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Mocking frameworks can be extremely useful for stubbing out objects in your unit tests to ensure you're only testing what you want to test and to allow you to control the environment/state/surroundings under which your tests execute. There are many different mocking frameworks available for use in .Net, each with their own variations on syntax and approach. There are two main categories of mocking frameworks, Constrained and Unconstrained, and I describe how each works in my post How .Net Mocking Frameworks Work Under the Hood.

This post is broken into several sections, starting with a general overview and then hitting on several specific use cases of interest:

  • Mock Options
  • Stubs vs Mocks
  • Using Mocks to Limit the Scope of Your Tests
  • General Framework Usage
    • Creating a Mock
    • Controlling Mock Behaviors
    • Change How Many Times to Use the Mock
    • Returning Prepared Values Based on the Input Values
    • Returning the Prepared Values Regardless of the Input Values
    • Advanced Argument Constraints
    • Providing a Method Implementation / Using the Input Params
    • Mocking Things Other Than Methods
    • Throwing an Exception Instead
  • Testing Non-Public Members

Mock Options

Most .Net mocking frameworks support three basic types of mock objects:

Strict Mock
A strict mock requires you to provide alternate implementations for each method/property that is used on the mock. If any methods/properties are used which you have not provided implementations for, an exception will be thrown.
Dynamic Mock
With a dynamic mock, any methods/properties which are called by your tests for which you have not provided an implementation will return the default value for the data type of the return value.  In other words, you'll get back a 0 for number types, false for Booleans and a null for any object types.
Partial Mock
A partial mock will use the underlying object's implementation if you don't provide an alternate implementation.  So if you're only wanting to replace some of the functionality (or properties), and keep the rest, you'll want to use this.  For example, if you only want to override the method IsDatabaseActive(), and leave the rest of the class as-is, you'll want to use a partial mock and only provide an alternate implementation for IsDatabaseActive(). (This type of mock is not valid for interfaces.)

IMPORTANT: Constrained mocking frameworks can only mock/stub virtual or abstract members of a real class, so make sure the members you care about are virtual -- OR, event better, mock/stub an Interface, in which case you can do whatever you want. (See my post How .Net Mocking Frameworks Work Under the Hood for more details.)

Stubs vs Mocks

A stub is simply an alternate implementation. A mock, however, is more than that. A mock sets up an expectation that

  • A specific method will be called
  • It will be called with the provided inputs
  • It will return the provided results

The distinction between the two has lead to a fair amount of confusion over time, so not all frameworks differenciate between the two, and even some that do (like RhinoMocks) blur the lines a bit. Some framework have choosen to only support Mocks in order to reduce confusion. Some give them differnt names, such as "Fakes", "Substitute", or "Shim".

In addition to setting up an alternate behavior for when the code is run, the frameworks also provide a mechanism for verifying members were/were not called during execution. For example, RhinoMocks provides a .VerifyAllExpectations() method to ensure all Mocks were called, as well as .AssertWasCalled(...) and .AssertWasNotCalled(...) methods to validate specific members were/were not used during the exectution.

Since mocking frameworks are intended for use within unit testing frameworks, Exceptions are used to signal when your code has violated the expectations. For instance, if you've created a strict mock and your code attempts to use a method that hasn't been specifically mocked, an exception will be thrown. Or, if you utilize one of the post-execution validation methods to check if a member was/wasn't used, an exception will be thrown if your expectations weren't met.

For example, the below test using RhinoMocks will fail due to an ExpectationViolationException being thrown since Expect(101) is not being called.

[Test]
public void TestExpectations()
{
    //Arrange
    int myRecordId = 100;
    var recordFromDatabase = new ImportantData
                                    {
                                        Name = "Orignal Name",
                                        RecordId = myRecordId
                                    };

    var mockDAO = MockRepository.GenerateMock<IDataAccess>();

    mockDAO.Expect(dao => dao.GetRecordFromDatabase(101))
            .Return(recordFromDatabase);

    var fancyBL = new FancyBusinessLogic { MyDataAccessObject = mockDAO };
        

    //Act
    fancyBL.GetImportantDataAndUpdateTheName(myRecordId);

    //Assert
    mockDAO.VerifyAllExpectations();
}

Using Mocks to Limit the Scope of Your Tests

Let's say I have a class that I want to test, but it relies on a database object to fetch/update, etc records from the database. I want to test my class in isolation from the database, so I mock the database object's interface, which allows me to control the flow of information within my implementation.  

To achieve this in the code sample below, I generate a stub for the database object's GetRecordFromDatabase() method so that when it's called with the recordId I care about, it will return my prepared value. This removes the dependency on the database layer (which is not even used since I'm mocking an Interface) and ensures my test is controlling the inputs and outputs so I'm getting exactly what I want for my specific test condition.

[Test]
public void TestGetImportantDataAndUpdateTheName()
{
    //Arrange
    int myRecordId = 100;
    var recordFromDatabase = new ImportantData {RecordId = myRecordId};
    var mockDAO = MockRepository.GenerateMock<IDataAccess>();
         
    mockDAO.Stub(dao => dao.GetRecordFromDatabase(myRecordId))
            .Return(recordFromDatabase);

    var objectUnderTest = new FancyBusinessLogic { MyDataAccessObject = mockDAO };

    //Act
    var myRecord = objectUnderTest.GetImportantDataAndUpdateTheName(myRecordId);

    //Assert
    Assert.AreEqual(myRecord.RecordId, myRecordId);
    Assert.AreEqual(myRecord.Name, "All Your Base Are Belong To Us");
}

General Framework Usage

Each framework has it's own syntax, so for this guide I'm covering the concepts but can't give specific since they variy widely. I've created a few framework-specific pages to help with the syntax of some of the more populate frameworks:

Creating a Mock

To generate mocks, you'll genearlly use a static factory to create the specific type of mock you want (strict, dynamic or partial). There are typically generic methods such as the following (from RhinoMocks):

  • GenerateMock<T> (for DynamicMocks)
  • GeneratePartialMock<T>
  • GenerateStrictMock<T>

where the T is the class/interface being mocked. Some frameworks will allow you differenciate between mocks and stubs at this time as well, but most do that on a member-by-member basis. The contructor parameters, if you need to provide any, will generally be passed as parameters to this factory method.

For some frameworks the return type directly implements the type you're mocking and the configuration of the mock (see below) is done via extention methods off that type. In other cases, such as Moq, the type returned is a specific type defined by the framework and the actual mocked type is a property on that object. For example, in the case of Moq, you have to use mock.Object to access the mock itself.

Controlling Mock Behaviors

In this section, I'll describe some of the basic things you can do to configure mocks and stubs that are available in all the mocking frameworks. Some frameworks, particularly the Unconstrained types, provide addition functionality beyone these, so make sure to get up to speed on the specific framework you've selected.

Almost all frameworks you a fluent-style builder pattern when configuring your mocks. This means you chain a set of method calls together to build out how you want the mock to work.

First and foremost, for frameworks that differenciate between mocks and stubs, you will need to start your configuration by stating if it's a mock or a stub (see the "Mocks vs Stubs" section above for the difference). Usually that with a .Mock(...), Expect(...) or .Stub(...) method, but the names vary greatly. Frameworks that don't differenciate between mocks and stubs will have a Setup(...), or Fake(...) method. The parameter to these methods is generally a delegate/lambda express for the member being mocked, like this: mock.Stub(x => x.MyFavoriteMethod()).

The primary configuration you'll provide for a mock is the return value. This is usually done via a method named Return(...), to which you provide the value you want returned from the mocked implementation. For example: mock.Mock(x => MyFavoriteMethod()).Return(false).

Change How Many Times to Use the Mock

Once you've setup the mock, you can change how many times it applies in the event your code calls the mocked member more than once. For many, but not all, frameworks, setting up a mock will will cause the mock object to return the provided value the first time it's called with the provided inputs.  Sometimes we want to change this behavior, so the frameworks provide mechanisms to adjust how many times the mock should be used. Note that with a mock, you're potentially setting up expectations for how many times it will be called.

In most frameworks, there's some sort of "Repeat" option you can use, such as these from RhinoMocks: .Repeat.Any(), .Repeat.Once(), .Repeat.Times(10).

mockDAO.Stub(dao => dao.GetRecordFromDatabase(myRecordId))
        .Repeat.Any()
        .Return(recordFromDatabase);

You may also have a Repeat.Never() option, which will set the expectation the member is never called.

Returning Prepared Values Based on the Input Values

When setting up a mock or stub for a method that takes parameters, the default behavior is to only trigger the mocked behavior when the input exactly match the input you used when configuring the mock. So if you define the mock like this: mockDAO.Stub(dao => dao.GetRecordFromDatabase(100)).Return(null), then this mock will only be used when the input value is 100. All other values will act as if the mock was not configured.

This allows you to have different mocked behanviors based on different inputs. So input 100 could return a valid record, while input 200 would return null or throw an exception.

Returning the Prepared Values Regardless of the Input Values

Sometimes you want a mock to apply regaurdless of the input values, and the frameworks provide various mechanisms to do that. The typical case is for you to provide something in the method when defining the mock (so that it knows which method overload you want to mock) and then apply a modifier like .IgnoreArguments(). This will apply the mock for any input.

mockDAO.Stub(dao => dao.GetRecordFromDatabase(myRecordId))
          .IgnoreArguments()
          .Return(recordFromDatabase);

Advanced Argument Constraints

Other times you don't want to be so all-or-none in your configurations, so all of the frameworks provide ways to be much more granular in your argument matching configurations, allowing you to provide very detailed conditions for when to use your return values by defining per-parameter constraints. For example, here I've used RhinoMocks syntax to stipulate any input greater than 0 should use this stub.

mockDAO.Stub(dao => dao.GetRecordFromDatabase(Arg<int>.Is.GreaterThanOrEqual(0)))
       .Return(recordFromDatabase);

Here's an example with more than one parameter: (There's a lot more than this – IntelliSense is your friend)

mockDAO.Stub(dao => dao.GetRecordFromDatabase(
                    Arg<int>.Is.GreaterThanOrEqual(0),
                    Arg<decimal>.Is.NotEqual(2.0),
                    Arg<List<string>>.List.ContainsAll(new List<string> {"foo", "bar"}),
                    Arg<object>.Is.NotNull,
                    Arg<object>.Is.Anything))
        .Return(recordFromDatabase);

Many frameworks allow you to provide a delegate that gets called and returns true/false as to if the parameter meet the critera for the mock. This allows you to maintain your own state and get very complex in your handling. Beware: The more complex you get, the more difficult it is for future maintainers to understand what you're attempting to test and is a codesmall that maybe your code is doing too much and should be simplified.

Providing a Method Implementation / Using the Input Params

Instead of just providing a simple value to return, you can provide a full implementation of the method, typically as a delegate/lambda function. This also allows you to get access to the input parameters.  If you want, you can define a delegate and just call the delegate.  I prefer to use lamdas unless the method is really long.

So instead of my previous stub for GetRecordForDatabase which pre-configured a return value, I can do it on the fly, such as this example using RhinoMock's .Do(...) syntax:

mockDAO.Stub(dao => dao.GetRecordFromDatabase(0))
                .IgnoreArguments()
                .Do((Func<int, ImportantData>)(input => new ImportantData{RecordId = input}));

Mocking Things Other Than Methods

You're not restricted to just mocking methods. You can mock properties (both getters and setters, though the setter syntax is generally not as straight forward) and events. Some Unconstrained frameworks may also allow you mock constructors, as well as static members of classes.

Throwing an Exception Instead

Instead of returning a value, you can use instruct the mock to force an exception, usually with a .Throw(...) method, like this:

mockDAO.Stub(dao => dao.GetRecordFromDatabase(0))
        .IgnoreArguments()
        .Repeat.Any()
        .Throw(new NullReferenceException());         

Testing Non-Public Members

With Constrained frameworks, you can't mock private or protected members, but you can mock internal members if you do a little extra work.

Most of the popular open-source constrained frameworks utilize Castle DynamicProxy to generate an assembly at runtime with the mock logic (see my post How .Net Mocking Frameworks Work Under the Hood for more details). Specifically, you must allow your test assembly and the proxy assembly to access internal members of the assembly under test.  This means adding two InternalsVisibleToAttributes to the AssemblyInfo.cs file of the assembly under test: one for the unit test assembly and one for Castles' DynamicProxyGenAssembly2.  If you're using signed assemblies, you must put the full public key in the attribute.

You can get the public key for an assembly by using the sn -Tp yourAssembly.dll command in the Visual Studio Command Prompt.

For example: (no wrapping -- can't be any spaces in the public key)

[assembly: InternalsVisibleTo("DynamicProxyGenAssembly2, PublicKey=0024000004800000940000000602000000240000525341310004000001000100c547cac37abd99c8db225ef2f6c8a3602f3b3606cc9891605d02baa56104f4cfc0734aa39b93bf7852f7d9266654753cc297e7d2edfe0bac1cdcf9f717241550e0a7b191195b7667bb4f64bcb8e2121380fd1d9d46ad2d92d2d15605093924cceaf74c4861eff62abf69b9291ed0a340e113be11e6a7d3113e92484cf7045cc7")]
[assembly: InternalsVisibleTo("jwright.Blog.UnitTesting, PublicKey=00……ec")]

You may also be interested in my posting about Mocking Objects with Restricted Access (internal/private), which has examples of using reflection to manipulate objects that can’t be mocked with Constrained frameworks.



Unit testing has become an accepted part of our lives as .NET programmers. To help focus our tests to only the code we want to validate, Mocking Frameworks are a powerful tool in our toolbox. Like many tools, if you have an understanding of how the tool works under the hood, you can bend it to your will (and also know where it'll break if you bend too much).

In this post, I provide an overview of the two main types of mocking frameworks: constrained frameworks (like RhinoMocks and Moq) and unconstrained frameworks (such as Typemock Isolator and Telerik JustMock). I'll dig into how the two actually do their magic and some of the pros and cons of each approach.

Terms: Constrained vs Uncontrained

A few years back I read The Art of Unit Testing by Roy Osherove, and in one of the chapters he uses the terms "Constrained" and "Unconstrained" to describe the two main types of mocking frameworks, which I feel are excellent names for the two.

Constrained Frameworks
I'll dig into the "why" later in this post, but contstrained frameworks are "constrained" by the rules of class inheritance within .Net. This means the framework must have visibility to the members you want to mock (ie: you can't mock private members) and the ability to override them (ie: you can't mock sealed or non-abstract/non-virtual members).
Unconstrained Frameworks
Frameworks that fall into this category don't have the same limitations as Constrained frameworks and can pretty much mock anything you care to throw at it.

Contrained Frameworks: Class Inheritance On-Demand

Chances are, if you've used mocking frameworks in .Net, you've likely used one in this category. This is because most of them are free and open-source, so their barriers to entry are set pretty low. Frameworks in this category include Moq, RhinoMocks, NSubstitute, and FakeItEasy. In almost all cases, these frameworks use the Castle DynamicProxy library to do the heavy lifting.

The general idea behind contrained frameworks is that a new proxy class is created at runtime that extends the class (or implements the interface) you're mocking, adding code that will check for the mock behaviors, as well as some infrastructure code.

For example, if you have this class:

    public class MyClass
    {
        public virtual int GenerateNumber(){
            return DateTime.Now.Millisecond;
        }
        
        public virtual int GenerateNumber(int floor){
            return floor + DateTime.Now.Millisecond;
        }
    }

You can generate a mock of GenerateNumber() with code like this (using RhinoMocks syntax):

    var mock = MockRepository.GenerateMock<MyClass>();
    mock.Expect(x => x.GenerateNumber()).Return(11);

Conceptually, when this code runs, it would generate a new proxy class that looked something like this:

    public MyClassMock: MyClass
    {
        public override int GenerateNumber() {
            return 11;
        }
    }

Now, imagine you generated a mock with this syntax:

    var mock = MockRepository.GenerateMock<MyClass>();
    mock.Expect(x => x.GenerateNumber(Arg<int>.Is.GreaterThan(10)))
        .Return(11)
        .Repeat.Twice;

The resulting generated code would start to get more complex -- something like this:

    public MyClassMock: MyClass
    {
        private callCounter = 0;

        public override int GenerateNumber(int floor) {
            
            //Check the conditions:
            if (floor > 10) //Arg<int>.Is.GreaterThan(10)
            {
                callCounter++;
                if (callCounter < 2)  //.Repeat.Twice
                {
                    return 11;
                }
            } 
            
            //All other cases, use the real implementation.
            return base.GenerateNumber(floor);            
        }
    }

If you create a strict mock, any unmocked methods would end up with something like this:

    public override int GenerateNumber() {
        throw new NotImplementedException("Not mocked");
    }

Or for a dynamic mock, like this:

    public override int GenerateNumber() {
        return default(int);
    }

But certainly, it ends up being a lot more complicated. For example, since you later will likely want to call mock.VerifyWasCalled(...), the proxy method will need to maintain information about when it was called and what parameters were provided. You as the caller could have multiple mock implementations based on input args, etc, etc. So there's a fair amount of additional logic that goes into these proxies, but you get the general idea.

The main thing to take away is that the proxy objects used to implement the mocks rely on inheritance to put a middle layer between the calling code and the class being mocked. Because of this, only things that can be inherited can be mocked. Classes that are sealed and members that are static or private cannot be mocked. Members that are internal can be mocked, but only if you add an [InternalsVisibleTo(...)] attribute for the proxy assembly. Since almost all of these frameworks use Castle DynamicProxy (which generates an in-memory assembly named "DynamicProxyGenAssembly2"), that would look like this:

[assembly: InternalsVisibleTo("DynamicProxyGenAssembly2, PublicKey=0024000004800000940000000602000000240000525341310004000001000100c547cac37abd99c8db225ef2f6c8a3602f3b3606cc9891605d02baa56104f4cfc0734aa39b93bf7852f7d9266654753cc297e7d2edfe0bac1cdcf9f717241550e0a7b191195b7667bb4f64bcb8e2121380fd1d9d46ad2d92d2d15605093924cceaf74c4861eff62abf69b9291ed0a340e113be11e6a7d3113e92484cf7045cc7")]

Impact on the design of your own classes

In many cases, in order to fully test your own classes and mock the external dependancies, you will find yourself in a situation where you have to make design tradeoffs to allow the member to be mocked. For example, you may need to make something virtual or internal that you would otherwise not expose for modification or use outside the class. So you must make the choice between testablity and usability of the classes. This can be a hard choice to make.

Uncontrained Frameworks: Make Your Own Man-In-The-Middle Attack

While Constrained frameworks must live within the rules of .Net inheritance, Unconstrained frameworks have almost no limitations as to what they can mock. This is because unconstrained frameworks don't create new classes -- instead, they modify the existing code, even if it's not your code, at runtime to inject their own logic.

To do this, they utilize the .Net Profiling APIs. Using these APIs, a "profiler" (in this case, the mocking framework) registers with the .Net CLR so that the CLR executes a callback within the profiler every time the CLR loads a class, compiles a function, etc. These frameworks will then wait for the JITCompilationStarted callback, which the CLR will use just before it JIT-compiles something. The callback can then use the ICorProfilerInfo::SetILFunctionBody to modify the MSIL for the method being JIT-compiled and inject it's own logic.

Frameworks in this category include Telerik JustMock, Typemock Isolator, and Microsoft Fakes. These are all (to the best of my knowledge) closed-source, commercial (licensed) software. Since most of these frameworks are closed-source, and their license do not permit me to decompile their assemblies, I'm making some guesses here as to how they are implemented, but for the purposes of understanding how they work, I believe this is fine.

Going back to our example class:

public class MyClass
{
    public virtual int GenerateNumber(){
        return DateTime.Now.Millisecond;
    }

    public virtual int GenerateNumber(int floor){
        return floor + DateTime.Now.Millisecond;
    }
}

You can generate a mock of GenerateNumber() with code like this (using Typemock Isolator syntax):

    var mock = Isolate.Fake.Instance<MyClass>();
    Isolate.WhenCalled(() => mock.GenerateNumber()).WillReturn(11);

When the MyClass.GenerateNumber() method is being compiled by the CLR JIT-er, it will call into the profiler (in this case, a Typemock Isolator service that is running), which will re-write the code to insert some hooks, like this:

    public MyClass
    {

        private bool _mockWasProvided() 
        {
            // Would determine if a mock was configured
            // for this specific method call or not.
            // This may need to consider argment filters,
            // "Repeat.Once" directives, etc.
            return true; 
        }

        private int _userProvidedMock() {
            // This would be the user-configured
            // mock behavior.
            return 11;
        }

        public virtual int GenerateNumber() {
            
            //START injected code
            if (_mockWasProvided())
            {
                return _userProvidedMock()
            }
            //END injected code
            
            return DateTime.Now.Millisecond;
        }
    }

Just like the examples I provided in the constrained frameworks, this can get pretty complicated, as it needs to deal with advanced input matching filters, capturing data needed to verify calls were actually made using Isolate.Verify.WasCalled(...);, etc.

An important thing to note: In order for these to work, a profiler application must be running and registered with the CLR's Profiler API before any of your code is loaded into the AppDomain, otherwise it will already be JIT'd and your mocks won't be injected. This generally requires a service or application from the mocking framework be installed on the system and running in the background. Since it's basically listening in to every CLR action, this will need to run with elevated privileges, which could represent a security risk. In effect, you are intenationally building your own man-in-the-middle attack against the code you want to mock. This also means you cannot run code that utilizes Unconstrained mocking frameworks unless you have the profiler installed.

Mocking Third-party and Static Code

One big advantage that comes with the Unconstrained frameworks is that you can mock pretty much anything, since the CLR will call into the profiler when any code is being JIT-compiled. This means you can provide mocks for static, sealed, private, etc, members. It also means you can create mocks for Third-party code or even the .Net Framework itself. As you can imagine, you can get yourself into some hot water here. If you provide a mock for a very commonly used .Net framework member and your mock has really bad performance, you're going to have a very negative impact on the performace of everything! For this reason, most of these frameworks don't let you mock out certain very commonly used / high-performance .Net framework code, such as the string constructor.

To put this into concreate examples, think about the ability to mock out calls to DateTime.Now so that you can write unit tests against the code without having to move that into a seperate, mockable (non-private, virtual) method. You could unit test for Leap Year scenarios, end-of-month reports, etc:

  Isolate.WhenCalled(() => DateTime.Now).WillReturn(new DateTime(2016, 2, 29));

Other common scenarios include:

  • SharePoint and EntityFramework SDKs
  • HttpContext objects in ASP.NET
  • classes using the Singleton pattern.

Putting it into Perspective

Constrained frameworks can cover a major portion of most peoples needs, especially for greenfield (new development work) projects and have the major benefit of being free. Unconstrained frameworks can cover just about anything and remove the need to expose parts of your code just so they can be tested. But they can be fairly expensive.

Here's the big breakdown, in table form.

Constrained Unconstrained
Members:
methods
properties
events
static
sealed
Access:
public
internal
private
Features:
Your own code
Third-Party Code
.NET Framework
Open Source
License Open Source Proprietary
Cost Free $$$*
Additional Software Profiler Application
Commercial Support Limited With Paid License
May force design tradeoffs
Best fit for code written with testing in mind. Great for testing "legacy" code.

* At the time I wrote this, prices were as follows:

  • Telerik JustMock: $399 per developer
  • Typemock Isolator: $399 per developer, $990 per 5 build servers
  • Visual Studio Enterprise (MS Fakes): $5,999 per developer ($2,569 renewals). MS Fakes is only available as part of VS Enterprise and is not sold seperately.


I've been a big fan of Rhino.Mocks for many years, but the original Rhino.Mocks maintainer (Oren Eini aka Ayende) stated in 2013 that he is no longer maintaining the project and has handed the project over to a new maintainer (Mike Meisinger), but Mike has not been actively maintaining the project. It's effectively dead and very unlikely to support future versions of .NET, including .NET core.

A while back I wrote a Quick Guide to Using Rhino.Mocks and in last week's post, I gave an overview of Moq for Rhino.Mocks Users. While Moq seems to be the current front runner as a Rhino.Mocks replacement, there's another player in town with NSubstitute. So in the same vane as my Moq post, I'm providing a guide to help people make the transition (or at least help with a review) from Rhino.Mocks to NSubstitue.

The format of this post will follow that of the others to help with cross-referencing between them.

Overall, I'm not a big fan of NSubstitute's syntax, as it looks too similar to just calls to the methods themselves, which:

  • makes it harder to understand, as the reader must know which calls are setting up stubs and which are real calls.
  • makes it much more likely you'll accidentally do the wrong thing and call a real method.

Now, if you're only mocking interfaces, then this wouldn't be an issue -- but rarely do I find a codebase that allows everything to be an interface.

Generating Different Mock Types

By default, NSubstitue's .For<T> method generates something close to a Dynamic mock -- that is, if you don't explicitly provide a return value, it will return the default value for the data type of the return value if it's a value type (0 for numbers, false for bools, empty string for string), but for object types, it's a bit more complicated. From their documentation:

...any properties or methods that return an interface, delegate, or purely virtual class* will automatically return substitutes themselves.

A pure virtual class is defined as one with all its public methods and properties defined as virtual or abstract and with a default, parameterless constructor defined as public or protected.

So it won't return null in those cases. Instead, it creates a new mock of the return type. Otherwise, it will return null.

Be careful, thought -- if you are using a real class (not an interface), it will call the underlying methods if they aren't virtual.

That said, there is the concept of a partial mock using .ForPartsOf<T> -- that is, a mock that can use the underlying object's implementation. However, to prevent calling the real methods for the methods you do want to mock, you must use the syntax described in the Advanced Argument Constraints section below.

Rhino.Mocks NSubstitute
Dynamic-ish Mock IFoo mock = MockRepository.GenerateMock<IFoo>(); IFoo substitute = Substitute.For<IFoo>();
Partial Mock IFoo mock = MockRepository.GeneratePartialMock<IFoo>(); IFoo substitute = Substitue.ForPartsOf<IFoo>();

There isn't an option to create a Strict mock in NSubstitute, and it looks like there won't ever be one.

Passing Constructor Arguments

If you need to pass arguments to the constructor of the class you're trying to mock, there are overloads to allow you to do that.

Rhino.Mocks NSubstitute
IFoo mock = MockRepository.GenerateMock<SomeClass>(param1, param2); Substitue.ForPartsOf<SomeClass>()(param1, param2);

Stubs vs Mocks (or not)

The syntax for NSubstitute is a little different than the others. In most cases, there's not an explicit method you call to create the mock/stub -- rather, you basically call the method you want to create a substitue for. Also, there's no distinction between a mock and a stub -- they're all just substitutions. But since they don't create _expectations_, I would classify them as stubs.

You use the .Returns() method to setup the stub. Note that you can provide multiple values to .Returns() which will set up a chain of calls, like this: .Returns(valueForFirstCall, valueForSecondCall, valueForThirdCall). This works for methods and properties.

For Methods:

Rhino.Mocks NSubstitute
Stub mock.Stub(x => x.SomeMethod()).Return(true); substitute.SomeMethod().Returns(true);
Mock mock.Expect(x => x.SomeMethod()).Return(true); (not supported)

For Properties:

Rhino.Mocks NSubstitute
Stub mock.Stub(x => x.SomeProperty).Return(true); substitute.SomeProperty.Returns(true);
or
substitute.SomeProperty = true;
Mock mock.Expect(x => x.SomeProperty).Return(true); (not supported)

Unlike Rhino.Mocks, however, if some other code sets the property value to something else, NSubstitute's stub will return the new value, not the value you stipulated in your stub. In other words, NSubstitue's properties will act like regular properties, and your stub just sets an initial value.

Verifying expectations

Since you're not creating expectations with NSubstitute, there are no mass validation options. Instead, you need to check each stub (which, being more explicit, is probably the better route anyway).

For Methods:

Rhino.Mocks NSubstitute
Called mock.AssertWasCalled(x => x.SomeMethod()); substitute.Received().SomeMethod();
Called a specific number of times mock.AssertWasCalled(x => x.SomeMethod(), options => options.Repeat.Times(2) ); substitute.Received(2).SomeMethod();
Not called mock.AssertWasNotCalled(x => x.SomeMethod()); substitute.DidNotReceive().SomeMethod();
or
substitute.DidNotReceiveWithAnyArgs().SomeMethod();

For Properties:

Rhino.Mocks NSubstitute
Get mock.AssertWasCalled(x => x.SomeProperty); var temp = substitue.Received().SomeProperty;
Set mock.AssertWasCalled(x => x.SomeProperty = true); substitue.Received().SomeProperty = true;
Not called mock.AssertWasNotCalled(x => x.SomeProperty = true); substitue.DidNotReceive().SomeProperty = true;
or
substitue.DidNotReceiveWithAnyArgs().SomeProperty = true;

Note that for the getter, you must set a variable to the return value to prevent a compiler error.

The WithAnyArgs versions will ignore whatever parameters (for methods) or set values (for properties) you use in your check and will verify against any inputs.

Advanced Argument Constraints

Both frameworks provide ways to put advanced constraints on the arguments that trigger a mock. Below are examples -- you'll need to consult the framework documentation for the full list of available constraints.

Rhino.Mocks

 mock.Stub(dao => dao.GetRecordFromDatabase(
                         Arg<int>.Is.GreaterThanOrEqual(0),
                         Arg<decimal>.Is.NotEqual(2.0),
                         Arg<List<string>>.List.ContainsAll(new List<string> { "foo", "bar" }),
                         Arg<object>.Is.NotNull,
                         Arg<object>.Is.Anything))
                 .Return(recordFromDatabase);

NSubstitute

  substitute.GetRecordFromDatabase(
                         Arg.Is<int>(i => i >= 0),
                         Arg.Is<decimal>(d => d != 2.0m),
                         Arg.Do<int>(x => capturedValue = x ),
                         Arg.Any<object>()))
         .Returns(recordFromDatabase);

Or, to ignore the arguments altogether, use .ReturnsForAnyArgs(). This is similar to Rhino.Mocks' .IgnoreArguments() method.

Providing a Method Implementation / Using the Input Params

For Rhino.Mocks, in order to provide an alternate implementation, you use the .Do() method to provide a delegate. In NSubstitue, you provide a delegate to the .Returns() call.

Rhino.Mocks

  mock.Stub(dao => dao.GetRecordFromDatabase(0))
                .IgnoreArguments()
                .Repeat.Any()
                .Do((Func<int, ImportantData>)(input => new ImportantData
                {
                    Name = "Orignal Name",
                    RecordId = input
                }));

NSubstitute

           
            substitute.GetRecordFromDatabase(0)
                      .ReturnsForAnyArgs(input => new ImportantData
                       {
                           Name = "Orignal Name",
                           RecordId = input.ArgAt<int>(0)
                       });

Throwing an Exception Instead

If the return type of the method is not void, you just provide a delegate to .Returns() that throws an exception. However, if the return type is void, then you use a .When().Do() syntax instead:

Rhino.Mocks NSubstitute
mock.Stub(x => x.SomeMethod()).Throw(new Exception("POW!")); substitute.SomeMethod().Returns(x => { throw new Exception("POW!"); });
or
substitute.When(x => x.VoidMethod()).Do(x => { throw new Exception("POW!"); });

Testing Non-Public Members

With Rhino.Mocks, you can’t mock private or internal members, but you can mock internal members if you add an InternalsVisibleTo attribute for the Castle dynamic proxy assembly. NSubstitute also uses the same proxy, so you'll still need to add the attribute. See the Moq Quickstart Guide for details on how to do this.