Is there a way to limit the instantiation of the nested class in C#? I want to prevent nested class being instantiated from any other class except the nesting class, but to allow full access to the nested class from other code.
Usually I create an interface for the functionality you want to expose to other classes, then make the nested class private and implement that interface. This way the nested class definition can stay hidden:
public class Outer
{
private class Nested : IFace
{
public Nested(...)
{
}
//interface member implementations...
}
public IFace GetNested()
{
return new Nested();
}
}
If you need to meet one of the following requirements:
You want the nested class to be sealed,
You don't want to copy all the nested class's method signatures to an interface like in Lee's answer,
I found a solution similar to the one posted by ak99372, but without using a static initializer:
public class Outer
{
private interface IPrivateFactory<T>
{
T CreateInstance();
}
public sealed class Nested
{
private Nested() {
// private constructor, accessible only to the class Factory.
}
public class Factory : IPrivateFactory<Nested>
{
Nested IPrivateFactory<Nested>.CreateInstance() { return new Nested(); }
}
}
public Nested GetNested() {
// We couldn't write these lines outside of the `Outer` class.
IPrivateFactory<Nested> factory = new Nested.Factory();
return factory.CreateInstance();
}
}
The idea is that the Nested class's constructor is accessible only to the Factory class, which is nested one level deeper. The Factory class explicitly implements the method CreateInstance from the private interface IPrivateFactory, so that only those who can see IPrivateFactory can call CreateInstance and get a new instance of Nested.
Code outside the Outer class can't freely create instances of Nested without asking Outer.GetNested(), because
Outer.Nested's constructor is privated, so they can't call it directly
Outer.Nested.Factory can be instantiated, but can't be cast to IPrivateFactory, so its CreateInstance() method can't be called.
Note that I wouldn't recommend using that pattern heavily in production code, but it's a trick I find useful to have up my sleeve on rare occasions.
In short, no, you cannot do that. There is an accessibity modifier "public" which means "accessible by anything inside me or outside me" and there is an accessibility modifier "private" which means "accessible by anything inside me". There is no modifier which means "accessible to the thing immediately outside me but not to anything outside it", which is what you would need to mark the constructor as. That's simply not a concept that the designers of the type system thought would be useful.
Can you describe why you want this crazy kind of accessibility? Perhaps there is a better way to get what you want.
Since there is nothing in C# syntax you'll have to implement something like "a contract" between them. You can take advantage of the fact that nested class can access private fields of its parent:
public class ParentClass
{
private static Func<FriendClass> _friendContract;
public class FriendClass
{
static FriendClass()
{
_friendContract= () => new FriendClass();
}
private FriendClass() { }
}
///Usage
public FriendClass MethodUse()
{
var fInstance = _friendContract();
//fInstance.DoSomething();
return fInstance;
}
}
Of course you can adjust the contract to handle different parameters
private static Func<Arg1,Arg2,FriendClass> _friendContract;
For the answer proposed by Joshua Smith I found it necessary to force the static constructor of FriendClass to run, achieved by calling an empty static Initalize() method on FriendClass from the static constructor of ParentClass.
With the new static abstract interface members of C# 11 you can limit the instantiation of nested classes quite neatly:
public class Outer
{
protected interface INestedFactory<T> where T : INestedFactory<T>
{
public static abstract T CreateInstance();
}
public class SomeNested : INestedFactory<SomeNested>
{
private SomeNested() { }
static SomeNested INestedFactory<SomeNested>.CreateInstance()
{
return new SomeNested();
}
}
protected void CreateNested<T>() where T : INestedFactory<T>
{
T.CreateInstance();
}
}
public class Outer
{
public class Nested
{
readonly Outer Outer;
public Nested(Outer outer /* , parameters */)
{
Outer = outer;
// implementation
}
// implementation
}
public Nested GetNested(/* parameters */) => new Nested(this /* , parameters */);
}
Note that you can access private members of Outer from Nested.
Related
Right now in my code, I have an abstract class, which contains a nested class. In the constructor of every class implementing my abstract class, I will have to construct an instance of the nested class, and pass a reference to the current object to the nested class, as demonstrated below.
public abstract class MainClass
{
public SpecializedClass SpecialStuff { get; init; }
/* General stuff happens here*/
public class SpecializedClass
{
private MainClass _parent;
public SpecializedClass(MainClass parent, object stuff)
{
this._parent = parent;
// Do stuff...
}
/* More specialized stuff happens here.*/
}
}
public class Foo : MainClass
{
public Foo()
{
this.SpecialStuff = new SpecializedClass(this, "stuff"); // <= This is the issue
// More stuff
}
}
As you can see, whenever I need to call new SpecializedClass(), I have to pass a reference to this. Is there a way to do this automatically, so the classes inheriting from MainClass don't need to keep passing references down?
There answer is basically no. The stack frame does not contain a reference to the calling object (if any), there is no way to get at it fundamentally unless you pass it in, or want to walk the stack (bad idea).
There are however situations where you can use expression trees for more complicated less trivial problems. however, they hardly apply here, and would be more printable characters anyway.
I guess another approach would be an extension methods, however you are just kicking the this ball up the road and creating more code for no good reason
Which leaves with a instance method or base class...
Lastly, although common, I would double check you actually need the calling object. This can easily brush up on design issues such as the Single Responsibility principle (SRP) and Separation of concerns (SOC) among others
All things being equal, if you need it then just pass it in.
No, this is not possible.
Think about it: You could run SpecialStuff = new SpecializedClass(this, "stuff"); in a static function. Then what would the value of this be?
It seems that you could push that functionality into the base abstract class.
Now you only need to call the SpecializedClass and pass this once
public abstract class MainClass
{
public SpecializedClass SpecialStuff { get; init; }
public MainClass(object stuff)
{
this.SpecialStuff = new SpecializedClass(this, stuff);
}
/* General stuff happens here*/
public class SpecializedClass
{
private MainClass _parent;
public SpecializedClass(MainClass parent, object stuff)
{
this._parent = parent;
// Do stuff...
}
/* More specialized stuff happens here.*/
}
}
public class Foo : MainClass
{
public Foo() : base("stuff")
{
// More stuff
}
}
Here is a piece of code:
private class myClass
{
public static void Main()
{
}
}
'or'
private class myClass
{
public void method()
{
}
}
I know, first one will not work. And second one will.
But why first is not working? Is there any specific reason for it?
Actually looking for a solution in this perspective, thats why made it bold. Sorry
It would be meaningful in this scenario; you have a public class SomeClass, inside which you want to encapsulate some functionality that is only relevant to SomeClass. You could do this by declaring a private class (SomePrivateClass in my example) within SomeClass, as shown below.
public class SomeClass
{
private class SomePrivateClass
{
public void DoSomething()
{
}
}
// Only SomeClass has access to SomePrivateClass,
// and can access its public methods, properties etc
}
This holds true regardless of whether SomePrivateClass is static, or contains public static methods.
I would call this a nested class, and it is explored in another StackOverflow thread.
Richard Ev gave a use case of access inside a nested classes. Another use case for nested classes is private implementation of a public interface:
public class MySpecialCollection<T> : IEnumerable<T>
{
public IEnumerator<T> GetEnumerator()
{
return new MySpecialEnumerator(...);
}
private class MySpecialEnumerator : IEnumerator<T>
{
public bool MoveNext() { ... }
public T Current
{
get { return ...; }
}
// etc...
}
}
This allows one to provide a private (or protected or internal) implementation of a public interface or base class. The consumer need not know nor care about the concrete implementation. This can also be done without nested classes by having the MySpecialEnumerator class be internal, as you cannot have non-nested private classes.
The BCL uses non-public implementations extensively. For example, objects returned by LINQ operators are non-public classes that implement IEnumerable<T>.
This code is syntactically correct. But the big question is: is it useful, or at least usable in the context where you want to use it? Probably not, since the Main method must be in a public class.
Main() method is where application execution begin, so the reason you cannot compile your first class (with public static void Main()) is because you already have Main method somewhere else in your application. The compiler don't know where to begin execute your application.
Your application must have only one Main method to compile with default behavior otherwise you need to add /main option when you compile it.
I' ve been doing some programming lately and faced an issue which i found weird in c#. (at least for me)
public class Foo
{
//whatever
public class FooSpecificCollection : IList<Bar>
{
//implementation details
}
public FooSpecificCollection GetFoosStuff()
{
//return the collection
}
}
I want the consumer of Foo to be able to obtain a reference to FooSpecificCollection, even perform some operations on it. Maybe even set it to some other property of Foo or smth like that, but not To be able to CREATE an instance of this class. (the only class that should be able to instatiate this collection should be Foo.
Is my request really that far-fetched? I know that people way smarter defined c# but shouldn't there be such an option that a parent class can create a nested class instance but nobody else can't.
So far I created a solution to make an abstract class, or interface available through the property and implement a concrete private class that is not available anywhere else.
Is this a correct way to handle such a situation.?
The way embedded classes work is that they, as members of the outer class, get access to private members of that outer class. But not the other way around (what is what you want).
You can shield the constructor of FooSpecificCollection, but then the Factory has to be part of FooSpecificCollection itself. It could enlist the outer class:
public class Foo
{
public class FooSpecificCollection : List<Bar>
{
private FooSpecificCollection () { }
public static FooSpecificCollection GetFoosStuff()
{
var collection = new FooSpecificCollection ();
PrepareFooSpecificCollection(collection);
return collection;
}
}
private static void PrepareFooSpecificCollection(FooSpecificCollection collection)
{
//prepare the collection
}
}
Make your nested class private and make the return value of GetFoosStuff IList<Bar> instead of FooSpecificCollection.
Also, there's a good chance that deriving from List<Bar> is a bug.
If you are creating a library for others to use, you could make the constructor internal. Anyone outside the library will not be able to access it. If you are concerned about calling the constructor in your own project, just don't call it outside the parent class.
We create classes all the time which are not directly related to other classes, but the constructors don't have to be hidden from non-related classes. We (the programmers) know the the objects are not related so we don't ever create an instance of one in the other.
There is a solution but I don't think I would use it in my App :)
The idea is to have derived class from FooSpecific which is private and can be used only inside Foo but has public constructor, so Foo can create its instances.
public class Foo
{
//whatever
public class FooSpecific
{
// Protected contructor.
protected FooSpecific()
{
}
// All other code in here.
}
// Private helper class used for initialization.
private class FooSpecificInitHelper : FooSpecific
{
public FooSpecificInitHelper()
{
}
}
// Method in foo to create instaces of FooSpecific.
private FooSpecific CreateFooSpecific()
{
return new FooSpecificInitHelper();
}
}
No, and it doesn't really make sense.
I mean the whole point is so that you could potentially return other instances; but who will be deriving from that class anyway? Certainly not any other classes (Because that would be wrong, and imply it shouldn't be hidden inside the main class), so ...
What are all the difference between an abstract class, and a class with only protected constructor(s)? They seem to be pretty similar to me, in that you can't instantiate either one.
EDIT:
How would you create an instance in a derived class, with a base class with a protected constructor? For instance:
public class ProtectedConstructor
{
protected ProtectedConstructor()
{
}
public static ProtectedConstructor GetInstance()
{
return new ProtectedConstructor(); // this is fine
}
}
public class DerivedClass : ProtectedConstructor
{
public void createInstance()
{
ProtectedConstructor p = new ProtectedConstructor(); // doesn't compile
}
public static ProtectedConstructor getInstance()
{
return new ProtectedConstructor(); // doesn't compile
}
}
You can instantiate a class with protected constructors from within the class itself - in a static constructor or static method. This can be used to implement a singleton, or a factory-type thing.
An abstract class cannot be instantiated at all - the intent is that one or more child classes will complete the implementation, and those classes will get instantiated
Edit:
if you call ProtectedConstructor.GetInstance(); instead of new ProtectedConstructor();, it works. Maybe protected constructors can't be called this way? But protected methods certainly can.
Here is an interesting article on the topic.
Most of the time, there is little practical difference, as both are only able to be generated via a subclass.
However, marking a class abstract has two benefits:
With protected constructors, it's still possible to create an instance of the class in two ways. You can use Activator.CreateInstance with BindingFlags.NonPublic, or you can use a factory method defined in the class (or a subclass) to create an instance of the class. A class marked abstract, however, cannot be created.
You are making your intention more clear by marking the class abstract. Personally, I find this the most compelling reason to do so.
From an outside , black-box perspective, yes they are similar in that you cannot instantiate either one. However, you can never instantiate an abstract class, where you can construct a class with only protected constructors from within the class itself, or from an inheritor.
An abstract class can have abstract methods; methods that consist only of the method signature, but no body, that child classes must implement.
Seriously, not one person mentioned that yet?
Your example is flawed because in the getInstance case because you construct a ProtectedConstructor class and expect to down cast it as a DerivedClass. Instead you need a slightly more complete implementation where the derived class has a constrcutor:
public class ProtectedConstructor
{
protected ProtectedConstructor(string arg)
{
// do something with arg
}
public static ProtectedConstructor GetInstance()
{
return new ProtectedConstructor("test");
}
}
public class DerivedClass : ProtectedConstructor
{
protected DerivedClass(string arg) : base(arg)
{
}
public void createInstance()
{
DerivedClass p = new DerivedClass("test");
}
public static DerivedClass getInstance()
{
return new DerivedClass("test");
}
}
Regardless the major difference usage of abstract classes is to define abstract methods that subclasses must implement but you don't want to provide a default implementation for. For example suppose you have some kind of Thread class that has a Run method. You want to ensure that every call to Run first setups up some logging then does the real work of the thread and then stops logging. You could write an abstract Thread class like this:
public abstract Thread
{
protected Thread()
{
}
public void Run()
{
LogStart();
DoRun();
LogEnd();
}
protected abstract DoRun();
private void LogStart()
{
Console.Write("Starting Thread Run");
}
private void LogEnd()
{
Console.Write("Ending Thread Run");
}
}
public class HelloWorldThread : Thread
{
public HelloWorldThread()
{
}
protected override DoRun()
{
Console.Write("Hello World");
}
}
Another thing to consider, that I didn't see other people mention, is that your code may be maintained in the future. If the maintainer adds a public constructor to a class, then it can be instantiated. This might break your design, so you should prevent it (or design to accommodate it).
To prevent other people from making these kinds of changes, you can comment your code. Or, as other people said, use "abstract" to explicitly document your intent.
Well, the first difference that comes to mind is that an abstract class can not be instantiated, but a class with protected constructors could be instantiated throw another public method.
A common example of this might be something like the Singleton pattern: http://en.wikipedia.org/wiki/Singleton_pattern
if you inherit an abstract class from another abstract class, you do not have to satisfy abstract methods, but you do with a normal class with protected ctors. Examples
public abstract class Parent
{
protected abstract void AMethod();
}
public abstract class Child: Parent
{
// does not implement AMethod, and that's ok
}
public class Child2: Parent
{
// does not implement AMethod, and that will cause a compile error
}
If your intent is to only allow static uses of the class (i.e. not to use it as a pure base class) then you should use the static keyword instead; the CLR will prevent instances of the class being created via any method including Reflection (AFAIK).
I am relatively new to C# and each time I begin to work on a C# project (I only worked on nearly mature projects in C#) I wonder why there are no inner classes?
Maybe I don't understand their goal. To me, inner classes -- at least private inner classes -- look a lot like "inner procedures" in Pascal / Modula-2 / Ada : they allow to break down a main class in smaller parts in order to ease the understanding.
Example : here is what is see most of the time :
public class ClassA
{
public MethodA()
{
<some code>
myObjectClassB.DoSomething(); // ClassB is only used by ClassA
<some code>
}
}
public class ClassB
{
public DoSomething()
{
}
}
Since ClassB will be used (at least for a while) only by ClassA, my guess is that this code would be better expressed as follow :
public class ClassA
{
public MethodA()
{
<some code>
myObjectClassB.DoSomething(); // Class B is only usable by ClassA
<some code>
}
private class ClassB
{
public DoSomething()
{
}
}
}
I would be glad to hear from you on this subject - Am I right?
Nested classes (probably best to avoid the word "inner" as nested classes in C# are somewhat different to inner classes in Java) can indeed be very useful.
One pattern which hasn't been mentioned is the "better enum" pattern - which can be even more flexible than the one in Java:
public abstract class MyCleverEnum
{
public static readonly MyCleverEnum First = new FirstCleverEnum();
public static readonly MyCleverEnum Second = new SecondCleverEnum();
// Can only be called by this type *and nested types*
private MyCleverEnum()
{
}
public abstract void SomeMethod();
public abstract void AnotherMethod();
private class FirstCleverEnum : MyCleverEnum
{
public override void SomeMethod()
{
// First-specific behaviour here
}
public override void AnotherMethod()
{
// First-specific behaviour here
}
}
private class SecondCleverEnum : MyCleverEnum
{
public override void SomeMethod()
{
// Second-specific behaviour here
}
public override void AnotherMethod()
{
// Second-specific behaviour here
}
}
}
We could do with some language support to do some of this automatically - and there are lots of options I haven't shown here, like not actually using a nested class for all of the values, or using the same nested class for multiple values, but giving them different constructor parameters. But basically, the fact that the nested class can call the private constructor gives a lot of power.
The Framework Design Guidelines has the best rules for using nested classes that I have found to date.
Here's a brief summary list:
Do use nested types when the relationship between type and nested type is such the member-accessibility semantics are desired.
Do NOT use public nested types as a logical group construct
Avoid using publicly exposed nested types.
Do NOT use nested types if the type is likely to be referenced outside of the containing type.
Do NOT use nested types if they need to be instantiated by client code.
Do NOT define a nested type as a member of an interface.
You should limit the responsibilities of each class so that each one stays simple, testable and reusable. Private inner classes go against that. They contribute to the complexity of the outer class, they are not testable and they are not reusable.
For me personally I only create private inner classes if I need to create in-process collections of an object that may require methods on them.
Otherwise, it could cause confusion for other developers working on the project to actually find these classes, as they are not very clear as to where they are.