Mr Andersson

Pimping PsUnit with constraint based assertions

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Using PsUnit one can set up scripted “unit tests” for PowerShell scripts. I have sort of just started using PsUnit at work and right now I feel that a lot of the basic features of other test frameworks are missing.

PsUnit has one function for doing constraint based assertions. This may be familar to those who are using NUnit. Basically the syntax for this is:
$result = DoSomething
Assert-That $result { $ActualValue -eq “expected value” }

Using NUnit and this constraint based model, a simple assertion can look like this:
Assert.That( myString, Is.EqualTo(“Hello”) );

I quite don’t like the “barebone” scriptblock you need to write to use Assert-That in PsUnit, so here is what I came up with (using my example of above):
$result = DoSomething
Assert-That $result (IsEqualTo “expected value”)

I also added another constraint which allows you to assert that some code throws something:
# The following assertion will always pass
Assert-That { throw “blah!” } (ThrowsException)
This complements the already built-in feature of PsUnit which allows you to add a parameter, $ExpectedException, to the test. If you have been using MSTest you would use the method attribute ExpectedException for this.

Here is another example which tweaks the constraint to a somewhat more specific type of exception:
# The following assertion will fail since throw in PowerShell creates a System.Management.Automation.ErrorRecord
Assert-That { throw “blah!” } (ThrowsException -Exception $([System.ArgumentException]))

A more usable test using the ThrowsException constraint could be:
Assert-That {
# The following web resource does not exist, thus an error should be the result
Get-Http “;
} (ThrowsException -Exception $([System.Net.WebException]))

The biggest improvement using these new constraints is the output in the test result.
While using PsUnit out-of-the-box…
$foo = “bar”
Assert-That $foo { $ActualValue -eq “foo” }
…results with “Assert-That returned false!” in the test result.
Using the constraint in this blog post…:
$foo = “bar”
Assert-That $foo (IsEqualTo “foo”)
…results with “ActualValue ‘bar’ is not equal to expected value ‘foo’“.

The same level of detail is provided by the ThrowsException constraint, which may result in “No exception was thrown, expected exception of type ‘System.ArgumentException’“.

Any ideas, suggestions etc.? Please don’t hesitate to drop a comment!


Written by anderssonjohan

November 11, 2009 at 20:41

Posted in scripting

Tagged with ,

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