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Posted 1 Jul 2005


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New Version Of Rhino Mocks

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1 Jul 200518 min read
A flexible mock objects framework and general discussion of mocking, now in a second version

Sample Image - Rhino_Mocks_Version_20.jpg

Rhino Mocks Version 2.0

A dynamic mock object framework for the .Net platform. It's purpose is to ease testing by allowing the developer to create mock implementations of custom objects and verify the interactions using unit testing.

Rhino.Mocks is an attempt to create easier way to build and use mock objects and allow better refactoring support from the current tools. It's a hybrid approach between the pure Record/Replay of EasyMock.Net's model and NMock's expectation based model. Rhino.Mocks originated from EasyMock.Net and attempt to improve on their model to create easy to use and power mocking framework. It's free for use and modification for free and commercial software.

I created Rhino.Mocks because I wanted better support from refactoring tools such as ReSharper and because I don't like the way NMock handle parameterized methods (you need to pass fake constraints to get it to recognize the correct method) and the lack of extensibility [I required hacks to get to the result that I wanted].

Licensing: Rhino Mocks is Free Software which is released under the BSD license.

You can get Rhino Mocks at its project page

What does Rhino Mocks offers?

  • Explicit record & replay model for expectations.
  • Working with strongly typed mocks.
  • Expectations based on:
    • Arguments matching
    • Constraints matching
    • Custom callback to verify the expected arguments using your own code
  • Setting actions on methods, return spesific value, or throw an exception.

Why reinvent the wheel?

  • Other frameworks, such as NMock, NMock2 or TypeMock.Net uses string baed approach for mocking call. This means that you cannot use the compiler and most refactoring tools in order to catch any mistakes until runtime, if then. After getting burned more than once by this, I decided that I want a mocking framework that would use the objects, and not strings, in order to set up expectations on the mocked object.
  • Why not use EasyMock.Net, then?
    • EasyMock.Net is a port of a Java library, and it uses Remoting proxies to do its work.
    • I didn't like the API that EasyMock.Net had, it had too much roots in Java to be comfortable to work with in .Net
    • I found several places where a the Remoting proxies were behaving exactly like the real objects. Specifically, they coulnd't be copied using Array.Copy();
    • I Never had much interest in Remoting, so I don't know a lot about it

I hope you will enjoy using Rhino Mocks.

In the first article, I had explained a little about mock objects, interaction base testing, state based test, etc. Here is a brief refreshment for those who don't want to read the full article.

  • Mock Object - an object that pretend to be another object, and allows to set expectations on its interactions with another object.
  • Interaction Based Testing - you specify certain sequence of interactions between objects, initiate an action, and then verify that the sequence of interactions happened as you specified it.
  • State Based Testing - you initiate an action, and then check for the expected results (return value, property, created object, etc).
  • Expectation - general name for validation that a particular method call is the expected one.
  • Record & Replay model - a model that allows for recording actions on a mock object, and then replaying and verifying them. All mocking frameworks uses this model. Some (NMock, TypeMock.Net, NMock2) uses it implicitly and some (EasyMock.Net, Rhino Mocks) uses it explicitly.
  • Ordering - The ability to specify that replaying a sequence of method calls will occur in a specific order (or disorder).

So, what has changed?

In version 2.0, I moved from the EasyMock.Net based code, which relied on Remoting's proxies; to inheritance approach using Castle.DynamicProxy. The resulting code base is much more flexible and easier to extend. While rebuilding it, I paid attention to several factors.

  • NMock2's syntax is very nice, and allows to set clear expectations on the called methods.
  • The annoyance of having to create a MockControl for each mock object that I wanted to use.
  • The annoyance of having a false positive test because I didn't verify the MockControls of each mocked object.
  • Preserving the ability to specify constraints, callbacks and parameter matching.
  • Preserve the explicit Record & Replay model, but make it easier to work with.
  • Making the last call idea explicit in the code.
  • Making it easy to create mocks and set & verify expectations on them.
  • Making it easy to extend the framework if there is need.
  • Fail as early as possible, and with descriptive error message.
  • Mock classes as well as interfaces, and make sure that both behave exactly like they are supposed to (and not sort of like it.)

Rhino Mocks capabilities:

  • mock interface and concrete classes, including those with parameterized constructors.

  • set expectations on the called methods by using strongly typed mocks instead of strings.
  • lends itself easily for refactoring & leaning on the compiler.
  • allows wide range of expectations to be set on mock object, or several mock objects.

Let's get down to the code, okay? [I tried to create an example for this article, but the example was both contrived and didn't portrayed the possibilities correctly. So most of the code samples here were taken from NHibernate Query Analyzer tests and are "real code".]

The purpose of mock objects is to allow you to test the interactions between components, this is very useful when you test code that doesn't lend itself easily to state base testing. Most of the examples here are tests to check that the save routine for a view is working as expected. The requirements for this method are:

  • If the project is a new project, then ask for a name for the project. [ Allow canceling save at this point ]
    • If the new project name already exists, ask if user want to overwrite it. [ If no, cancel the save operation ]
    • If the user approve overwrite, delete old project.
  • Save project.

So we have five tests here:

  1. Project is new project, and the user cancels on new name.
  2. Project is new project, the user gives a new name, and the project is saved.
  3. Project is new project, the user gives a name that already exists and don't approve overwrite.
  4. Project is new project, the user gives a name that already exist and approve overwrite.
  5. Project already exists, so just save it.

Trying to test that using state based testing is going to be both awkward and painful, using interaction based testing, it would be a breeze (other types of tests would be just as painful to test using interaction testing but easy using state testing.)

That is enough talking, let's see some code:

public void SaveProjectAs_CanBeCanceled()
    using(MockRepository mocks = new MocksRepository())
        IProjectView projectView = (IProjectView)projectView.CreateMock(typeof(IProjectView));
        Project prj = new Project("Example Project");
        IProjectPresenter presenter = new ProjectPresenter(prj,projectView);

We create a MockRepository and create a mock project view, project and a project presenter, then we set the expectations. After we finished with setting up the expectations, we move to the replay state, and call the SaveProjectAs() method, which should return false if the user canceled the save process.

This example clarify several key concepts with Rhino Mocks.

  • We set expectation on object using the object's methods, and not strings, this reduce the chances of creating a mistake that will only (hopefully) be caught at runtime, when you run the tests.
  • We are using explicit call to move from Record state to Replay state.
  • MockRepository implement IDisposable; so when we reach the end for the using statement.

This is about as simple example as can be had, the real test move creating the MockRepository, the project, the view and presenter to the setup method, since they are require for each test. You can see that we expected two method to be called, with specific arguments, and that we set a result for each. This method uses parameter  matching expectations, Rhino Mocks supports several more. More info: Method Calls.

Ordered / Unordered:

Method calls in Rhino Mocks can be ordered or unordered. The default state for a recorder is unordered recording, this means that during replay, methods can come at any order. If the recorder is changed to ordered, then the methods must be called in the exact same order as they were recorded. Here is a code sample:

public void SaveProjectAs_NewNameWithoutConflicts()
            Return( newProjectName);
        projectView.Title = newProjectName;
        projectView.HasChanges = false;

In the above code example we ask Rhino Mocks to verify that the calls come in the exact same order. Notice that we specify expectations on several mock objects, and Rhino Mocks will handle the ordering between them. This mean that if I set the project view title before I get a project from the repository, the test will fail. In Rhino Mocks, the default is to use unordered matching, but it support unlimited depth of nesting between ordered and unordered, this means that you can create really powerful expectations. Here is a contrived example:

public void MovingFundsUsingTransactions()
    using(MockRepository mocks = new MockRepository())
        IDatabaseManager databaseManager = mocks.CreateMock(typeof(IDatabaseManager));
        IBankAccount accountOne = mocks.CreateMock(typeof(IBackAccount)),
            accountTwo = mocks.CreateMock(typeof(IBankAccount)); 
        Bank bank = new Bank(databaseManager);

This code verify that the transfer of funds from one account to another is wrapped in a transaction, but the implementation is free to withdraw from the first account first, or to deposit into the second account first, both are legal, as long as both actions happens. The reverse is true as well, you may specified unordered sequence of ordered events (I want A then B then C to happen, and I want D then E then F to happen, but I don't care which sequence comes first).

Ordering have two caveats:

  • To exit an ordering in replay state, you must call all the recorded methods. In  the above example, we can move from the inner Unordered ordering (the one containing the withdraw and deposit) only after both methods were called. This fall in line with the way the recording code looks, so it shouldn't cause any surprises.
  • You must exit all ordering before you can start replaying. This is enforced by the framework (you get an exception if you try).

The IDisposable pattern:

The using(...) syntax & IDisposable are very convenient when it comes to make sure you're not forgetting to clean up resources, in Rhino Mocks, it's used in two places:

  • MockRepository implement IDisposable so you won't forget to call VerifyAll(), or call ReplayAll().
    • If an exception has been raised during the test because of Rhino Mocks, the repository won't validate, and you'll get the exception that was thrown.
    • But if an exception was thrown that didn't originate in Rhino Mocks (from an Assert that failed, for instance), then the repository will attempt to verify that all expectations has been met, and will (probably) fail. This result in an exception that masks the real cause for failure. Keep this in mind if you have a failing test.
    • The recommended approach is to create the repository in the Setup method, and verify it in the Teardown method, this way you'll get the correct exception. Just call VerifyAll() (or Dispose(), which does the same thing) in your teardown.
  • Ordered() and Unordered() methods on MockRepository, these two methods set the MockRepository to expect the following method in ordered or unordered way. Those calls should be wrapped in a using declaration (or a manual Dispose() call). You cannot start replaying with an open ordering call.

Method Calls:

When Rhino Mocks intercept a call from a mocked object it determines if the call is expected by checking the method signature, the object this method was called on, whatever the expectation on this method match the arguments passed for the method. The matching of the mocked object and the method signature is not very interesting, but the expectations on a method call is. Rhino Mocks support the following expectations:

  • Matching Parameters - The same arguments that were passed during record state must be passed during the replay state in order to get a match. One caveat, though, for arrays, the comparison is done for the objects contained in the array. This is the default behavior of Rhino Mocks, and require no action on your part.
  • Ignore Parameters - Expect just the method call on this object, any arguments will be accepted. This is useful if you don't care about the arguments. To activate this behavior, you need to call IgnoreArguments() on the IMethodOptions interface (More info: Method Options Interface).
  • Constraints Matching - Each argument is validated against a constraint, to accept the call, all constraints must pass. (More info: Constraints.)

  • Callbacks - A user supplied delegate is called to check if a method call is valid, if the delegate return true, then the method is accepted. (More info: Callbacks.)


A callback is a user supplied delegate that is called whenever Rhino Mocks needs to evaluate whatever a method call is expected or not. This is useful in some scenarios when you want to do a complex validation on a method arguments, or have a complex interactions between objects that you need to mock in the tests. I added this feature because I want to test some threading code which had the semantics of: Start Job, and notify me when you're done. The only way to re-create that without bringing the real thread (and kill tests isolations) was to plug my own code during the replay state and call the tested object myself.

Some things to consider before you decide to use callbacks:

  • It can be abused very easily. It can be very hard to understand tests that use callbacks, because you get calls from supposedly innocent code. Do it only if you need it.
  • You callback may (and likely will be) called several times; keep that in mind when you write it. Either wrap it all in a if ( firstTimeCalled ) { /*do work*/)  or make sure that repeated call for the delegate won't have some nasty side effects.

If it so open to abuse, why add it?

  • Because when you need it, you really need it, and I would prefer that I'd some nice way to do it, and not a really ugly hacks.

  • Because I respect those who will use this framework not to take the power and abuse it.

The technical details - In order to be a valid callback, the callback must return a Boolean, and have the same arguments as the mocked methods. You register a delegate using the following code:

IProjectRepository repository = (IProjectRepository) mocks.CreateMock(typeof(IProjectRepository));
IProjectView view = (IProjectView )mocks.CreateMock(typeof(IProjectView ));
Expect.On(view).(view.Ask(null,null)).Callback(new AskDelegate(DemoAskDelegateMethod)).Return(null);

Notice that you cannot change the return value for the method, but must pass it explicitly.

Recursive Expectations:

One final word of warning regarding callbacks. If your callback will initiate an action that cause another call on a mocked object, this will fail when mixed with Ordered(). The reason for that is that the framework cannot decide whatever to accept the call or not, and calling another mocked method while evaluating means that the currently expected call is the one still being evaluated. This will fail the test. Using Unordered(), it will work, since the second expectation is not dependant on the first one being accepted first. In short, Ordered() is not re-entrant :-)

Method Options Interface:

The IMethodOptions allows you to set various options on a method call. Here is an example of telling Rhino Mocks to ignore the arguments of a method:

IProjectRepository repository = (IProjectRepository)mocks.CreateMock(typeof(IProjectRepository));
IProjectView view = (IProjectView ) mocks.CreateMock(typeof(IProjectView ));

As you can see, we use the Expect.On().Call() for methods that has return values, and LastCall.On() for methods that return void to get the IMethodOptions interface. I find the Expect.On().Call() syntax a bit clearer, but there is no practical difference between the two. I would recommend using Expect wherever possible (anything that return a value). For properties setters, or methods returning void, the Expect syntax is not applicable, since there is no return value. Thus, the need for the LastCall.On(). The idea of Last Call is pervasive in the record state, you can only set the method options for the last call  - even Expect.On().Call() syntax is merely a wrapper around LastCall.On().

Expect.On().Call()  & LastCall.On() allows you to set the following options:

  • The return value of a method, if it has one.

  • The exception the method will throw:

    Throw(new Exception("Demo"));
  • The number of times this method is expected to repeat (there are a number of convenience methods there):

  • To ignore the method arguments:

  • To set the constraints of the method:

  • To set the callback for this method:

    Callback(new AskDelegate(VerifyAskArguments));

Note: For methods that return a value, you must specify either a return value or an exception to throw. You will not be able to continue recording or move to replay state otherwise.

Note II: Method chaining really makes writing this cod easier.


Constraints are a way to verify that a method arguments match a certain criteria. Rhino Mocks include a number of built in constraints, and allows you to define you own custom one, which will integrate cleanly into the framework. You specify constraints on method arguments using the following syntax:


You need to pass the exact same number of constraints as the number of arguments of the method. When a method is called during replay state, Rhino Mocks evaluate each constraint against the parameter in the same index, and accept the method if all the constraints were met. As a note, I got the idea of the current constraints syntax from NMock2, it is much leaner approach to create constraints.

Rhino Mocks built in constraints:

Rhino mocks ships with the following constraints:


Example: Accepted Value(s): Unaccepted Value(s):
Object Anything Is.Anything() Anything at all {0,"","whatever",null, etc} (Nothing at all!)
Equal Is.Equal(3) (int)3 3f, 4,"", new object()
Not Equal Is.NotEqual(3) or !Is.Equal(3) 3f,43,null,DateTime.Now (int)3
Null Is.Null() null 5,"",new object()
Not Null Is.NotNull() DateTime.Now, "asbcd", 0xHead null
Type Of Is.TypeOf(typeof(string)) "","Hello",String.Empty null, 3, DateTime.Now
Greater Than Is.GreaterThan(10) 15,100,300 3,4,5, 10
Greater Than Or Equal Is.GreaterThanOrEqual(10) 10,15,100 4,2,8,9
Less Than Is.LessThan(10) 1,2,9 10,32,100
Less Than Or Eqaul Is.LessThanOrEqual(10) 1,9,10 11,33,43
Property Equal To Value Property.Value("Length",0) new ArrayList(),String.Empty "Hello", 5
Null Property.IsNull("InnerException") new Exception("exception without inner exception") new Exception("Exception with inner Exception", new Exception("Inner")
Not Null Property.IsNotNull("InnerException") new Exception("Exception with inner Exception", new Exception("Inner") new Exception("exception without inner exception")
List Is In List
[the parameter is a collection that contains this value]
List.IsIn(4) new int[]{1,2,3,4}, new int[]{4,5,6} new object[]{"",3}
One Of
[parameter equal to one of the objects in this list]
List.OneOf(new int[]{3,4,5}) 3,4,5 9,1,""
Equal List.Equal(new int[]{4,5,6}) new int[]{4,5,6}, new object[]{4,5,6} new int[]{4,5,6,7}
Text Starts With Text.StartsWith("Hello") "Hello, World", "Hello, Rhino Mocks" "", "Bye, Bye"
Ends With Text.EndsWith("World") "World","Champion Of The World" "world", "World Seria"
Contains Text.Contains("or") "The Horror Movie...", "Either that or this" "Movie Of The Year"
[perform regular expression validation]
Text.Like("Rhino|rhino|Rhinoceros|rhinoceros" ) "Rhino Mocks", "Red Rhinoceros" "Hello world", "Foo bar", Another boring example string"
Operator Overloading And - operator & Text.StartsWith("Hello") & Text.Contains("bar") "Hello, Foobar" "Hello, World", "Foo bar"
Or - operator | Text.StartsWith("Hello") & Text.Contains("bar") "Hello, Foobar", "Hello, World", "Foo bar" "boring string"
Not - operator ! !Text.StartsWith("Hello") "Foo bar", "index.html" "Hello, Rhino Mocks"

Operator Overloading warning: Pay attention to operator precedence if you are using several operators in a single statement, otherwise you may get unexpected results.

Creating custom constraints: Creating you custom constraint is easy, just derive a class from AbstractConstraint and return true from the Eval() method.

Setup Result:

Sometimes you have a method on your mocked object where you don't care how / if it was called, you may want to set a return value (or an exception to be thrown), but for this specific test, you just don't care. For example, you may have some interaction that you already verified, or you are testing some other class and just need to get the right value from a method. The way to do it in Rhino Mocks is to use SetupResult.On().Call(). Here is the code:

public void SetupResultUsingOrdered()
    for (int i = 0; i < 30; i++)

When we use SetupResult.On().Call() we tell Rhino Mocks: "I don't care about this method, it need to do X, so just do it, but otherwise ignore it." Using SetupResult.On().Call() completely bypass the expectations model in Rhino Mocks. In the above example, we define demo.Prop to return "Ayende", and we can all it no matter in what the currently expected method is.

What about methods returning void?
You would use LastCall.On().Repeat.Any(), which has identical semantics (ignore ordering, etc).

Note: If you want to have a method that can repeat any number of time, but still obey ordering, you can use: LastCall.On().Repeat.Times(0,int.MaxValue), this does obey all the normal rules.

Mocking classes:

Rhino Mocks supports mocking concrete classes as well as interfaces. In fact, it can even mock classes that doesn't have a default constructor! To mock a class, simply pass its type to MockRepository.CreateMock() along with any parameters for the constructor.

public void AbuseArrayList()
    using(MockRepository mocks = new MockRepository())
        ArrayList list = (ArrayList)mocks.CreateMock(typeof(ArrayList));

If you want to call the non default constructor, just add the parameters after the type. Like this:

ArrayList list = (ArrayList)mocks.CreateMock(typeof(ArrayList),500);

Some things to be aware of:

  • You cannot create a mock object from a sealed class.
  • You cannot intercept calls to non-virtual methods.


  • Currently Rhino Mocks can't mock interfaces and classes that use out or ref parameters.


This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

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Comments and Discussions

QuestionSetPropertyAndIgnoreArgument on "new virtual" property Pin
darkneoncyber28-Aug-12 23:46
Memberdarkneoncyber28-Aug-12 23:46 
QuestionStrong name signed assembly? Pin
Nathan Hop12-Aug-10 17:02
MemberNathan Hop12-Aug-10 17:02 
QuestionWhat is Replaying For? Pin
Jeff C. Benson23-Dec-09 5:44
MemberJeff C. Benson23-Dec-09 5:44 
GeneralI have a large class that I only want to set an expectation on one method Pin
Ryan Andrus20-Jun-08 9:11
MemberRyan Andrus20-Jun-08 9:11 
GeneralMocking static methods Pin
Sanchit Bahal4-Dec-07 18:28
MemberSanchit Bahal4-Dec-07 18:28 
Questioncallback in rhino mock Pin
HoomanAmini29-Nov-07 8:05
MemberHoomanAmini29-Nov-07 8:05 
QuestionGenerics? Pin
cybermoose9-Jul-07 23:59
Membercybermoose9-Jul-07 23:59 
AnswerRe: Generics? Pin
Ayende @ Rahien10-Jul-07 8:25
MemberAyende @ Rahien10-Jul-07 8:25 
GeneralMocking a class method with void return and no args Pin
Sean Feldman26-Jun-07 9:57
MemberSean Feldman26-Jun-07 9:57 
GeneralNewer version exists at author's site Pin
Drew Noakes22-May-07 22:34
MemberDrew Noakes22-May-07 22:34 
GeneralJust Started Using Pin
Mr_Hosed18-Feb-07 16:49
MemberMr_Hosed18-Feb-07 16:49 
QuestionProblem with using notation in 2.0 Pin
Swiss Thomas16-Oct-06 0:32
MemberSwiss Thomas16-Oct-06 0:32 
AnswerRe: Problem with using notation in 2.0 Pin
Ayende @ Rahien16-Oct-06 2:07
MemberAyende @ Rahien16-Oct-06 2:07 
GeneralExcellent! Pin
Judah Gabriel Himango20-Jul-06 13:11
sponsorJudah Gabriel Himango20-Jul-06 13:11 

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