fc4963d261c8a12436518206dc093344d5ad293

Assumptions and assertions

Users can use assumptions and assertions to express preconditions (assumptions) and postconditions (assertions) about their tests. When IntelliTest generates a set of parameter values and "explores" the code, it might violate an assumption of the test. When that happens, it will not generate a test for that path but will silently ignore it.

Assertions are a well known concept in regular unit test frameworks, so IntelliTest already "understands" the built-in Assert classes provided by each supported test framework. However, most frameworks do not provide an Assume class. In that case, IntelliTest provides the PexAssume class. If you do not want to use an existing test framework, IntelliTest also has the PexAssert class.

public void Test1(object o) {
    // precondition: o should not be null


In particular, the non-nullness assumption can be encoded as a custom attribute:

public void Test2([PexAssumeNotNull] object o)
// precondition: o should not be null


A precondition of a method expresses the conditions under which the method will succeed.

Usually, the precondition is enforced by checking the parameters and the object state, and throwing an ArgumentException or InvalidOperationException if it is violated.

In IntelliTest, a precondition of a parameterized unit test is expressed with PexAssume.


A postcondition of a method expresses the conditions which should hold during and after execution of the method, assuming that its preconditions were initially valid.

Usually, the postcondition is enforced by calls to Assert methods.

With IntelliTest, a postcondition of a parameterized unit test is expressed with PexAssert.

Test failures

When does a generated test case fail?

  1. If it does not terminate within the configured path bounds, it is considered as a failure unless the TestExcludePathBoundsExceeded option is set

  2. If the test throws a PexAssumeFailedException, it succeeds. However, it is usually filtered out unless TestEmissionFilter is set to All

  3. If the test violates an assertion; for example, by throwing an assertion violation exception of a unit testing framework, it fails

If none of the above produce a decision, a test succeeds if and only if it does not throw an exception. Assertion violations are treated in the same way as exceptions.

Setup and tear down

As part of the integration with test frameworks, IntelliTest supports detecting and running setup and tear down methods.


using Microsoft.Pex.Framework;
using NUnit.Framework;

namespace MyTests
    public partial class MyTestClass
        public void Init()
            // monitored

        public void MyTest(int i)

        public void Dispose()
            // monitored

Further reading

Got feedback?

Post your ideas and feature requests on UserVoice.

Tell us what you think with Send-a-Smile Send-a-Smile on the title bar.

Go back Back to the index