Mocking a method to throw an exception moq but otherwise act like the mocked object

Question

I have a Transfer class  simplified it looks like this   public class Transfer       public virtual IFileConnection source   get  set        public virtual IFileConnection destination   get  set         public virtual void GetFile IFileConnection connection           string remoteFilename  string localFilename                connection Get remoteFilename  localFilename              public virtual void PutFile IFileConnection connection           string localFilename  string remoteFilename                connection Get remoteFilename  localFilename              public virtual void TransferFiles string sourceName  string destName                source   internalConfig GetFileConnection  source            destination   internalConfig GetFileConnection  destination            var tempName   Path GetTempFileName            GetFile source  sourceName  tempName           PutFile destination  tempName  destName             The simplified version of the IFileConnection interface looks like this   public interface IFileConnection       void Get string remoteFileName  string localFileName       void Put string localFileName  string remoteFileName       The real class is supposed to handle a System IO IOException that is thrown when the IFileConnection concrete classes loses connectivity with the remote  sending out emails and what not   I would like to use Moq to create a Transfer class  and use it as my concrete Transfer class in all properties and methods  except when the GetFile method is invoked - then I want it to throw a System IO IOException and make sure the Transfer class handles it properly   Am I using the right tool for the job   Am I going about this the right way   And how would I write the setup for that unit test for NUnit

User · Accepted Answer

This is how I managed to do what I was trying to do:

[Test]
public void TransferHandlesDisconnect()
{
    // ... set up config here
    var methodTester = new Mock<Transfer>(configInfo);
    methodTester.CallBase = true;
    methodTester
        .Setup(m => 
            m.GetFile(
                It.IsAny<IFileConnection>(), 
                It.IsAny<string>(), 
                It.IsAny<string>()
            ))
        .Throws<System.IO.IOException>();

    methodTester.Object.TransferFiles("foo1", "foo2");
    Assert.IsTrue(methodTester.Object.Status == TransferStatus.TransferInterrupted);
}

If there is a problem with this method, I would like to know; the other answers suggest I am doing this wrong, but this was exactly what I was trying to do.

User · Answer

I think this is what you want  I already tested this code and works  The tools used are   all these tools can be downloaded as Nuget packages   http   fluentassertions codeplex com   http   autofixture codeplex com   http   code google com p moq   https   nuget org packages AutoFixture AutoMoq  var fixture   new Fixture   Customize new AutoMoqCustomization     var myInterface   fixture Freeze lt Mock lt IFileConnection gt  gt      var sut   fixture CreateAnonymous lt Transfer gt      myInterface Setup x   gt  x Get It IsAny lt string gt     It IsAny lt string gt               Throws lt System IO IOException gt      sut Invoking x   gt           x TransferFiles              myInterface Object               It IsAny lt string gt                  It IsAny lt string gt                        ShouldThrow lt System IO IOException gt       Edited   Let me explain   When you write a test  you must know exactly what you want to test  this is called   subject under test  SUT    if my understanding is correctly  in this case your SUT is  Transfer  So with this in mind  you should not mock your SUT  if you substitute your SUT  then you wouldn t be actually testing the real code  When your SUT has external dependencies  very common  then you need to substitute them in order to test in isolation your SUT  When I say substitute I m referring to use a mock  dummy  mock  etc depending on your needs  In this case your external dependency is IFileConnection so you need to create mock for this dependency and configure it to throw the exception  then just call your SUT real method and assert your method handles the exception as expected   var fixture   new Fixture   Customize new AutoMoqCustomization      This linie initializes a new Fixture object  Autofixture library   this object is used to create SUT s without having to explicitly have to worry about the constructor parameters  since they are created automatically or mocked  in this case using Moq var myInterface   fixture Freeze lt Mock lt IFileConnection gt  gt      This freezes the IFileConnection dependency  Freeze means that Autofixture will use always this dependency when asked  like a singleton for simplicity  But the interesting part is that we are creating a Mock of this dependency  you can use all the Moq methods  since this is a simple Moq object var sut   fixture CreateAnonymous lt Transfer gt      Here AutoFixture is creating the SUT for us myInterface Setup x   gt  x Get It IsAny lt string gt     It IsAny lt string gt      Throws lt System IO IOException gt     Here you are configuring the dependency to throw an exception whenever the Get method is called  the rest of the methods from this interface are not being configured  therefore if you try to access them you will get an unexpected  exception sut Invoking x   gt  x TransferFiles myInterface Object  It IsAny lt string gt     It IsAny lt string gt      ShouldThrow lt System IO IOException gt      And finally  the time to test your SUT  this line uses the FluenAssertions library  and it just calls the TransferFiles real method from the SUT and as parameters it receives the mocked IFileConnection so whenever you call the IFileConnection Get in the normal flow of your SUT TransferFiles method  the mocked object will be invoking throwing the configured exception and this is the time to assert that your SUT is handling correctly the exception  in this case  I am just assuring that the exception was thrown by using the ShouldThrow lt System IO IOException gt     from the FluentAssertions library    References recommended   http   martinfowler com articles mocksArentStubs html  http   misko hevery com code-reviewers-guide   http   misko hevery com presentations   http   www youtube com watch v wEhu57pih5w amp feature player embedded  http   www youtube com watch v RlfLCWKxHJ0 amp feature player embedded

User · Answer

Here s how you can mock your FileConnection  Mock lt IFileConnection gt  fileConnection   new Mock lt IFileConnection gt                                                              MockBehavior Strict   fileConnection Setup item   gt  item Get It IsAny lt string gt  It IsAny lt string gt                   Throws new IOException       Then instantiate your Transfer class and use the mock in your method call  Transfer transfer   new Transfer    transfer GetFile fileConnection Object  someRemoteFilename  someLocalFileName     Update   First of all you have to mock your dependencies only  not the class you are testing Transfer class in this case   Stating those dependencies in your constructor make it easy to see what services your class needs to work  It also makes it possible to replace them with fakes when you are writing your unit tests  At the moment it s impossible to replace those properties with fakes   Since you are setting those properties using another dependency  I would write it like this   public class Transfer       public Transfer IInternalConfig internalConfig                source   internalConfig GetFileConnection  source            destination   internalConfig GetFileConnection  destination                 you should consider making these private or protected fields     public virtual IFileConnection source   get  set        public virtual IFileConnection destination   get  set         public virtual void GetFile IFileConnection connection           string remoteFilename  string localFilename                connection Get remoteFilename  localFilename              public virtual void PutFile IFileConnection connection           string localFilename  string remoteFilename                connection Get remoteFilename  localFilename              public virtual void TransferFiles string sourceName  string destName                var tempName   Path GetTempFileName            GetFile source  sourceName  tempName           PutFile destination  tempName  destName             This way you can mock internalConfig and make it return IFileConnection mocks that does what you want

[c#] Mocking a method to throw an exception (moq), but otherwise act like the mocked object?

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