Static helper classes

IntelliTest provides a set of static helper class that can be used when authoring parameterized unit tests:

  • PexAssume: used to define assumptions on inputs, and is useful for filtering undesirable inputs
  • PexAssert: a simple assertion class for use if your test framework does not provide one
  • PexChoose: a stream of additional test inputs that IntelliTest manages
  • PexObserve: logs concrete values and, optionaly, validates them in the generated code

Some classes allow you to interact with the IntelliTest reasoning engine at a low-level:

  • PexSymbolicValue: utilities to inspect or modify symbolic constraints on variables


A static class used to express assumptions, such as preconditions, in parameterized unit tests. The methods of this class can be used to filter out undesirable test inputs.

If the assumed condition does not hold for some test input, a PexAssumeFailedException is thrown. This will cause the test to be silently ignored.


The following parameterized test will not consider j=0:

public void TestSomething(int i, int j) {
     PexAssume.AreNotEqual(j, 0);
     int k = i/j;


The code above is almost equivalent to:

     if (j==0)

except that a failing PexAssume results in no test cases. In the case of an if statement, IntelliTest generates a separate test case to cover the then branch of the if statement.

PexAssume also contains specialzed nested classes for assumptions on string, arrays, and collections.


A static class used to express assertions, such as postconditions, in parameterized unit tests.

If the asserted condition does not hold for some test input, a PexAssertFailedException is thrown, which causes the test to fail.


The following asserts that the absolute value of an integer is positive:

public void TestSomething(int i) {
     int j = Maths.Abs(i);
     PexAssert.IsTrue(j >= 0);


A static class that supplies auxiliary input values to a test, which can be used to implement Parameterized Mocks.

The PexChoose class does not help in determining whether a test passes or fails for particular input values. PexChoose simply provides input values, which are also referred to as choices. It is still up to the user to restrict the input values, and to write assertions that define when a test passes or fails.

Modes of operation

The PexChoose class can operate in two modes:

  • While IntelliTest is performing a symbolic analysis of the test and the tested code during input generation, the chooser returns arbitrary values and IntelliTest tracks how each value is used in the test and the tested code. IntelliTest will generate relevant values to trigger different execution paths in the test and the tested code.

  • The generated code for particular test cases sets up the choice provider in a specific way, so that the re-execution of such a test case will make specific choices to trigger a particular execution path.


  • Simple call PexChoose.Value to generate a new value:
public int Foo() {
    return PexChoose.Value<int>("foo");


A static class to log named values.

When IntelliTest explores the code, PexObserve is used to record computed values using their formatted string representations. The values are associated with unique names.

PexObserve.Value<string>("result", result);


// product code
public static class MathEx {
     public static int Square(int value) { return value * value; }

// fixture
public partial class MathExTests {
     public int SquareTest(int a) {
        int result = MathEx.Square(a); 
        // storing result
        return result;


A static class used to ignore constraints on parameters, and to print the symbolic information associated with values.


Normally, IntelliTest tries to cover all execution paths of the code during execution. However, especially when computing assumption and assertion conditions, it should not explore all possible cases.


This example shows the implementation of the PexAssume.Arrays.ElementsAreNotNull method. In the method, you ignore the constraints on the lengh of the array value to avoid IntelliTest trying to generate different sizes of array. The constraints are ignored only here. If the tested code behaves differently for different array lengths, IntelliTest cannot generate different sized arrays from the constraints of the tested code.

public static void AreElementsNotNull<T>(T[] value)
    where T : class
    // the followings prevents the exploration of all array lengths
    int len = PexSymbolicValue.Ignore<int>(value.Length);

    // building up a boolean value as follows prevents exploration
    // of all combinations of non-null (instead, there are just two cases)
    bool anyNull = false;
    for (int i = 0; i < len; ++i)
        anyNull |= value[i] == null;

    // was any element null?
    if (anyNull)
        PexAssume.Fail("some element of array is a null reference");

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