How do I use Assert to verify that an exception has been thrown

Question

How do I use Assert  or other Test class   to verify that an exception has been thrown

User · Answer

You can achieve this with a simple one-line.

If your operation foo.bar() is async:

await Assert.ThrowsExceptionAsync<Exception>(() => foo.bar());

If foo.bar() is not async

Assert.ThrowsException<Exception>(() => foo.bar());

User · Answer

It is an attribute on the test method    you don t use Assert  Looks like this    ExpectedException typeof ExceptionType    public void YourMethod should throw exception

User · Answer

Be wary of using ExpectedException  as it can lead to several pitfalls as demonstrated here  http   geekswithblogs net sdorman archive 2009 01 17 unit-testing-and-expected-exceptions aspx And here  http   xunit github io docs comparisons html If you need to test for exceptions  there are less frowned upon ways  You can use the try act fail catch assert  method  which can be useful for frameworks that don t have direct support for exception tests other than ExpectedException  A better alternative is to use xUnit NET  which is a very modern  forward looking  and extensible unit testing framework that has learned from all the others mistakes  and improved  One such improvement is Assert Throws  which provides a much better syntax for asserting exceptions  You can find xUnit NET at github  http   xunit github io

User · Answer

In a project i  m working on we have another solution doing this   First I don  t like the ExpectedExceptionAttribute becuase it does take in consideration which method call that caused the Exception   I do this with a helpermethod instead   Test   TestMethod  public void AccountRepository ThrowsExceptionIfFileisCorrupt          var file   File Create  Accounts bin         file WriteByte 1        file Close          IAccountRepository repo   new FileAccountRepository         TestHelpers AssertThrows lt SerializationException gt      gt repo GetAll                     HelperMethod  public static TException AssertThrows lt TException gt  Action action  where TException   Exception               try                       action                      catch  TException ex                        return ex                    Assert Fail  Expected exception was not thrown             return null          Neat  isn  t it

User · Answer

If you re using MSTest  which originally didn t have an ExpectedException attribute  you could do this   try        SomeExceptionThrowingMethod       Assert Fail  no exception thrown      catch  Exception ex        Assert IsTrue ex is SpecificExceptionType

User · Answer

Since you mention using other test classes  a better option than the ExpectedException attribute is to use Shoudly s Should Throw   Should Throw lt DivideByZeroException gt       gt    MyDivideMethod 1  0         Let s say we have a requirement that the customer must have an address to create an order  If not  the CreateOrderForCustomer method should result in an ArgumentException  Then we could write    TestMethod  public void NullUserIdInConstructor       var customer   new Customer name     Justin   address    null      Should Throw lt ArgumentException gt       gt        var order   CreateOrderForCustomer customer          This is better than using an ExpectedException attribute because we are being specific about what should throw the error  This makes requirements in our tests clearer and also makes diagnosis easier when the test fails   Note there is also a Should ThrowAsync for asynchronous method testing

User · Answer

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

User · Answer

I do not recommend using the ExpectedException attribute  since it s too constraining and error-prone  or to write a try catch block in each test  since it s too complicated and error-prone    Use a well-designed assert method -- either provided by your test framework or write your own   Here s what I wrote and use     public static class ExceptionAssert       private static T GetException lt T gt  Action action  string message     where T   Exception               try                       action                      catch  T exception                        return exception                    throw new AssertFailedException  Expected exception     typeof T  FullName      but none was propagated       message              public static void Propagates lt T gt  Action action  where T   Exception               Propagates lt T gt  action                  public static void Propagates lt T gt  Action action  string message  where T   Exception               GetException lt T gt  action  message              public static void Propagates lt T gt  Action action  Action lt T gt  validation  where T   Exception               Propagates action  validation                  public static void Propagates lt T gt  Action action  Action lt T gt  validation  string message  where T   Exception               validation GetException lt T gt  action  message              Example uses        TestMethod      public void Run PropagatesWin32Exception ForInvalidExeFile                  test setup that might propagate Win32Exception          ExceptionAssert Propagates lt Win32Exception gt                    gt  CommandExecutionUtil Run Assembly GetExecutingAssembly   Location  new string 0              more asserts or something              TestMethod      public void Run PropagatesFileNotFoundException ForExecutableNotFound                  test setup that might propagate FileNotFoundException          ExceptionAssert Propagates lt FileNotFoundException gt                    gt  CommandExecutionUtil Run  NotThere exe   new string 0                e   gt  StringAssert Contains e Message   NotThere exe              more asserts or something          NOTES  Returning the exception instead of supporting a validation callback is a reasonable idea except that doing so makes the calling syntax of this assert very different than other asserts I use   Unlike others  I use  propagates  instead of  throws  since we can only test whether an exception propagates from a call  We can t test directly that an exception is thrown   But I suppose you could image throws to mean  thrown and not caught   FINAL THOUGHT  Before switching to this sort of approach I considered using the ExpectedException attribute when a test only verified the exception type and using a try catch block if more validation was required   But  not only would I have to think about which technique to use for each test  but changing the code from one technique to the other as needs changed was not trivial effort   Using one consistent approach saves mental effort     So in summary  this approach sports  ease-of-use  flexibility and robustness  hard to do it wrong

User · Answer

As an alternative you can try testing exceptions are in fact being thrown with the next 2 lines in your test   var testDelegate        gt  MyService Method params   Assert Throws lt Exception gt  testDelegate

User · Answer

In case of using NUnit  try this   Assert That      gt                        Your Method To Test               Throws TypeOf lt Your Specific Exception gt    With Message EqualTo  Your Specific Message

User · Answer

Well i ll pretty much sum up what everyone else here said before   Anyways  here s the code i built according to the good answers    All is left to do is copy and use          lt summary gt      Checks to make sure that the input delegate throws a exception of type TException       lt  summary gt       lt typeparam name  TException  gt The type of exception expected  lt  typeparam gt       lt param name  methodToExecute  gt The method to execute to generate the exception  lt  param gt  public static void AssertRaises lt TException gt  Action methodToExecute  where TException   System Exception       try               methodToExecute              catch  TException            return              catch  System Exception ex                Assert Fail  Expected exception of type     typeof TException      but type of     ex GetType       was thrown instead               Assert Fail  Expected exception of type     typeof TException      but no exception was thrown

User · Answer

There is an awesome library called NFluent which speeds up and eases the way you write your assertions   It is pretty straightforward to write an assertion for throwing an exception        Test      public void given when then                 Check ThatCode      gt  MethodToTest                 Throws lt Exception gt                 WithMessage  Process has been failed

User · Answer

The helper provided by  Richiban above works great except it doesn t handle the situation where an exception is thrown  but not the type expected  The following addresses that   using System  using Microsoft VisualStudio TestTools UnitTesting   namespace YourProject Tests       public static class MyAssert                    lt summary gt              Helper for Asserting that a function throws an exception of a particular type               lt  summary gt          public static void Throws lt T gt   Action func   where T   Exception                       Exception exceptionOther   null              var exceptionThrown   false              try                               func Invoke                              catch   T                                 exceptionThrown   true                            catch  Exception e                    exceptionOther   e                             if    exceptionThrown                                 if  exceptionOther    null                        throw new AssertFailedException                          String Format  An exception of type  0  was expected  but not thrown  Instead  an exception of type  1  was thrown    typeof T   exceptionOther GetType                             exceptionOther                                                               throw new AssertFailedException                      String Format  An exception of type  0  was expected  but no exception was thrown    typeof T

User · Answer

This is going to depend on what test framework are you using   In MbUnit  for example  you can specify the expected exception with an attribute to ensure that you are getting the exception you really expect    ExpectedException typeof ArgumentException

User · Answer

Even though this is an old question  I would like to add a new thought to the discussion  I have extended the Arrange  Act  Assert pattern to be Expected  Arrange  Act  Assert  You can make an expected exception pointer  then assert it was assigned to  This feels cleaner than doing your Asserts in a catch block  leaving your Act section mostly just for the one line of code to call the method under test  You also don t have to Assert Fail    or return from multiple points in the code  Any other exception thrown will cause the test to fail  because it won t be caught  and if an exception of your expected type is thrown  but the it wasn t the one you were expecting  Asserting against the message or other properties of the exception help make sure your test won t pass inadvertently    TestMethod  public void Bar InvalidDependency ThrowsInvalidOperationException            Expectations     InvalidOperationException expectedException   null      string expectedExceptionMessage    Bar did something invalid            Arrange     IDependency dependency   DependencyMocks Create        Foo foo   new Foo dependency           Act     try               foo Bar              catch  InvalidOperationException ex                expectedException   ex                Assert     Assert IsNotNull expectedException       Assert AreEqual expectedExceptionMessage  expectedException Message

User · Answer

In VS built-in unit testing if you simply want to verify that  any exception  is thrown  but you don t know the type  you can use a catch all    TestMethod   ExpectedException typeof Exception   AllowDerivedTypes   true   public void ThrowExceptionTest

User · Answer

MSTest  v2  now has an Assert ThrowsException function which can be used like this   Assert ThrowsException lt System FormatException gt       gt                                Story actual   PersonalSite Services Content ExtractHeader String Empty                      You can install it with nuget  Install-Package MSTest TestFramework

User · Answer

My preferred method for implementing this is to write a method called Throws  and use it just like any other Assert method  Unfortunately   NET doesn t allow you to write a static extension method  so you can t use this method as if it actually belongs to the build in Assert class  just make another called MyAssert or something similar  The class looks like this   using System  using Microsoft VisualStudio TestTools UnitTesting   namespace YourProject Tests       public static class MyAssert               public static void Throws lt T gt   Action func   where T   Exception                       var exceptionThrown   false              try                               func Invoke                              catch   T                                 exceptionThrown   true                             if    exceptionThrown                                 throw new AssertFailedException                      String Format  An exception of type  0  was expected  but not thrown   typeof T                                                            That means that your unit test looks like this    TestMethod    public void ExceptionTest         String testStr   null      MyAssert Throws lt NullReferenceException gt       gt  testStr ToUpper         Which looks and behaves much more like the rest of your unit test syntaxes

User · Answer

You can download a package from Nuget using  PM  Install-Package MSTestExtensions that adds Assert Throws   syntax in the style of nUnit xUnit to MsTest   High level instructions  download the assembly and inherit from BaseTest and you can use the Assert Throws   syntax   The main method for the Throws implementation looks as follows   public static void Throws lt T gt  Action task  string expectedMessage  ExceptionMessageCompareOptions options  where T   Exception       try               task              catch  Exception ex                AssertExceptionType lt T gt  ex           AssertExceptionMessage ex  expectedMessage  options           return             if  typeof T  Equals new Exception   GetType                   Assert Fail  Expected exception but no exception was thrown               else               Assert Fail string Format  Expected exception of type  0  but no exception was thrown    typeof T               Disclosure  I put together this package   More Info  http   www bradoncode com blog 2012 01 asserting-exceptions-in-mstest-with html

User · Answer

Check out nUnit Docs for examples about     ExpectedException  typeof  ArgumentException

User · Answer

For  Visual Studio Team Test  it appears you apply the ExpectedException attribute to the test s method   Sample from the documentation here  A Unit Testing Walkthrough with Visual Studio Team Test   TestMethod   ExpectedException typeof ArgumentException        A userId of null was inappropriately allowed     public void NullUserIdInConstructor        LogonInfo logonInfo   new LogonInfo null   P ss0word

User · Answer

if you use NUNIT  you can do something like this   Assert Throws lt ExpectedException gt       gt  methodToTest        It is also possible to store the thrown exception in order to validate it further   ExpectedException ex   Assert Throws lt ExpectedException gt       gt  methodToTest     Assert AreEqual   Expected message text    ex Message    Assert AreEqual  5  ex SomeNumber     See  http   nunit org docs 2 5 exceptionAsserts html

[c#] How do I use Assert to verify that an exception has been thrown?

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