How do you unit test private methods

Question

I m building a class library that will have some public  amp  private methods  I want to be able to unit test the private methods  mostly while developing  but also it could be useful for future refactoring    What is the correct way to do this

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If you want to unit test a private method  something may be wrong  Unit tests are  generally speaking  meant to test the interface of a class  meaning its public  and protected  methods  You can of course  hack  a solution to this  even if just by making the methods public   but you may also want to consider    If the method you d like to test is really worth testing  it may be worth to move it into its own class  Add more tests to the public methods that call the private method  testing the private method s functionality   As the commentators indicated  you should only do this if these private methods s functionality is really a part in with the public interface  If they actually perform functions that are hidden from the user  i e  the unit test   this is probably bad

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In the rare cases I have wanted to test private functions  I have usually modified them to be protected instead  and the I have written a subclass with a public wrapper function   The Class        protected void APrivateFunction                      Subclass for testing         Test  public void TestAPrivateFunction         APrivateFunction          or whatever testing code you want here

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I don t agree with the  you should only be interested in testing the external interface  philosophy   It s a bit like saying that a car repair shop should only have tests to see if the wheels turn   Yes  ultimately I m interested in the external behavior but I like my own  private  internal tests to be a bit more specific and to the point   Yes  if I refactor  I may have to change some of the tests  but unless it s a massive refactor  I ll only have to change a few and the fact that the other  unchanged  internal tests still work is a great indicator that the refactoring has been successful   You can try to cover all internal cases using only the public interface and theoretically it s possible to test every internal method  or at least every one that matters  entirely by using the public interface but you may have to end up standing on your head to achieve this and the connection between the test cases being run through the public interface and the internal portion of the solution they re designed to test may be difficult or impossible to discern   Having pointed  individual tests that guarantee that the internal machinery is working properly is well worth the minor test changes that come about with refactoring - at least that s been my experience   If you have to make huge changes to your tests for every refactoring  then maybe this doesn t make sense  but in that case  maybe you ought to rethink your design entirely   A good design should be flexible enough to allow for most changes without massive redesigns

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You should not be testing the private methods of your code in the first place  You should be testing the  public interface  or API  the public things of your classes  The API are all the public methods you expose to outside callers   The reason is that once you start testing the private methods and internals of your class you are coupling the implementation of your class  the private things  to your tests  This means that when you decide to change your implementation details you will also have to change your tests   You should for this reason avoid using InternalsVisibleToAtrribute    Here is a great talk by Ian Cooper which covers this subject  Ian Cooper  TDD  where did it all go wrong

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I ve also used the InternalsVisibleToAttribute method  It s worth mentioning too that  if you feel uncomfortable making your previously private methods internal in order to achieve this  then maybe they should not be the subject of direct unit tests anyway   After all  you re testing the behaviour of your class  rather than it s specific implementation - you can change the latter without changing the former and your tests should still pass

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A way to do this is to have your method protected and write a test fixture which inherits your class to be tested  This way  you are nor turning your method public  but you enable the testing

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Private types  internals and private members are so because of some reason  and often you don   t want to mess with them directly  And if you do  chances are that you   ll break later  because there is no guarantee that the guys who created those assemblies will keep the private internal implementations as such   But  at times  when doing some hacks exploration of compiled or third party assemblies  I have myself ended up wanting to initialize a private class or a class with a private or internal constructor  Or  at times  when dealing with pre-compiled legacy libraries that I can   t change - I end up writing some tests against a private method   Thus born the AccessPrivateWrapper - http   amazedsaint blogspot com 2010 05 accessprivatewrapper-c-40-dynamic html -  it s is a quick wrapper class that   ll make the job easy using C  4 0 dynamic features and reflection   You can create internal private types like        Note that the wrapper is dynamic     dynamic wrapper   AccessPrivateWrapper FromType          typeof SomeKnownClass  Assembly  ClassWithPrivateConstructor           Access the private members     wrapper PrivateMethodInPrivateClass

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I want to create a clear code example here which you can use on any class in which you want to test private method    In your test case class just include these methods and then employ them as indicated                    var Class name of class you want to test private methods in      note  the actual class and the private variable to store the       class instance in  should at least be different case so that      they do not get confused in the code   Here the class name is      is upper case while the private instance variable is all lower      case         private  class name of class you want to test private methods in              This uses reflection to be able to get private methods to test       param  methodName       return ReflectionMethod         protected static function getMethod  methodName         class   new ReflectionClass  Class name of class you want to test private methods in         method    class- gt getMethod  methodName        method- gt setAccessible true       return  method                  Uses reflection class to call private methods and get return values        param  methodName       param array  params       return mixed           usage       this- gt  callMethod   someFunctionName   array param1 param2 param3           params are in        order in which they appear in the function declaration          protected function  callMethod  methodName   params array           method   self  getMethod  methodName       return  method- gt invokeArgs  this- gt class name of class you want to test private methods in   params          this-  callMethod   someFunctionName   array param1 param2 param3     Just issue the parameters in the order that they appear in the original private function

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I tend not to use compiler directives because they clutter things up quickly  One way to mitigate it if you really need them is to put them in a partial class and have your build ignore that  cs file when making the production version

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A way to do this is to have your method protected and write a test fixture which inherits your class to be tested  This way  you are nor turning your method public  but you enable the testing

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If you are using  net  you should use the InternalsVisibleToAttribute

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For anyone who wants to run private methods without all the fess and mess  This works with any unit testing framework using nothing but good old Reflection   public class ReflectionTools          If the class is non-static     public static Object InvokePrivate Object objectUnderTest  string method  params object   args                Type t   objectUnderTest GetType            return t InvokeMember method              BindingFlags InvokeMethod               BindingFlags NonPublic               BindingFlags Instance               BindingFlags Static              null              objectUnderTest              args                if the class is static     public static Object InvokePrivate Type typeOfObjectUnderTest  string method  params object   args                MemberInfo   members   typeOfObjectUnderTest GetMembers BindingFlags NonPublic   BindingFlags Static           foreach var member in members                        if  member Name    method                                return typeOfObjectUnderTest InvokeMember method  BindingFlags NonPublic   BindingFlags Static   BindingFlags InvokeMethod  null  typeOfObjectUnderTest  args                                   return null            Then in your actual tests  you can do something like this   Assert AreEqual     ReflectionTools InvokePrivate      typeof StaticClassOfMethod         PrivateMethod        Expected Result     Assert AreEqual     ReflectionTools InvokePrivate      new ClassOfMethod          PrivateMethod        Expected Result

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I tend not to use compiler directives because they clutter things up quickly  One way to mitigate it if you really need them is to put them in a partial class and have your build ignore that  cs file when making the production version

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Here s an example  first the method signature   private string   SplitInternal         return Regex Matches Format                                                     Cast lt Match gt                             Select m   gt  m Value                           Where s   gt   string IsNullOrEmpty s                            ToArray        Here s the test        lt summary gt     A test for SplitInternal     lt  summary gt   TestMethod     DeploymentItem  Git XmlLib vs2008 dll    public void SplitInternalTest         string path    pair path to  Key  0   Items Item Name  1   Date       object   values   new object     2   Martin         XPathString xp   new XPathString path  values        PrivateObject param0   new PrivateObject xp       XPathString Accessor target   new XPathString Accessor param0       string   expected   new string              pair path to  Key  0              Items            Item Name  1              Date             string   actual      actual   target SplitInternal        CollectionAssert AreEqual expected  actual

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1  If you have a legacy code then the only way to test private methods is by reflection   2  If it is new code then you have the following options    Use reflection  to complicated  Write unit test in the same class  makes the production code ugly by having test code also in it  Refactor and make the method public in some kind of util class Use  VisibleForTesting annotation and remove private   I prefer the annotation method  simplest and least complicated  The only issue is that we have increased the visibility which I think is not a big concern  We should always be coding to interface  so if we have an interface MyService and an implementation MyServiceImpl then we can have the corresponding test classes that is MyServiceTest  test interface methods  and MyServiceImplTest  test private methods   All clients should anyway be using the interface so in a way even though the visibility of the private method has been increased it should not really matter

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For anyone who wants to run private methods without all the fess and mess  This works with any unit testing framework using nothing but good old Reflection   public class ReflectionTools          If the class is non-static     public static Object InvokePrivate Object objectUnderTest  string method  params object   args                Type t   objectUnderTest GetType            return t InvokeMember method              BindingFlags InvokeMethod               BindingFlags NonPublic               BindingFlags Instance               BindingFlags Static              null              objectUnderTest              args                if the class is static     public static Object InvokePrivate Type typeOfObjectUnderTest  string method  params object   args                MemberInfo   members   typeOfObjectUnderTest GetMembers BindingFlags NonPublic   BindingFlags Static           foreach var member in members                        if  member Name    method                                return typeOfObjectUnderTest InvokeMember method  BindingFlags NonPublic   BindingFlags Static   BindingFlags InvokeMethod  null  typeOfObjectUnderTest  args                                   return null            Then in your actual tests  you can do something like this   Assert AreEqual     ReflectionTools InvokePrivate      typeof StaticClassOfMethod         PrivateMethod        Expected Result     Assert AreEqual     ReflectionTools InvokePrivate      new ClassOfMethod          PrivateMethod        Expected Result

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CC -Dprivate public    CC  is the command line compiler on the system I use  -Dfoo bar does the equivalent of  define foo bar  So  this compilation option effectively changes all private stuff to public

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If you are using  net  you should use the InternalsVisibleToAttribute

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For JAVA language  Here  you can over-ride a particular method of the testing class with mock behavior    For the below code   public class ClassToTest        public void methodToTest                 Integer integerInstance   new Integer 0           boolean returnValue  methodToMock integerInstance           if returnValue                        System out println  methodToMock returned true                      else                       System out println  methodToMock returned true                      System out println              private boolean methodToMock int value                return true            Test class would be   public class ClassToTestTest        Test     public void testMethodToTest             new Mockup lt ClassToTest gt                  Mock             private boolean methodToMock int value                   return true

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You could generate the test method for the private method from Visual studio 2008  When you create a unit test for a private method  a Test References folder is added to your test project and an accessor is added to that folder  The accessor is also referred to in the logic of the unit test method  This accessor allows your unit test to call private methods in the code that you are testing  For details have a look at  http   msdn microsoft com en-us library bb385974 aspx

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There are 2 types of private methods  Static Private Methods and Non Static Private methods Instance Methods   The following 2 articles explain how to unit test private methods with examples    Unit Testing Static Private Methods Unit Testing Non Static Private Methods

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I ve also used the InternalsVisibleToAttribute method  It s worth mentioning too that  if you feel uncomfortable making your previously private methods internal in order to achieve this  then maybe they should not be the subject of direct unit tests anyway   After all  you re testing the behaviour of your class  rather than it s specific implementation - you can change the latter without changing the former and your tests should still pass

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CC -Dprivate public    CC  is the command line compiler on the system I use  -Dfoo bar does the equivalent of  define foo bar  So  this compilation option effectively changes all private stuff to public

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I ve also used the InternalsVisibleToAttribute method  It s worth mentioning too that  if you feel uncomfortable making your previously private methods internal in order to achieve this  then maybe they should not be the subject of direct unit tests anyway   After all  you re testing the behaviour of your class  rather than it s specific implementation - you can change the latter without changing the former and your tests should still pass

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Well you can unit test private method in two ways   you can create instance of PrivateObject class the syntax is as follows  PrivateObject obj  new PrivateObject PrivateClass     now with this obj you can call the private method of PrivateCalss  obj PrivateMethod  Parameters     You can use reflection   PrivateClass obj   new PrivateClass       Class containing private obj Type t   typeof PrivateClass    var x   t InvokeMember  PrivateFunc        BindingFlags InvokeMethod   BindingFlags NonPublic   BindingFlags Public             BindingFlags Instance  null  obj  new object     5

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I ve also used the InternalsVisibleToAttribute method  It s worth mentioning too that  if you feel uncomfortable making your previously private methods internal in order to achieve this  then maybe they should not be the subject of direct unit tests anyway   After all  you re testing the behaviour of your class  rather than it s specific implementation - you can change the latter without changing the former and your tests should still pass

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CC -Dprivate public    CC  is the command line compiler on the system I use  -Dfoo bar does the equivalent of  define foo bar  So  this compilation option effectively changes all private stuff to public

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On CodeProject  there is an article that briefly discusses pros and cons of testing private methods   It then provides some reflection code to access private methods  similar to the code Marcus provides above    The only issue I ve found with the sample is that the code doesn t take into account overloaded methods   You can find the article here   http   www codeproject com KB cs testnonpublicmembers aspx

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MS Test has a nice feature built in that makes private members and methods available in the project by creating a file called VSCodeGenAccessors   System Diagnostics DebuggerStepThrough         System CodeDom Compiler GeneratedCodeAttribute  Microsoft VisualStudio TestTools UnitTestGeneration    1 0 0 0        internal class BaseAccessor                protected Microsoft VisualStudio TestTools UnitTesting PrivateObject m privateObject           protected BaseAccessor object target  Microsoft VisualStudio TestTools UnitTesting PrivateType type                        m privateObject   new Microsoft VisualStudio TestTools UnitTesting PrivateObject target  type                      protected BaseAccessor Microsoft VisualStudio TestTools UnitTesting PrivateType type                                this null  type                               internal virtual object Target                       get                               return m privateObject Target                                   public override string ToString                         return this Target ToString                       public override bool Equals object obj                        if  typeof BaseAccessor  IsInstanceOfType obj                                 obj     BaseAccessor  obj   Target                            return this Target Equals obj                      public override int GetHashCode                         return this Target GetHashCode                      With classes that derive from BaseAccessor  such as    System Diagnostics DebuggerStepThrough     System CodeDom Compiler GeneratedCodeAttribute  Microsoft VisualStudio TestTools UnitTestGeneration    1 0 0 0    internal class SomeClassAccessor   BaseAccessor        protected static Microsoft VisualStudio TestTools UnitTesting PrivateType m privateType   new Microsoft VisualStudio TestTools UnitTesting PrivateType typeof global  Namespace SomeClass         internal SomeClassAccessor global  Namespace Someclass target            base target  m privateType                   internal static string STATIC STRING               get                       string ret     string  m privateType GetStaticField  STATIC STRING                  return ret                    set                       m privateType SetStaticField  STATIC STRING   value                        internal int memberVar              get                       int ret     int  m privateObject GetField  memberVar                  return ret                    set                       m privateObject SetField  memberVar   value                        internal int PrivateMethodName int paramName                object   args   new object                 paramName           int ret    int  m privateObject Invoke  PrivateMethodName   new System Type                     typeof int    args             return ret

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I m surprised nobody has said this yet  but a solution I have employed is to make a static method inside the class to test itself   This gives you access to everything public and private to test with   Furthermore  in a scripting language  with OO abilities  like Python  Ruby and PHP   you can make the file test itself when run   Nice quick way of making sure your changes didn t break anything   This obviously makes a scalable solution to testing all your classes   just run them all    you can also do this in other languages with a void main which always runs its tests as well

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In the rare cases I have wanted to test private functions  I have usually modified them to be protected instead  and the I have written a subclass with a public wrapper function   The Class        protected void APrivateFunction                      Subclass for testing         Test  public void TestAPrivateFunction         APrivateFunction          or whatever testing code you want here

User · Answer

You could generate the test method for the private method from Visual studio 2008  When you create a unit test for a private method  a Test References folder is added to your test project and an accessor is added to that folder  The accessor is also referred to in the logic of the unit test method  This accessor allows your unit test to call private methods in the code that you are testing  For details have a look at  http   msdn microsoft com en-us library bb385974 aspx

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In my opinion you should only unit test your classe s public API    Making a method public  in order to unit test it  breaks encapsulation exposing implementation details   A good public API solves an immediate goal of the client code and solves that goal completely

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I m surprised nobody has said this yet  but a solution I have employed is to make a static method inside the class to test itself   This gives you access to everything public and private to test with   Furthermore  in a scripting language  with OO abilities  like Python  Ruby and PHP   you can make the file test itself when run   Nice quick way of making sure your changes didn t break anything   This obviously makes a scalable solution to testing all your classes   just run them all    you can also do this in other languages with a void main which always runs its tests as well

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Also note that the InternalsVisibleToAtrribute has a requirement that your assembly be strong named  which creates it s own set of problems if you re working in a solution that had not had that requirement before   I use the accessor to test private methods   See this question that for an example of this

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Sometimes  it can be good to test private declarations  Fundamentally  a compiler only has one public method  Compile  string outputFileName  params string   sourceSFileNames    I m sure you understand that would be difficult to test such a method without testing each  hidden  declarations   That s why we have created Visual T   to make easier tests  It s a free  NET programming language  C  v2 0 compatible    We have added   -  operator  It just behave like     operator  except you can also access any hidden declaration from your tests without changing anything in your tested project   Take a look at our web site  download it for free

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You could also declare it as public or internal  with InternalsVisibleToAttribute  while building in debug-Mode            lt summary gt          This Method is private           lt  summary gt   if DEBUG     public  else     private  endif     static string MyPrivateMethod                 return  false           It bloats the code  but it will be private in a release build

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MS Test has a nice feature built in that makes private members and methods available in the project by creating a file called VSCodeGenAccessors   System Diagnostics DebuggerStepThrough         System CodeDom Compiler GeneratedCodeAttribute  Microsoft VisualStudio TestTools UnitTestGeneration    1 0 0 0        internal class BaseAccessor                protected Microsoft VisualStudio TestTools UnitTesting PrivateObject m privateObject           protected BaseAccessor object target  Microsoft VisualStudio TestTools UnitTesting PrivateType type                        m privateObject   new Microsoft VisualStudio TestTools UnitTesting PrivateObject target  type                      protected BaseAccessor Microsoft VisualStudio TestTools UnitTesting PrivateType type                                this null  type                               internal virtual object Target                       get                               return m privateObject Target                                   public override string ToString                         return this Target ToString                       public override bool Equals object obj                        if  typeof BaseAccessor  IsInstanceOfType obj                                 obj     BaseAccessor  obj   Target                            return this Target Equals obj                      public override int GetHashCode                         return this Target GetHashCode                      With classes that derive from BaseAccessor  such as    System Diagnostics DebuggerStepThrough     System CodeDom Compiler GeneratedCodeAttribute  Microsoft VisualStudio TestTools UnitTestGeneration    1 0 0 0    internal class SomeClassAccessor   BaseAccessor        protected static Microsoft VisualStudio TestTools UnitTesting PrivateType m privateType   new Microsoft VisualStudio TestTools UnitTesting PrivateType typeof global  Namespace SomeClass         internal SomeClassAccessor global  Namespace Someclass target            base target  m privateType                   internal static string STATIC STRING               get                       string ret     string  m privateType GetStaticField  STATIC STRING                  return ret                    set                       m privateType SetStaticField  STATIC STRING   value                        internal int memberVar              get                       int ret     int  m privateObject GetField  memberVar                  return ret                    set                       m privateObject SetField  memberVar   value                        internal int PrivateMethodName int paramName                object   args   new object                 paramName           int ret    int  m privateObject Invoke  PrivateMethodName   new System Type                     typeof int    args             return ret

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For JAVA language  Here  you can over-ride a particular method of the testing class with mock behavior    For the below code   public class ClassToTest        public void methodToTest                 Integer integerInstance   new Integer 0           boolean returnValue  methodToMock integerInstance           if returnValue                        System out println  methodToMock returned true                      else                       System out println  methodToMock returned true                      System out println              private boolean methodToMock int value                return true            Test class would be   public class ClassToTestTest        Test     public void testMethodToTest             new Mockup lt ClassToTest gt                  Mock             private boolean methodToMock int value                   return true

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In the rare cases I have wanted to test private functions  I have usually modified them to be protected instead  and the I have written a subclass with a public wrapper function   The Class        protected void APrivateFunction                      Subclass for testing         Test  public void TestAPrivateFunction         APrivateFunction          or whatever testing code you want here

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I think a more fundamental question should be asked is that why are you trying to test the private method in the first place  That is a code smell that you re trying to test the private method through that class  public interface whereas that method is private for a reason as it s an implementation detail  One should only be concerned with the behaviour of the public interface not on how it s implemented under the covers    If I want to test the behaviour of the private method   by using common refactorings  I can extract its code into another class  maybe with package level visibility so ensure it s not part of a public API   I can then test its behaviour in isolation   The product of the refactoring means that private method is now a separate class that has become a collaborator to the original class  Its behaviour will have become well understood via its own unit tests    I can then mock its behaviour when I try to test the original class so that I can then concentrate on test the behaviour of that class  public interface rather than having to test a combinatorial explosion of the public interface and the behaviour of all its private methods    I see this analogous to driving a car  When I drive a car I don t drive with the bonnet up so I can see that the engine is working  I rely on the interface the car provides  namely the rev counter and the speedometer to  know the engine is working  I rely on the fact that the car actually moves when I press the gas pedal  If I want to test the engine I can do checks on that in isolation   D  Of course testing private methods directly may be a last resort if you have a legacy application but I would prefer that legacy code is refactored to enable better testing  Michael Feathers has written a great book on this very subject  http   www amazon co uk Working-Effectively-Legacy-Robert-Martin dp 0131177052

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Private types  internals and private members are so because of some reason  and often you don   t want to mess with them directly  And if you do  chances are that you   ll break later  because there is no guarantee that the guys who created those assemblies will keep the private internal implementations as such   But  at times  when doing some hacks exploration of compiled or third party assemblies  I have myself ended up wanting to initialize a private class or a class with a private or internal constructor  Or  at times  when dealing with pre-compiled legacy libraries that I can   t change - I end up writing some tests against a private method   Thus born the AccessPrivateWrapper - http   amazedsaint blogspot com 2010 05 accessprivatewrapper-c-40-dynamic html -  it s is a quick wrapper class that   ll make the job easy using C  4 0 dynamic features and reflection   You can create internal private types like        Note that the wrapper is dynamic     dynamic wrapper   AccessPrivateWrapper FromType          typeof SomeKnownClass  Assembly  ClassWithPrivateConstructor           Access the private members     wrapper PrivateMethodInPrivateClass

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MbUnit got a nice wrapper for this called Reflector     Reflector dogReflector   new Reflector new Dog     dogReflector Invoke  DreamAbout   DogDream Food     You can also set and get values from properties  dogReflector GetProperty  Age      Regarding the  test private  I agree that   in the perfect world  there is no point in doing private unit tests  But in the real world you might end up wanting to write private tests instead of refactoring code

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1  If you have a legacy code then the only way to test private methods is by reflection   2  If it is new code then you have the following options    Use reflection  to complicated  Write unit test in the same class  makes the production code ugly by having test code also in it  Refactor and make the method public in some kind of util class Use  VisibleForTesting annotation and remove private   I prefer the annotation method  simplest and least complicated  The only issue is that we have increased the visibility which I think is not a big concern  We should always be coding to interface  so if we have an interface MyService and an implementation MyServiceImpl then we can have the corresponding test classes that is MyServiceTest  test interface methods  and MyServiceImplTest  test private methods   All clients should anyway be using the interface so in a way even though the visibility of the private method has been increased it should not really matter

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Well you can unit test private method in two ways   you can create instance of PrivateObject class the syntax is as follows  PrivateObject obj  new PrivateObject PrivateClass     now with this obj you can call the private method of PrivateCalss  obj PrivateMethod  Parameters     You can use reflection   PrivateClass obj   new PrivateClass       Class containing private obj Type t   typeof PrivateClass    var x   t InvokeMember  PrivateFunc        BindingFlags InvokeMethod   BindingFlags NonPublic   BindingFlags Public             BindingFlags Instance  null  obj  new object     5

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MbUnit got a nice wrapper for this called Reflector     Reflector dogReflector   new Reflector new Dog     dogReflector Invoke  DreamAbout   DogDream Food     You can also set and get values from properties  dogReflector GetProperty  Age      Regarding the  test private  I agree that   in the perfect world  there is no point in doing private unit tests  But in the real world you might end up wanting to write private tests instead of refactoring code

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I tend not to use compiler directives because they clutter things up quickly  One way to mitigate it if you really need them is to put them in a partial class and have your build ignore that  cs file when making the production version

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If you are using  net  you should use the InternalsVisibleToAttribute

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Declare them internal  and then use the InternalsVisibleToAttribute to allow your unit test assembly to see them

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I don t agree with the  you should only be interested in testing the external interface  philosophy   It s a bit like saying that a car repair shop should only have tests to see if the wheels turn   Yes  ultimately I m interested in the external behavior but I like my own  private  internal tests to be a bit more specific and to the point   Yes  if I refactor  I may have to change some of the tests  but unless it s a massive refactor  I ll only have to change a few and the fact that the other  unchanged  internal tests still work is a great indicator that the refactoring has been successful   You can try to cover all internal cases using only the public interface and theoretically it s possible to test every internal method  or at least every one that matters  entirely by using the public interface but you may have to end up standing on your head to achieve this and the connection between the test cases being run through the public interface and the internal portion of the solution they re designed to test may be difficult or impossible to discern   Having pointed  individual tests that guarantee that the internal machinery is working properly is well worth the minor test changes that come about with refactoring - at least that s been my experience   If you have to make huge changes to your tests for every refactoring  then maybe this doesn t make sense  but in that case  maybe you ought to rethink your design entirely   A good design should be flexible enough to allow for most changes without massive redesigns

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If you want to unit test a private method  something may be wrong  Unit tests are  generally speaking  meant to test the interface of a class  meaning its public  and protected  methods  You can of course  hack  a solution to this  even if just by making the methods public   but you may also want to consider    If the method you d like to test is really worth testing  it may be worth to move it into its own class  Add more tests to the public methods that call the private method  testing the private method s functionality   As the commentators indicated  you should only do this if these private methods s functionality is really a part in with the public interface  If they actually perform functions that are hidden from the user  i e  the unit test   this is probably bad

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If you want to unit test a private method  something may be wrong  Unit tests are  generally speaking  meant to test the interface of a class  meaning its public  and protected  methods  You can of course  hack  a solution to this  even if just by making the methods public   but you may also want to consider    If the method you d like to test is really worth testing  it may be worth to move it into its own class  Add more tests to the public methods that call the private method  testing the private method s functionality   As the commentators indicated  you should only do this if these private methods s functionality is really a part in with the public interface  If they actually perform functions that are hidden from the user  i e  the unit test   this is probably bad

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MS Test has a nice feature built in that makes private members and methods available in the project by creating a file called VSCodeGenAccessors   System Diagnostics DebuggerStepThrough         System CodeDom Compiler GeneratedCodeAttribute  Microsoft VisualStudio TestTools UnitTestGeneration    1 0 0 0        internal class BaseAccessor                protected Microsoft VisualStudio TestTools UnitTesting PrivateObject m privateObject           protected BaseAccessor object target  Microsoft VisualStudio TestTools UnitTesting PrivateType type                        m privateObject   new Microsoft VisualStudio TestTools UnitTesting PrivateObject target  type                      protected BaseAccessor Microsoft VisualStudio TestTools UnitTesting PrivateType type                                this null  type                               internal virtual object Target                       get                               return m privateObject Target                                   public override string ToString                         return this Target ToString                       public override bool Equals object obj                        if  typeof BaseAccessor  IsInstanceOfType obj                                 obj     BaseAccessor  obj   Target                            return this Target Equals obj                      public override int GetHashCode                         return this Target GetHashCode                      With classes that derive from BaseAccessor  such as    System Diagnostics DebuggerStepThrough     System CodeDom Compiler GeneratedCodeAttribute  Microsoft VisualStudio TestTools UnitTestGeneration    1 0 0 0    internal class SomeClassAccessor   BaseAccessor        protected static Microsoft VisualStudio TestTools UnitTesting PrivateType m privateType   new Microsoft VisualStudio TestTools UnitTesting PrivateType typeof global  Namespace SomeClass         internal SomeClassAccessor global  Namespace Someclass target            base target  m privateType                   internal static string STATIC STRING               get                       string ret     string  m privateType GetStaticField  STATIC STRING                  return ret                    set                       m privateType SetStaticField  STATIC STRING   value                        internal int memberVar              get                       int ret     int  m privateObject GetField  memberVar                  return ret                    set                       m privateObject SetField  memberVar   value                        internal int PrivateMethodName int paramName                object   args   new object                 paramName           int ret    int  m privateObject Invoke  PrivateMethodName   new System Type                     typeof int    args             return ret

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On CodeProject  there is an article that briefly discusses pros and cons of testing private methods   It then provides some reflection code to access private methods  similar to the code Marcus provides above    The only issue I ve found with the sample is that the code doesn t take into account overloaded methods   You can find the article here   http   www codeproject com KB cs testnonpublicmembers aspx

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Here s an example  first the method signature   private string   SplitInternal         return Regex Matches Format                                                     Cast lt Match gt                             Select m   gt  m Value                           Where s   gt   string IsNullOrEmpty s                            ToArray        Here s the test        lt summary gt     A test for SplitInternal     lt  summary gt   TestMethod     DeploymentItem  Git XmlLib vs2008 dll    public void SplitInternalTest         string path    pair path to  Key  0   Items Item Name  1   Date       object   values   new object     2   Martin         XPathString xp   new XPathString path  values        PrivateObject param0   new PrivateObject xp       XPathString Accessor target   new XPathString Accessor param0       string   expected   new string              pair path to  Key  0              Items            Item Name  1              Date             string   actual      actual   target SplitInternal        CollectionAssert AreEqual expected  actual

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I use PrivateObject class  But as mentioned previously better to avoid testing private methods   Class target   new Class    PrivateObject obj   new PrivateObject target   var retVal   obj Invoke  PrivateMethod    Assert AreEqual retVal

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On CodeProject  there is an article that briefly discusses pros and cons of testing private methods   It then provides some reflection code to access private methods  similar to the code Marcus provides above    The only issue I ve found with the sample is that the code doesn t take into account overloaded methods   You can find the article here   http   www codeproject com KB cs testnonpublicmembers aspx

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It might not be useful to test private methods  However  I also sometimes like to call private methods from test methods  Most of the time in order to prevent code duplication for test data generation     Microsoft provides two mechanisms for this   Accessors   Goto the class definition s source code Right-click on the name of the class Choose  Create Private Accessor  Choose the project in which the accessor should be created    You will end up with a new class with the name foo accessor  This class will be dynamically generated during compilation and privides all members public available    However  the mechanism is sometimes a bit intractable when it comes to changes of the interface of the original class  So  most of the times I avoid using this   PrivateObject class The other way is to use Microsoft VisualStudio TestTools UnitTesting PrivateObject     Wrap an already existing instance PrivateObject accessor   new PrivateObject  objectInstanceToBeWrapped        Retrieve a private field MyReturnType accessiblePrivateField    MyReturnType  accessor GetField   privateFieldName         Call a private method accessor Invoke   PrivateMethodName   new Object

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Sometimes  it can be good to test private declarations  Fundamentally  a compiler only has one public method  Compile  string outputFileName  params string   sourceSFileNames    I m sure you understand that would be difficult to test such a method without testing each  hidden  declarations   That s why we have created Visual T   to make easier tests  It s a free  NET programming language  C  v2 0 compatible    We have added   -  operator  It just behave like     operator  except you can also access any hidden declaration from your tests without changing anything in your tested project   Take a look at our web site  download it for free

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MS Test has a nice feature built in that makes private members and methods available in the project by creating a file called VSCodeGenAccessors   System Diagnostics DebuggerStepThrough         System CodeDom Compiler GeneratedCodeAttribute  Microsoft VisualStudio TestTools UnitTestGeneration    1 0 0 0        internal class BaseAccessor                protected Microsoft VisualStudio TestTools UnitTesting PrivateObject m privateObject           protected BaseAccessor object target  Microsoft VisualStudio TestTools UnitTesting PrivateType type                        m privateObject   new Microsoft VisualStudio TestTools UnitTesting PrivateObject target  type                      protected BaseAccessor Microsoft VisualStudio TestTools UnitTesting PrivateType type                                this null  type                               internal virtual object Target                       get                               return m privateObject Target                                   public override string ToString                         return this Target ToString                       public override bool Equals object obj                        if  typeof BaseAccessor  IsInstanceOfType obj                                 obj     BaseAccessor  obj   Target                            return this Target Equals obj                      public override int GetHashCode                         return this Target GetHashCode                      With classes that derive from BaseAccessor  such as    System Diagnostics DebuggerStepThrough     System CodeDom Compiler GeneratedCodeAttribute  Microsoft VisualStudio TestTools UnitTestGeneration    1 0 0 0    internal class SomeClassAccessor   BaseAccessor        protected static Microsoft VisualStudio TestTools UnitTesting PrivateType m privateType   new Microsoft VisualStudio TestTools UnitTesting PrivateType typeof global  Namespace SomeClass         internal SomeClassAccessor global  Namespace Someclass target            base target  m privateType                   internal static string STATIC STRING               get                       string ret     string  m privateType GetStaticField  STATIC STRING                  return ret                    set                       m privateType SetStaticField  STATIC STRING   value                        internal int memberVar              get                       int ret     int  m privateObject GetField  memberVar                  return ret                    set                       m privateObject SetField  memberVar   value                        internal int PrivateMethodName int paramName                object   args   new object                 paramName           int ret    int  m privateObject Invoke  PrivateMethodName   new System Type                     typeof int    args             return ret

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In my opinion you should only unit test your classe s public API    Making a method public  in order to unit test it  breaks encapsulation exposing implementation details   A good public API solves an immediate goal of the client code and solves that goal completely

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I tend not to use compiler directives because they clutter things up quickly  One way to mitigate it if you really need them is to put them in a partial class and have your build ignore that  cs file when making the production version

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I want to create a clear code example here which you can use on any class in which you want to test private method    In your test case class just include these methods and then employ them as indicated                    var Class name of class you want to test private methods in      note  the actual class and the private variable to store the       class instance in  should at least be different case so that      they do not get confused in the code   Here the class name is      is upper case while the private instance variable is all lower      case         private  class name of class you want to test private methods in              This uses reflection to be able to get private methods to test       param  methodName       return ReflectionMethod         protected static function getMethod  methodName         class   new ReflectionClass  Class name of class you want to test private methods in         method    class- gt getMethod  methodName        method- gt setAccessible true       return  method                  Uses reflection class to call private methods and get return values        param  methodName       param array  params       return mixed           usage       this- gt  callMethod   someFunctionName   array param1 param2 param3           params are in        order in which they appear in the function declaration          protected function  callMethod  methodName   params array           method   self  getMethod  methodName       return  method- gt invokeArgs  this- gt class name of class you want to test private methods in   params          this-  callMethod   someFunctionName   array param1 param2 param3     Just issue the parameters in the order that they appear in the original private function

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Declare them internal  and then use the InternalsVisibleToAttribute to allow your unit test assembly to see them

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I use PrivateObject class  But as mentioned previously better to avoid testing private methods   Class target   new Class    PrivateObject obj   new PrivateObject target   var retVal   obj Invoke  PrivateMethod    Assert AreEqual retVal

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Declare them internal  and then use the InternalsVisibleToAttribute to allow your unit test assembly to see them

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Here is good article about unit testing of private methods  But I m not sure what s better  to make you application designed specially for testing it s like creating tests for testing only  or use reflexion for testing  Pretty sure most of us will choose second way

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Also note that the InternalsVisibleToAtrribute has a requirement that your assembly be strong named  which creates it s own set of problems if you re working in a solution that had not had that requirement before   I use the accessor to test private methods   See this question that for an example of this

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You could also declare it as public or internal  with InternalsVisibleToAttribute  while building in debug-Mode            lt summary gt          This Method is private           lt  summary gt   if DEBUG     public  else     private  endif     static string MyPrivateMethod                 return  false           It bloats the code  but it will be private in a release build

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In the rare cases I have wanted to test private functions  I have usually modified them to be protected instead  and the I have written a subclass with a public wrapper function   The Class        protected void APrivateFunction                      Subclass for testing         Test  public void TestAPrivateFunction         APrivateFunction          or whatever testing code you want here

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On CodeProject  there is an article that briefly discusses pros and cons of testing private methods   It then provides some reflection code to access private methods  similar to the code Marcus provides above    The only issue I ve found with the sample is that the code doesn t take into account overloaded methods   You can find the article here   http   www codeproject com KB cs testnonpublicmembers aspx

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There are 2 types of private methods  Static Private Methods and Non Static Private methods Instance Methods   The following 2 articles explain how to unit test private methods with examples    Unit Testing Static Private Methods Unit Testing Non Static Private Methods

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CC -Dprivate public    CC  is the command line compiler on the system I use  -Dfoo bar does the equivalent of  define foo bar  So  this compilation option effectively changes all private stuff to public

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Here is good article about unit testing of private methods  But I m not sure what s better  to make you application designed specially for testing it s like creating tests for testing only  or use reflexion for testing  Pretty sure most of us will choose second way

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I think a more fundamental question should be asked is that why are you trying to test the private method in the first place  That is a code smell that you re trying to test the private method through that class  public interface whereas that method is private for a reason as it s an implementation detail  One should only be concerned with the behaviour of the public interface not on how it s implemented under the covers    If I want to test the behaviour of the private method   by using common refactorings  I can extract its code into another class  maybe with package level visibility so ensure it s not part of a public API   I can then test its behaviour in isolation   The product of the refactoring means that private method is now a separate class that has become a collaborator to the original class  Its behaviour will have become well understood via its own unit tests    I can then mock its behaviour when I try to test the original class so that I can then concentrate on test the behaviour of that class  public interface rather than having to test a combinatorial explosion of the public interface and the behaviour of all its private methods    I see this analogous to driving a car  When I drive a car I don t drive with the bonnet up so I can see that the engine is working  I rely on the interface the car provides  namely the rev counter and the speedometer to  know the engine is working  I rely on the fact that the car actually moves when I press the gas pedal  If I want to test the engine I can do checks on that in isolation   D  Of course testing private methods directly may be a last resort if you have a legacy application but I would prefer that legacy code is refactored to enable better testing  Michael Feathers has written a great book on this very subject  http   www amazon co uk Working-Effectively-Legacy-Robert-Martin dp 0131177052

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Declare them internal  and then use the InternalsVisibleToAttribute to allow your unit test assembly to see them

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If you want to unit test a private method  something may be wrong  Unit tests are  generally speaking  meant to test the interface of a class  meaning its public  and protected  methods  You can of course  hack  a solution to this  even if just by making the methods public   but you may also want to consider    If the method you d like to test is really worth testing  it may be worth to move it into its own class  Add more tests to the public methods that call the private method  testing the private method s functionality   As the commentators indicated  you should only do this if these private methods s functionality is really a part in with the public interface  If they actually perform functions that are hidden from the user  i e  the unit test   this is probably bad

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It might not be useful to test private methods  However  I also sometimes like to call private methods from test methods  Most of the time in order to prevent code duplication for test data generation     Microsoft provides two mechanisms for this   Accessors   Goto the class definition s source code Right-click on the name of the class Choose  Create Private Accessor  Choose the project in which the accessor should be created    You will end up with a new class with the name foo accessor  This class will be dynamically generated during compilation and privides all members public available    However  the mechanism is sometimes a bit intractable when it comes to changes of the interface of the original class  So  most of the times I avoid using this   PrivateObject class The other way is to use Microsoft VisualStudio TestTools UnitTesting PrivateObject     Wrap an already existing instance PrivateObject accessor   new PrivateObject  objectInstanceToBeWrapped        Retrieve a private field MyReturnType accessiblePrivateField    MyReturnType  accessor GetField   privateFieldName         Call a private method accessor Invoke   PrivateMethodName   new Object

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If you are using  net  you should use the InternalsVisibleToAttribute

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You should not be testing the private methods of your code in the first place  You should be testing the  public interface  or API  the public things of your classes  The API are all the public methods you expose to outside callers   The reason is that once you start testing the private methods and internals of your class you are coupling the implementation of your class  the private things  to your tests  This means that when you decide to change your implementation details you will also have to change your tests   You should for this reason avoid using InternalsVisibleToAtrribute    Here is a great talk by Ian Cooper which covers this subject  Ian Cooper  TDD  where did it all go wrong

[.net] How do you unit test private methods?

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