i have the following class
public class Manager
{
public SharedObject sharedobj {get;set;}
public SomeObject someobj {get;set;}
public AnotherObject anotherobj {get;set;}
//...
}
i want sharedobj to be only accessible from any instance that is in the Manager class instance
making it static is not an option since i will have multiple instances of the Manager class, so the only thing in my mind is sending the sharedobj to the constructors of each object that will use it, i was wondering if there is a better way to achive such thing without having to reference the sharedobj in each instance that will need to use it.
another way i thought of is using the protected keyword and make the other classes inherit the manager
protected (C# Reference):
The protected keyword is a member access modifier. A protected member is accessible within its class and by derived class instances. For a comparison of protected with the other access modifiers, see Accessibility Levels.
This is assuming you want each to have his own shared object that you can access if you have the manager.
On a side note, you could also use Dependency Injection (a.k.a DI) to do it. Here's the easiest, shortest explanation of DI I've seen so far: http://www.jamesshore.com/Blog/Dependency-Injection-Demystified.html .
It's a bit more advanced approach with it's own positives and negatives like everything else, but at least read the short link I've mentioned so you'll have an idea of what it is.
You will have to pass it to each object, just as you wrote. Either in the constructor or simply make a setter in SomeObject and AnotherObject, and use that. I would go with what you wrote, and pass it through the constructors.
On a sidenote, if nothing else but the objects within Manager should use SharedObject, then it probably should be private instead of public in the Manager class.
Related
I have two repository classes (RepositoryFactory and BaseRepository) implementing different interfaces within the same project. The BaseRepository class has a private method that would be now needed also in the other class, with the same implementation.
Instead of duplicate the method in order to keep it private, I was thinking to a possible alternative, although so far I could not find a good solution since by definition a private method has scope only in its own class.
Using inheritance and change the method to "protected" would also not be an option, since the two classes are not linked semantically. I cannot use a public property giving back the result of the method since the return type is void.
You can use reflection. Here's an example:
MethodInfo privMethod = objInstance.GetType().GetMethod("PrivateMethodName", BindingFlags.NonPublic | BindingFlags.Instance);
privMethod.Invoke(objInstance, new object[] { methodParameters });
It's not possible to do what you want in C#. The closest you can have is internal, which makes the member visible to an entire assembly. It might also be possible to make the two classes private and nested inside another class, but this isn't always appropriate.
Mads Torgersen, who works on C#, has this to say about it:
I've seen a number of proposals trying to grapple with some notion of "class set accessibility." The complication of course is that, unlike existing accessibilities, there is not already a natural group (everyone, assembly, derived classes, single class) to tie it to, so even with another accessibility modifier you still also need syntax (or something) to define the group.
There are several ways to slice it. I haven't seen a proposal that is obviously right, but I think the problem is relevant, and I will take this up with the design team.
(source)
You can, but it looks awkward. This takes advantage of nested classes being able to access private stuff from the containing class. However, even if something is possible doesn't mean you should do it. If you just change the modifier to internal you get the same behavior and since the two classes are coupled together then it makes sense to ship them in the same assembly, so internal modifier is the correct answer.
public class BaseRepository
{
public sealed class RepositoryFactory
{
public static BaseRepository Create()
{
var repo = new BaseRepository();
repo.MethodRequiredByRepositoryFactory();
return repo;
}
}
private void MethodRequiredByRepositoryFactory() { }
}
Reference
Possible by using reflection
Create a console application in Visual Studio.
Add 2 namespaces
2.1. System
2.2. System.Reflection
Now create a class and inside that class create one method that will be private as follows:
I need to know how to prevent a class from being Instantiated in .net?
I know few methods like making the class Abstract and Static.
Is there any more way to achieve this?
Making the class static is the best approach, if you absolutely don't want any instances. This stops anyone from creating instances. The class will be both sealed and abstract, and won't have any constructors.
Additionally, the language will notice that it's a static class and stop you from using it in various places which imply instances, e.g. type arguments and variables. This indicates the intention more clearly than just having a private constructor - which could mean that there are instances, created within that class (e.g. for a singleton implementation).
Oh, and making the class static will stop you from introducing any pointless instance members in the class, too :)
See MSDN for more information about static classes.
Mark the constructor(s) private, protected or if in used from another assembly, internal
Marking the constructor private. Of course, this doesn't prevent the class from instantiating itself through a static method, for example...
More practically, what's the purpose of disallowing class instantiation. If it's to have a singleton, then a private constructor is appropriate. If it's to force subclassing, making the class abstract is better; if it's to have a class with utility methods, making it static is one way (then you can only have static methods).
I need to know how to prevent a class from being Instantiated in .net?
Your question is not clear.
Do you mean instantiated at runtime? Make the class abstract or static.
Do you mean that the constructor is not accessible in code? Make the constructor private. But note that someone could still use reflection to grab a handle on a constructor and instantiate an instance at runtime.
So, which do you mean?
If the question is:
How can you make your class not be instanced without having your class
be static or abstract?
Then the answer to this is to implement the singleton pattern, which in .NET 4+ this is done easily with:
public sealed class myClass
{
private static readonly Lazy<myClass> lazyInstance =
new Lazy<myClass>(() => new myClass());
public static Instance
{
get
{
return lazyInstance.Value;
}
}
private myClass()
{
// constructor logic here
}
}
The singleton pattern allows you to pass your class around as a reference to methods, while still ensuring that you have only a single instance of your class. It also makes testing much easier as you can have a ImyClass instance which myClass implements, this is very helpful when making mock objects.
Without .NET4 you can still implement the singleton pattern, for example:
private static readonly myClass instance = new myClass();
public static Instance
{
get
{
return instance;
}
}
// rest of code remains the same
Which doesn't have deferred loading until it's called, there's lots of other ways as well (I think about 6 different ways), but the above two are the most common ones.
In summary the question is likely asking if you know the singleton pattern and if you recognise it's importance over static classes for unit tests and mock objects.
As others have already pointed out, static fields, even those marked readonly can be set with reflection, in addition the private constructor can be called using reflection. If you need to prevent these, either you need to make the calling code run in a less trusted app-domain space, or you will need to implement your class as static.
Generally though, people don't bother with such levels of reflection to get around your constraints unless they 'really need to' for example, writing obscure / extreme fringe case unit tests.
I have a class with a public property, that I want to restrict access to _for_some_modules_.
(The modules that use this class reside in different assemblies, so internal does not help.)
My first thought was to subclass, and make the derived property accessor private or protected, but this is not possible. The derived property has to have the same access rights. (See http://msdn.microsoft.com/en-us/library/75e8y5dd.aspx)
Any suggestions? I assume it is a common task to make a more restricted variant of a class?
Thanks!
You can use the InternalsVisibleToAttribute to make the internal members of the class visible to other assemblies (as many as you like). The documentation page has an example.
I assume it is a common task to make a
more restricted variant of a class?
This is not a common task since it violates the Liskov substitution principle - you can't use the sub class the same way as you would use the base class in regards to the property you restrict access to. You should consider refactoring your class hierarchy.
You could solve the problem through composition - make the class A internal only and write a public wrapper class that has a member of type A and delegates and controls access to the A's properties / methods.
Making more restricted subclasses is actually not common because it would break consumers of the base class that assumed they had access to the public members. In general, your classes should start out restrictive and get less so as they specialize, not vice-versa.
The concept you're looking for is called a "friend" class in other languages, but C# (purposely) doesn't implement them. The InternalsVisibleToAttribte is as close as it gets, but that is applied at the assembly level, so it may not work for you.
Without more information on why you are trying to restrict access this way, it's hard to give any good general-purpose alternatives. The access modifiers like public/private/etc aren't designed to be a security mechanism, since Reflection will get you access to read/write everything regardless. They're more of a hint to the consumers as to what is safe to use -- public members will usually remain stable across new versions, while private (implementation-detail) members are more likely to change.
You can always do something like this:
class MyBaseClass
{
protected string MyRestrictedProperty { get; set; }
}
class MyClass : MyBaseClass
{
public string MyPublicProperty
{
get { return MyRestrictedProperty; }
set { MyRestrictedProperty = value; }
}
}
I'm doing some internal domain-specific library development at the moment, and incidentally the stuff i'm trying to model mimicks "class" and "object" relations fairly well. So objects of my C# class MyClass should sort of act like a domain specific class for objects of my C# class MyObject who play the part of object or instance. Now I would like the code in MyObject to access methods of MyClass, which should not be accessible to other classes/code in the project. Any ideas how to enforce this, asside from documenting it at hoping my fellow developers will respect this.
I hope I made my question clear enough, otherwise let me know.
Best regards!
You could always split MyClass and MyObject up into another project, and define MyClass and/or MyObject as an internal class. That way it can only be accessed by other objects in that assembly.
See: http://msdn.microsoft.com/en-us/library/7c5ka91b(VS.80).aspx
The standard approach here is to declare the members internal and make sure MyClass and MyObject are part of the same assembly. That assembly should contain little else.
Additional: This is the tool that was designed for this purpose. Other languages have other means to fine-tune accessibility (C++: friend) but in .NET a simpler model was chosen.
And you don't have to take the 'nothing else' so strictly, the 2 classes could share an assembly with other related classes. you would then have to verify the no-access rule(s) manually inside that library.
I'd suggest a private nested class. That way, even if your fellow devs are writing code in the same namespace, they'll never be able to access the class.
Once the class declaration is fully enclosed within another class declaration, the class is considered nested and can only be accessed through the containing class.
Pehaps your MyObject should descend from MyClass and declare the methods in MyClas as protected.
If you don't want your consumers to invoke certain implementation specific methods you could try abstracting to interfaces or abstract base classes. That way the consumer will only 'see' the properties and methods you want them to see.
You do not have to use inheritance to provide shared functionality and you do not have to rely on member accesibility to prevent others from using methods you'd rather not expose.
For example:
public interface IDomainSpecific
{
void DoStuff();
}
public interface IDomainService
{
void HelpMeDoStuff();
}
public class DomainObject1 : IDomainSpecific
{
private readonly IDomainService _service;
public DomainObject1( IDomainService service )
{
_service = service;
}
public DoStuff()
{
// Do domain specific stuff here
// and use the service to help
_service.HelpMeDoStuff();
}
}
This uses classic constructor injection and works best when you already use dependency injection in your application, though it works perfectly well with factories as well.
The point is to keep responsibilities crystal clear. There's no chance of anybody invoking anything they shouldn't because the 'DomainObject' never knows what concrete type implements the shared service. The shared service is not exposed on the domain object either. The added bonus is testability and the possibility of swapping the service with another implementation without ever needing to touch the DomainObject.
Ok so I'm currently working with a set of classes that I don't have control over in some pretty generic functions using these objects. Instead of writing literally tens of functions that essentially do the same thing for each class I decided to use a generic function instead.
Now the classes I'm dealing with are a little weird in that the derived classes share many of the same properties but the base class that they are derived from doesn't. One such property example is .Parent which exists on a huge number of derived classes but not on the base class and it is this property that I need to use.
For ease of understanding I've created a small example as follows:
class StandardBaseClass {} // These are simulating the SMO objects
class StandardDerivedClass : StandardBaseClass {
public object Parent { get; set; }
}
static class Extensions
{
public static object GetParent(this StandardDerivedClass sdc) {
return sdc.Parent;
}
public static object GetParent(this StandardBaseClass sbc)
{
throw new NotImplementedException("StandardBaseClass does not contain a property Parent");
}
// This is the Generic function I'm trying to write and need the Parent property.
public static void DoSomething<T>(T foo) where T : StandardBaseClass
{
object Parent = ((T)foo).GetParent();
}
}
In the above example calling DoSomething() will throw the NotImplemented Exception in the base class's implementation of GetParent(), even though I'm forcing the cast to T which is a StandardDerivedClass.
This is contrary to other casting behaviour where by downcasting will force the use of the base class's implementation.
I see this behaviour as a bug. Has anyone else out there encountered this?
I see this behaviour as a bug.
This behavior is correct. Since your method DoSomething is constraining T to StandardBaseClass, you only have access to the specific methods of StandardBaseClass, not any methods or properties of a derived class. Since StandardBaseClass does not have a Parent property, this is invalid, and should be invalid, by design.
There are two potential options here - You can use reflection to pull out the Parent property, or use C# 4's dynamic type, and treat this as a dynamic object. Both bypass the standard type checking in the compiler, however, so will require you to do extra type checking at runtime to verify that the Parent property exists.
Create an interface that contains the Parent property. Have each class that has a Parent property implement that interace. You will then be able to create a generic method that accepts a parameter of type IHaveParent, and it will do the right thing.
For anyone that is interested an succinct answer to this situation is answered by Stephen Cleary on msdn here:
http://social.msdn.microsoft.com/Forums/en-AU/csharpgeneral/thread/95833bb3-fbe1-4ec9-8b04-3e05165e20f8?prof=required
To me this is a divergence in the class hierarchy. By this this I mean that either the base class has parent, or the derived classes with Parent are derived from an abstract child of the base.
Lol as John says, an interface as opposed to an abstract class is sufficient too.
You idea won't work because the compiler can never guarantee that the base class actually would have such a property. And it won't just select the "right" one based on if it has it or not.
The only way you can do this is using reflection and then test at runtime if the requested property exists on the inspected class. You have to judge yourself if that is a viable way to do for your project (reflection is slow and requires maximum rights).
This is correct, as the compiler only knows that it can bind to your type as a StandardBaseClass. The binding is not done at runtime (where it could potentially decide to use the StandardDerivedClass overload.
If you know that it's a StandardDerivedClass, then why not just cast it as such?
object Parent = ((StandardDerivedClass)foo).Parent;
It's a bit ugly, but you can accomplish this using a Registration system, where you register delegates for different possible derived classes that expose the 'shared' property/method and then use something like a Dictionary<Type,Func<SomeT>> to store the delegates. If you know all of the derived types ahead of time and don't have to load plug-ins or the like, you can also use the classic ugly if/else-if structure. Either way you're basically creating your own substitute for what should have been supported by the virtual method table.