Best way to test exceptions with Assert to ensure they will be thrown

Question

Do you think that this is a good way for testing exceptions  Any suggestions   Exception exception   null  try        I m sure that an exeption will happen here   catch  Exception ex       exception   ex     Assert IsNotNull exception     I m using MS Test

User · Answer

Suggest using NUnit s clean delegate syntax   Example for testing ArgumentNullExeption     Test   TestCase null   public void FooCalculation InvalidInput ShouldThrowArgumentNullExeption string text        var foo   new Foo        Assert That      gt  foo Calculate text   Throws ArgumentNullExeption          Or      Assert That      gt  foo Calculate text   Throws Exception TypeOf lt ArgumentNullExeption gt

User · Answer

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

User · Answer

With most  net unit testing frameworks you can put an  ExpectedException  attribute on the test method  However this can t tell you that the exception happened at the point you expected it to  That s where xunit net can help   With xunit you have Assert Throws  so you can do things like this        Fact      public void CantDecrementBasketLineQuantityBelowZero                 var o   new Basket            var p   new Product  Id   1  NetPrice   23 45m           o AddProduct p  1           Assert Throws lt BusinessException gt       gt  o SetProductQuantity p  -3             Fact  is the xunit equivalent of  TestMethod

User · Answer

As an alternative to using ExpectedException attribute  I sometimes define two helpful methods for my test classes   AssertThrowsException   takes a delegate and asserts that it throws the expected exception with the expected message     AssertDoesNotThrowException   takes the same delegate and asserts that it does not throw an exception   This pairing can be very useful when you want to test that an exception is thrown in one case  but not the other   Using them my unit test code might look like this   ExceptionThrower callStartOp   delegate    testObj StartOperation           Check exception is thrown correctly    AssertThrowsException callStartOp  typeof InvalidOperationException    StartOperation   called when not ready      testObj Ready   true      Check exception is now not thrown    AssertDoesNotThrowException callStartOp     Nice and neat huh   My AssertThrowsException   and AssertDoesNotThrowException   methods are defined on a common base class as follows   protected delegate void ExceptionThrower          lt summary gt      Asserts that calling a method results in an exception of the stated type with the stated message       lt  summary gt       lt param name  exceptionThrowingFunc  gt Delegate that calls the method to be tested  lt  param gt       lt param name  expectedExceptionType  gt The expected type of the exception  e g  typeof FormatException   lt  param gt       lt param name  expectedExceptionMessage  gt The expected exception message  or fragment of the whole message  lt  param gt  protected void AssertThrowsException ExceptionThrower exceptionThrowingFunc  Type expectedExceptionType  string expectedExceptionMessage        try               exceptionThrowingFunc            Assert Fail  Call did not raise any exception  but one was expected               catch  NUnit Framework AssertionException                   Ignore and rethrow NUnit exception         throw            catch  Exception ex                Assert IsInstanceOfType expectedExceptionType  ex   Exception raised was not the expected type             Assert IsTrue ex Message Contains expectedExceptionMessage    Exception raised did not contain expected message  Expected       expectedExceptionMessage        got       ex Message                        lt summary gt      Asserts that calling a method does not throw an exception       lt  summary gt       lt remarks gt      This is typically only used in conjunction with  lt see cref  AssertThrowsException   gt    e g  once you have tested that an ExceptionThrower     method throws an exception then your test may fix the cause of the exception and then call this to make sure it is now fixed        lt  remarks gt       lt param name  exceptionThrowingFunc  gt Delegate that calls the method to be tested  lt  param gt  protected void AssertDoesNotThrowException ExceptionThrower exceptionThrowingFunc        try               exceptionThrowingFunc              catch  NUnit Framework AssertionException                   Ignore and rethrow any NUnit exception         throw            catch  Exception ex                Assert Fail  Call raised an unexpected exception      ex Message

User · Answer

I m new here and don t have the reputation to comment or downvote  but wanted to point out a flaw in the example in Andy White s reply   try       SomethingThatCausesAnException        Assert Fail  Should have exceptioned above       catch  Exception ex           whatever logging code     In all unit testing frameworks I am familiar with  Assert Fail works by throwing an exception  so the generic catch will actually mask the failure of the test   If SomethingThatCausesAnException   does not throw  the Assert Fail will  but that will never bubble out to the test runner to indicate failure   If you need to catch the expected exception  i e   to assert certain details  like the message   properties on the exception   it s important to catch the specific expected type  and not the base Exception class   That would allow the Assert Fail exception to bubble out  assuming you aren t throwing the same type of exception that your unit testing framework does   but still allow validation on the exception that was thrown by your SomethingThatCausesAnException   method

User · Answer

Now  2017  you can do it easier with the new MSTest V2 Framework   Assert ThrowsException lt Exception gt       gt  myClass MyMethodWithError        async version await Assert ThrowsExceptionAsync lt SomeException gt          gt  myObject SomeMethodAsync

User · Answer

As of v 2 5  NUnit has the following method-level Asserts for testing exceptions   Assert Throws  which will test for an exact exception type   Assert Throws lt NullReferenceException gt       gt  someNullObject ToString       And Assert Catch  which will test for an exception of a given type  or an exception type derived from this type   Assert Catch lt Exception gt       gt  someNullObject ToString       As an aside  when debugging unit tests which throw exceptions  you may want to prevent VS from breaking on the exception   Edit  Just to give an example of Matthew s comment below  the return of the generic  Assert Throws and Assert Catch is the exception with the type of the exception  which you can then examine for further inspection      The type of ex is that of the generic type parameter  SqlException  var ex   Assert Throws lt SqlException gt       gt  MethodWhichDeadlocks     Assert AreEqual 1205  ex Number

User · Answer

Unfortunately MSTest STILL only really has the ExpectedException attribute  just shows how much MS cares about MSTest  which IMO is pretty awful because it breaks the Arrange Act Assert pattern and it doesnt allow you to specify exactly which line of code you expect the exception to occur on     When I m using   forced by a client  to use MSTest I always use this helper class   public static class AssertException       public static void Throws lt TException gt  Action action  where TException   Exception               try                       action                      catch  Exception ex                        Assert IsTrue ex GetType      typeof TException    Expected exception of type     typeof TException      but type of     ex GetType       was thrown instead                 return                    Assert Fail  Expected exception of type     typeof TException      but no exception was thrown                public static void Throws lt TException gt  Action action  string expectedMessage  where TException   Exception               try                       action                      catch  Exception ex                        Assert IsTrue ex GetType      typeof TException    Expected exception of type     typeof TException      but type of     ex GetType       was thrown instead                 Assert AreEqual expectedMessage  ex Message   Expected exception with a message of      expectedMessage      but exception with message of      ex Message      was thrown instead                 return                    Assert Fail  Expected exception of type     typeof TException      but no exception was thrown               Example of usage   AssertException Throws lt ArgumentNullException gt       gt  classUnderTest GetCustomer null

User · Answer

Mark the test with the ExpectedExceptionAttribute  this is the term in NUnit or MSTest  users of other unit testing frameworks may need to translate

[c#] Best way to test exceptions with Assert to ensure they will be thrown

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