Unit testing code contracts

Question

I'm just having a play with the code contracts in .Net 4.0 and must be missing something obvious as they are not behaving as I would expect.

I have always used a simple if... then.. throw statement to perform any validation at the start of a function.

if (hours < 0 || hours > 8)
    throw new ArgumentOutOfRangeException("hours", "Hours must be between 0 and 8");

I have simply replaced this with

Contract.Requires<ArgumentOutOfRangeException>(hours >= 0 && hours <= 8, "Hours must be between 0 and 8");

but it never seems to throw an issue on my unit tests.

    public static DurationUnit HoursAsDuration(int hours)
    {
        Contract.Requires<ArgumentOutOfRangeException>(hours >= 0 && hours <= 8, "Hours must be between 0 and 8");

        switch (hours)
        {
            case 1:
            case 2:
                return DurationUnit.Quarter;
            case 3:
            case 4:
                return DurationUnit.Half;
            case 5:
            case 6:
                return DurationUnit.ThreeQuarter;
            case 7:
            case 8:
                return DurationUnit.Full;
            default:
                return DurationUnit.None;
        }
    }

    [Test]
    public void CanConvertToDuration()
    {
        Assert.AreEqual(DurationUnit.None, DateTimeUtility.HoursAsDuration(0));
        Assert.AreEqual(DurationUnit.Quarter, DateTimeUtility.HoursAsDuration(1));
        Assert.AreEqual(DurationUnit.Quarter, DateTimeUtility.HoursAsDuration(2));
        Assert.AreEqual(DurationUnit.Half, DateTimeUtility.HoursAsDuration(3));
        Assert.AreEqual(DurationUnit.Half, DateTimeUtility.HoursAsDuration(4));
        Assert.AreEqual(DurationUnit.ThreeQuarter, DateTimeUtility.HoursAsDuration(5));
        Assert.AreEqual(DurationUnit.ThreeQuarter, DateTimeUtility.HoursAsDuration(6));
        Assert.AreEqual(DurationUnit.Full, DateTimeUtility.HoursAsDuration(7));
        Assert.AreEqual(DurationUnit.Full, DateTimeUtility.HoursAsDuration(8));

        //Would expect this to cause an issue
        Assert.AreEqual(DurationUnit.None, DateTimeUtility.HoursAsDuration(9));
    }

The test returns true but I would have expected the code contract to stop the value of "9" getting into the switch statement. Is this the expected behaviour?

score 1 · Accepted Answer · answered Mar 02 '11 at 11:40

If your function's specification, expressed in English, is that it accepts any value for hours and throws an exception when hours is not in the range 0..8, then its contract (expressed in Code Contracts language) is not that it requires hours to be between 0 and 8. The proper translation is that the function doesn't require anything, and it ensures that if hours was in the wrong range, an exception has been raised, and it ensures that if hours was in the correct range, the proper computation was done.

I would expect there is a way to express these things in Code Contracts, but I am not familiar with this contract language, only with another one. The philosophy is the same though: if you want the check to be part of the production build, then the condition of the check is not a pre-condition. On the other hand, the contract may (should) express that the check is made and that each case is handled appropriately.

Thanks Pascal that does clear things up a bit. I was of the belief that code contracts could replace all the parameter checks but they will still need to remain if I want them in the production build. — fluent, Mar 03 '11 at 09:08
Update : Porges below has pointed me in the correct direction with editing the project settings to enable the contracts in the production build. — fluent, Mar 03 '11 at 09:18

fluent · Answer 2 · 2011-03-02T11:28:45.087

0

Ok, I now have the code throwing the expected error by enabling the "Perform Runtime Contract Checking" option within the project properties in visual studio.

Does this mean that when released to production that code contracts are effectively ignored?

I'm obviosuly misunderstanding how code contracts are supposed to be used so if someone could point me in the direction of a best practice article i'd be grateful.

The msdn page states :

"Most methods in the contract class are conditionally compiled; that is, the compiler emits calls to these methods only when you define a special symbol, CONTRACTS FULL, by using the #define directive. CONTRACTS FULL lets you write contracts in your code without using #ifdef directives; you can produce different builds, some with contracts, and some without."

Does this mean that I would still be better of using if.. then.. throw.. for anything public facing? The parameter validation checks really help cut down on data corruption as it flags the point of failure as early as possible.

edited Mar 02 '11 at 11:28

answered Mar 02 '11 at 11:16

fluent

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I would say "yes" for all the questions in your answer, and I would expand a little bit, but StackOverflow comments are a bit limiting, so I wrote another answer instead. – Pascal Cuoq Mar 02 '11 at 11:42
1

rggardner: There are many options in the project properties :) You can enable the runtime enforcement for a Release build or disable it as you wish. It depends on what you want to do with the Contracts. You can also only enable it on public-facing methods, etc. – porges Mar 03 '11 at 00:17
Thanks Porges, very useful to know. All I need to do now is work the correct settings into my build files. – fluent Mar 03 '11 at 09:17

score -1 · Answer 3 · answered Mar 02 '11 at 10:23

-1

I think you need to swap the tests on your hours parameter around.

You are supposed to define the rule that would pass.

so i think this would work for you...

Contract.Requires<ArgumentOutOfRangeException>(hours < 0 && hours > 8, "Hours must be between 0 and 8");

answered Mar 02 '11 at 10:23

Adam Stewart

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Thanks, you spotted an error in my post which was a little misleading. The original if.. then.. throw was wrong which I have now corrected. However the problem still exists. – fluent Mar 02 '11 at 10:27
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-1: This does not make any sense. You essentially state that you _require_ `hours` to be less than zero and greater than 8 at the same time. – Daniel A.A. Pelsmaeker Mar 22 '12 at 12:54

Unit testing code contracts

3 Answers3