use Assert to verify that an exception has been thrown?

  • In C#
  • Sunday, August 07, 2016
  • 1178 Views

use Assert to verify that an exception has been thrown?

[TestMethod]
[ExpectedException(typeof(ArgumentException),
    "A userId of null was inappropriately allowed.")]
public void NullUserIdInConstructor()
{
   LogonInfo logonInfo = new LogonInfo(null, "P@ss0word");
}

Usually your testing framework will have an answer for this. But if it's not flexible enough, you can always do this:

try {
    somethingThatShouldThrowAnException();
    Assert.Fail(); // If it gets to this line, no exception was thrown
} catch (GoodException) { }

As @Jonas points out, this DOES NOT work for catching a base Exception:

try {
    somethingThatShouldThrowAnException();
    Assert.Fail(); // raises AssertionException
} catch (Exception) {
    // Catches the assertion exception, and the test passes
}

If you absolutely must catch Exception, you need to rethrow the Assert.Fail(). But really, this is a sign you shouldn't be hand-writing this; check your test framework for options, or see if you can throw a more meaningful exception to test for.

catch (AssertionException) { throw; }

You should be able to adapt this approach to whatever you like -- including specifying what kinds of exceptions to catch. If you only expect certain types, finish the catch blocks off with:

} catch (GoodException) {
} catch (Exception) {
    // not the right kind of exception
    Assert.Fail();
}

I have a couple of different patterns that I use. I use the ExpectedException attribute most of the time when an exception is expected. This suffices for most cases, however, there are some cases when this is not sufficient. The exception may not be catchable - since it's thrown by a method that is invoked by reflection - or perhaps I just want to check that other conditions hold, say a transaction is rolled back or some value has still been set. In these cases I wrap it in a try/catch block that expects the exact exception, does an Assert.Fail if the code succeeds and also catches generic exceptions to make sure that a different exception is not thrown.

First case:

[TestMethod]
[ExpectedException(typeof(ArgumentNullException))]
public void MethodTest()
{
     var obj = new ClassRequiringNonNullParameter( null );
}

Second case:

[TestMethod]
public void MethodTest()
{
    try
    {
        var obj = new ClassRequiringNonNullParameter( null );
        Assert.Fail("An exception should have been thrown");
    }
    catch (ArgumentNullException ae)
    {
        Assert.AreEqual( "Parameter cannot be null or empty.", ae.Message );
    }
    catch (Exception e)
    {
        Assert.Fail(
             string.Format( "Unexpected exception of type {0} caught: {1}",
                            e.GetType(), e.Message )
        );
    }
}