Practical uses for the internal keyword in C

Question

Could you please explain what the practical usage is for the internal keyword in C    I know that the internal modifier limits access to the current assembly  but when and in which circumstance should I use it

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If Bob needs BigImportantClass then Bob needs to get the people who own project A to sign up to guarantee that BigImportantClass will be written to meet his needs, tested to ensure that it meets his needs, is documented as meeting his needs, and that a process will be put in place to ensure that it will never be changed so as to no longer meet his needs.

If a class is internal then it doesn't have to go through that process, which saves budget for Project A that they can spend on other things.

The point of internal is not that it makes life difficult for Bob. It's that it allows you to control what expensive promises Project A is making about features, lifetime, compatibility, and so on.

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There are cases when it makes sense to make members of classes internal  One example could be if you want to control how the classes are instantiated  let s say you provide some sort of factory for creating instances of the class  You can make the constructor internal  so that the factory  that resides in the same assembly  can create instances of the class  but code outside of that assembly can t   However  I can t see any point with making classes or members internal without specific reasons  just as little as it makes sense to make them public  or private without specific reasons

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When you have classes or methods which don t fit cleanly into the Object-Oriented Paradigm  which do dangerous stuff  which need to be called from other classes and methods under your control  and which you don t want to let anyone else use   public class DangerousClass       public void SafeMethod           internal void UpdateGlobalStateInSomeBizarreWay

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This example contains two files  Assembly1 cs and Assembly2 cs  The first file contains an internal base class  BaseClass  In the second file  an attempt to instantiate BaseClass will produce an error      Assembly1 cs    compile with   target library internal class BaseClass       public static int intM   0        Assembly1 a cs    compile with   reference Assembly1 dll class TestAccess       static void Main                BaseClass myBase   new BaseClass         CS0122          In this example  use the same files you used in example 1  and change the accessibility level of BaseClass to public  Also change the accessibility level of the member IntM to internal  In this case  you can instantiate the class  but you cannot access the internal member      Assembly2 cs    compile with   target library public class BaseClass       internal static int intM   0        Assembly2 a cs    compile with   reference Assembly1 dll public class TestAccess       static void Main                     BaseClass myBase   new BaseClass         Ok        BaseClass intM   444        CS0117          source  http   msdn microsoft com en-us library 7c5ka91b VS 80  aspx

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Utility or helper classes methods that you would like to access from many other classes within the same assembly  but that you want to ensure code in other assemblies can t access   From MSDN  via archive org       A common use of internal access is in component-based development because it enables a group of components to cooperate in a private manner without being exposed to the rest of the application code  For example  a framework for building graphical user interfaces could provide Control and Form classes that cooperate using members with internal access  Since these members are internal  they are not exposed to code that is using the framework    You can also use the internal modifier along with the InternalsVisibleTo assembly level attribute to create  friend  assemblies that are granted special access to the target assembly internal classes     This can be useful for creation of unit testing assemblies that are then allowed to call internal members of the assembly to be tested   Of course no other assemblies are granted this level of access  so when you release your system  encapsulation is maintained

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The internal keyword is heavily used when you are building a wrapper over non-managed code   When you have a C C   based library that you want to DllImport you can import these functions as static functions of a class  and make they internal  so your user only have access to your wrapper and not the original API so it can t mess with anything  The functions being static you can use they everywhere in the assembly  for the multiple wrapper classes you need   You can take a look at Mono Cairo  it s a wrapper around cairo library that uses this approach

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Internal classes enable you to limit the API of your assembly   This has benefits  like making your API simpler to understand   Also  if a bug exists in your assembly  there is less of a chance of the fix introducing a breaking change  Without internal classes  you would have to assume that changing any class s public members would be a breaking change   With internal classes  you can assume that modifying their public members only breaks the internal API of the assembly  and any assemblies referenced in the InternalsVisibleTo attribute    I like having encapsulation at the class level and at the assembly level   There are some who disagree with this  but it s nice to know that the functionality is available

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How about this one  typically it is recommended that you do not expose a List object to external users of an assembly  rather expose an IEnumerable  But it is lot easier to use a List object inside the assembly  because you get the array syntax  and all other List methods  So  I typically have a internal property exposing a List to be used inside the assembly   Comments are welcome about this approach

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Being driven by  use as strict modifier as you can  rule I use internal everywhere I need to access  say  method from another class until I explicitly need to access it from another assembly   As assembly interface is usually more narrow than sum of its classes interfaces  there are quite many places I use it

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Saw an interesting one the other day  maybe week  on a blog that I can t remember   Basically I can t take credit for this but I thought it might have some useful application   Say you wanted an abstract class to be seen by another assembly but you don t want someone to be able to inherit from it   Sealed won t work because it s abstract for a reason  other classes in that assembly do inherit from it   Private won t work because you might want to declare a Parent class somewhere in the other assembly    namespace Base Assembly     public abstract class Parent         internal abstract void SomeMethod             This works just fine since it s in the same assembly    public class ChildWithin   Parent         internal override void SomeMethod                      namespace Another Assembly       Kaboom  because you can t override an internal method   public class ChildOutside   Parent            public class Test              Just fine     private Parent  parent       public Test                 Still fine        parent   new ChildWithin                  As you can see  it effectively allows someone to use the Parent class without being able to inherit from

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A very interesting use of internal - with internal member of course being limited only to the assembly in which it is declared - is getting  friend  functionality to some degree out of it   A friend member is something that is visible only to certain other assemblies outside of the assembly in which its declared   C  has no built in support for friend  however the CLR does   You can use InternalsVisibleToAttribute to declare a friend assembly  and all references from within the friend assembly will treat the internal members of your declaring assembly as public within the scope of the friend assembly   A problem with this is that all internal members are visible  you cannot pick and choose   A good use for InternalsVisibleTo is to expose various internal members to a unit test assembly thus eliminating the needs for complex reflection work arounds to test those members   All internal members being visible isn t so much of a problem  however taking this approach does muck up your class interfaces pretty heavily and can potentially ruin encapsulation within the declaring assembly

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Noise reduction  the less types you expose the more simple your library is  Tamper proofing   Security is another  although Reflection can win against it

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the only thing i have ever used the internal keyword on is the license-checking code in my product  -

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The idea is that when you are designing a library only the classes that are intended for use from outside  by clients of your library  should be public  This way you can hide classes that   Are likely to change in future releases  if they were public you would break client code  Are useless to the client and may cause confusion Are not safe  so improper use could break your library pretty badly    etc   If you are developing inhouse solutions than using internal elements is not that important I guess  because usually the clients will have constant contact with you and or access to the code  They are fairly critical for library developers though

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I have a project which uses LINQ-to-SQL for the data back-end  I have two main namespaces  Biz and Data  The LINQ data model lives in Data and is marked  internal   the Biz namespace has public classes which wrap around the LINQ data classes   So there s Data Client  and Biz Client  the latter exposes all relevant properties of the data object  e g    private Data Client  client  public int Id   get   return  client Id    set    client Id   value        The Biz objects have a private constructor  to force the use of factory methods   and an internal constructor which looks like this   internal Client Data Client client        this  client   client      That can be used by any of the business classes in the library  but the front-end  UI  has no way of directly accessing the data model  ensuring that the business layer always acts as an intermediary   This is the first time I ve really used internal much  and it s proving quite useful

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Another reason to use internal is if you obfuscate your binaries  The obfuscator knows that it s safe to scramble the class name of any internal classes  while the name of public classes can t be scrambled  because that could break existing references

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When you have methods  classes  etc which need to be accessible within the scope of the current assembly and never outside it   For example  a DAL may have an ORM but the objects should not be exposed to the business layer all interaction should be done through static methods and passing in the required paramters

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If you are writing a DLL that encapsulates a ton of complex functionality into a simple public API  then    internal    is used on the class members which are not to be exposed publicly   Hiding complexity  a k a  encapsulation  is the chief concept of quality software engineering

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I find internal to be far overused  you really should not be exposing certain functionailty only to certain classes that you would not to other consumers   This in my opinion breaks the interface  breaks the abstraction  This is not to say it should never be used  but a better solution is to refactor to a different class or to be used in a different way if possible  However  this may not be always possible   The reasons it can cause issues is that another developer may be charged with building another class in the same assembly that yours is  Having internals lessens the clarity of the abstraction  and can cause problems if being misused  It would be the same issue as if you made it public  The other class that is being built by the other developer is still a consumer  just like any external class  Class abstraction and encapsulation isnt just for protection for from external classes  but for any and all classes   Another problem is that a lot of developers will think they may need to use it elsewhere in the assembly and mark it as internal anyways  even though they dont need it at the time  Another developer then may think its there for the taking  Typically you want to mark private until you have a definative need    But some of this can be subjective  and I am not saying it should never be used  Just use when needed

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Keep in mind that any class defined as public will automatically show up in the intellisense when someone looks at your project namespace   From an API perspective  it is important to only show users of your project the classes that they can use   Use the internal keyword to hide things they shouldn t see   If your Big Important Class for Project A is intended for use outside your project  then you should not mark it internal   However  in many projects  you ll often have classes that are really only intended for use inside a project   For example  you may have a class that holds the arguments to a parameterized thread invocation   In these cases  you should mark them as internal if for no other reason than to protect yourself from an unintended API change down the road

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As rule-of-thumb there are two kinds of members    public surface  visible from an external assembly  public  protected  and internal protected   caller is not trusted  so parameter validation  method documentation  etc  is needed  private surface  not visible from an external assembly  private and internal  or internal classes    caller is generally trusted  so parameter validation  method documentation  etc  may be omitted

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One use of the internal keyword is to limit access to concrete implementations from the user of your assembly   If you have a factory or some other central location for constructing objects the user of your assembly need only deal with the public interface or abstract base class   Also  internal constructors allow you to control where and when an otherwise public class is instantiated

[c#] Practical uses for the "internal" keyword in C#

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