Test State and Phases

When properties are executed, an object representing the state of the test is supplied to them. This object contains all the necessary information required to evaluate the property. The state object is mutable; properties modify it as they are run.

namespace LinqCheck
    using System;
    using System.Collections.Generic;

Test Phases

graph TD GEN[Generate test case] PASS{Conditions satisfied?} SHRINK[Produce shrunk test case] FAIL{Property failed?} DONE((Done)) GEN --> PASS PASS -->|yes|GEN PASS -->|no|SHRINK PASS -->|all cases passed|DONE SHRINK --> FAIL FAIL --> |no|SHRINK FAIL --> |yes|DONE

Properties can be evaluated in three different contexts. These correspond to the phases of the test execution. The whole process is outlined on the figure on the right.

At first when a property is executed, it produces new test cases and evaluates that all specified conditions hold. If they do, a new case is generated until a predefined number of cases has passed.

If, however, one of the cases fails, then we jump to the shrinking step. That step first produces a (lazy) list of shrunk versions of each input parameter. Then it produces one combination of shrunk input parameters at a time, and checks if that combination makes the property fail. If it does, then that test case is reported to the user, and we are done. On the other hand, if the property does not fail, we go back and produce a new shrunk alternative.

Eventually, if all the shrunk test cases pass, we use the case that caused the property to fail in the first place. It is more probable, however, that some of the shrunk alternative has already made the property to fail before this happens.

We define three test phases that control the property evaluation:

  • Generate - Property generates new random test values and evaluates them normally.
  • StartShrink - Property produces an enumerable of shrunk alternatives for each variable generated in the failing case.
  • Shrink - Property takes its input from the enumerable of shrunk variables, and checks if it fails with those.

The enumeration of test phases is defined below.

	public enum TestPhase { Generate, StartShrink, Shrink }

Test State

The phase is naturally the first piece of data we define for the state object.

    public class TestState
        public readonly TestPhase Phase;

Input for Random Generation

In the first phase we need the input for the random value generators. The following three fields contain the random number generator, its seed, and the current size used with generators.

        public readonly Random Random;
		public int Seed;
        public int Size;

Property Label

The property label is used when outputting information to console. The label is stored along with the state object.

        public string Label;

Test Counters

The two fields below contain counters how many tests have passed and have been discarded so far.

        public int SuccessfulTests;
        public int DiscardedTests;

Test Case Classification

When the orderby clause is used in the property, the test cases are classified to groups based on the specified attribute. The groups are stored in the Classes dictionary defined below.

        public SortedDictionary<string, int> Classes = 
			new SortedDictionary<string,int> ();

Generated Values

The generated variables are stored in the Values list. Note that all the values are essentially untyped; they are stored as objects. Since all properties have an associated type, we can cast the values to appropriate type when properties need them.

		public readonly List<object> Values;

Another piece of information needed is the index of the item in the Values list we are generating/using currently. It is stored int the field below.

		public int CurrentValue;

Shrunk Test Data

The shrunk test data is kept in the ShrunkValues list. It contains all the shrunk values of generated variables. Note that they are stored in enumerables, so that the actual values are generated lazily as the enumerables are traversed.

		public readonly List<IEnumerable<object>> ShrunkValues;


We provide two simple constructors. They initialize all the fields according to specified values or to their default values.

		public TestState (TestPhase phase, int seed, int size, string label) :
            this (phase, seed, size, label, null, null)

        public TestState(TestPhase phase, int seed, int size, string label, 
			List<object> values, List<IEnumerable<object>> shrunkValues)
            Phase = phase;
            Random = new Random(seed);
			Seed = seed;
            Size = size;
			Label = label;
            Values = values ?? new List<object>();
            ShrunkValues = shrunkValues;

Resetting State

We need to reset the values between each property run in the generation phase. The following method clears the Values list and initializes the index back to zero.

        public void ResetValues ()
            if (Phase == TestPhase.Generate)
                CurrentValue = 0;

Few Remarks about the State

One must be quite careful when updating the state in the properties. The state is mutable and can cause problems if it is not behaving deterministically. Most of the fields in the object do not change after the state is initialized, but the random number generator, and generated values obviously do. So, we need to manage their side-effects carefully.