Practical uses for the "internal" keyword in C#

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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

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.

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.

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.

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.

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

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.

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.

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.

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() { }
}