How can i prevent a class from from being inherited
without using Sealed Keyword?
Thanks in advance.
In your class's constructor:
public MyUnsealedClass()
{
if (this.GetType() != typeof(MyUnsealedClass))
throw new Exception("Don't do that");
}
Why not use the sealed keyword though?
Another way is you can make a static method that returns an object of your type and then make the constructor private. This has the advantage that it will create a compile time error instead of a run time error.
You can use private constructors
class Base
{
private Base() {}
}
class Derived : Base
{
// derp
}
Then provide a utility to creaet Base objects (like static methods on Base that have access to the private ctor
class Base
{
private Base() {}
public static Base CreateBase() { return new Base(); }
}
Also, if you want to be able to derive from this class, but you don't want other people doing that, you can make your class internal (or even the ctor internal)
class Base
{
internal Base() { }
}
class Derived : Base
{
}
// in another assembly
class MyOwnDerived : Base
{
// derp
}
You can achieve this by using private constructor, something along the lines:
public class NonInheritableClass
{
static NonInheritableClass GetObject()
{
return new NonInheritableClass();
}
private NonInheritableClass()
{
}
}
Throwing an exception in the constructor will work.
I agree though - why not use sealed? It's there for that reason.
Unless you' are trying to do something else, and if that's the case, there is probably a better solution too.
Related
I have an abstract class:
public abstract class ExampleBase : IExampleBase
{
protected ExampleBase()
{
this.SetupData();
}
protected abstract Dictionary<int, Adress> RelevantData { get; set; }
protected abstract void SetupData();
public void ProcessData()
{
// use RelevantData
}
}
And a derived class:
public class Example : ExampleBase
{
public Example()
{
}
protected override void SetupData()
{
this.RelevantData = new Dictionary<int, Adress>
{ 1, new Adress { ... } },
{ 2, new Adress { ... } }
}
}
In the base class, ReSharper tells me
Virtual member call in constructor
I understand that it's dangerous to call the method because of the execution order.. but how can I resolve this issue?
Context: I want to set up data in each derived class which will then be processed in the base class. I wanted to call the SetupData() method in the base class since it's the same in every derived class.
Derived class:
Set up the data
Base class:
Process the data
You don't. You accept the fact this is dangerous, and (try to) prevent this. This is a design flaw!
You could prevent this for example by moving the call to the highest level class, or make every class responsible for it's own, thus removing the unsafe part of the method call. Then you don't need another class (a base class) to take responsibility for its deriving classes.
If that isn't possible. Make very clear using comments or any other method available that the developer should take this problem into account when updating the code.
Call SetupData in the constructor of Example (and every other derived class) not ExampleBase and make Example a sealed class.
The problem is that SetupData could access something that would be initialized by the Example constructor. But the Example constructor is called only after ExampleBase constructor has finished.
Your base class constructor is called first. If your override method in your subclass depends on anything done in its constructor it won't work. Personally I'd look for a different design, maybe passing the abstract class into the derived class rather than using inheritance.
So a couple lines of code in every derived class
If you need to control the process order then you can do this
public abstract class MyBase
{
public void ProcessData()
{
bool processData = true;
}
public MyBase()
{
bool myBase = true;
}
public MyBase(int pass)
{
bool myBase = true;
}
}
public class Example : MyBase
{
public void GetData() {}
public Example()
: base(1)
{
bool example = true;
GetData();
ProcessData();
}
}
I want to access class members that are in Class1 from another class (Class2) but I want to access it just from that class and to forbid access form any other class, form, etc. Is there any way to do this?
The only way to do this is to nest the classes, and then make the data private:
public class Class1
{
private object data;
public class Class2
{
public void Foo(Class1 parent)
{
Console.WriteLine(parent.data);
}
}
}
You can use nested class with private scope:
public class Class2
{
public Class2()
{
Class1 c1 = new Class1();
Console.WriteLine(c1.Id);
}
private class Class1
{
public int Id { get; set; }
}
}
Actually, you can do this with a bit of lateral thinking. One way is to create a method on Class1 that accepts an instance of Class2 and returns delegates to access the members which are marked private. Then declare Class2 as sealed to prevent someone sneaking in with inheritance.
Another way would be to inspect the call stack with a StackFrame in each member (fields are out for this) and throw an Exception if the caller isn't the desired type.
Lord knows.why you'd do this though! :)
You want a c# equivalent of "friend", but there isn't really one. This previous question gives possible solutions.
You can make an interface, that expose properties for access to your private members. And implement it explicitly. Than use that interface in other class.
2-nd way is to use reflection.
3-rd is nested classes.
you may implement a property
[IsAllowed(true)]
public class a
{
}
then at your,method you may change the sign of each method requiring the instance.
public class b {
public void method ( object aClassInstace){
.... Here check the property
}
}
A readonly field should be used when you have a variable that will be known at object-instatiation which should not be changed afterwards.
However one is not allowed to assign readonly fields from constructors of subclasses.
This doesn't even work if the superclass is abstract.
Does anyone have a good explanation why this either isn't a good idea, or lacks in the C# languange?
abstract class Super
{
protected readonly int Field;
}
class Sub : Super
{
public Sub()
{
this.Field = 5; //Not compileable
}
}
PS: You can of course reach the same result by having assignment of the readonly fields in a protected constructor in the superclass.
The only reason I can see for this is because "it was just designed that way", as per the spec:
Direct assignments to readonly fields can only occur as part of that
declaration or in an instance constructor or static constructor in the
same class.
The point of being read only is that it cannot be changed, if derived classes could modify then this would no longer be true and will violate encapsulation (by modifying the internals of another class).
public class Father
{
protected readonly Int32 field;
protected Father (Int32 field)
{
this.field = field;
}
}
public class Son : Father
{
public Son() : base(5)
{
}
}
You may try something like this instead!
I would model this by an abstract/virtual property in C#.
abstract class Super {
protected abstract int Field { get; }
}
class Sub : Super {
protected override int Field { get { return 5; } }
}
In my opinion that's a better solution than to have a constructor that includes each and every readonly field as parameter. For one because the compiler is able to inline this as well and also because the constructor solution will look like this in the derived class:
class Sub : Super {
public Sub() : base(5) { } // 5 what ?? -> need to check definition of super class constructor
}
Also that may not work if you already have a constructor that takes a single int value.
I suppose the main reason is an additional complexity for all .NET language implementations
also, there is always a simple workaround:
abstract class Super
{
protected readonly int Field;
protected Super(int field)
{
this.Field = field;
}
}
class Sub : Super {
public Sub():base(5)
{
}
}
I would prefer to use the protected constructor in superclass (as mentioned by alexm), reachly with xml comments.
This should eliminate the problem what DonAndre told in his code comment.
Here is an example. I have two classes, one inherited, and both have a function with the same name, but different arguments:
public class MyClass
{
//public class members
public MyClass()
{
//constructor code
}
public void Copy(MyClass classToCopy)
{
//copy code
}
}
public class InheritedClass : MyClass
{
//public class members
public InheritedClass():base()
{
//constructor code
}
public void Copy(InheritedClass inheritedClassToCopy)
{
//copy code
}
}
My question is how do I make the base class' copy method (MyClass.Copy) non-inheritable or non-visible in InheritedClass? I don't want to be able to do this:
MyClass a;
InheritedClass b;
b.Copy(a);
Does this make sense, or should I keep this functionality in there? Can what I'm asking even be done?
Does this make sense, or should I keep this functionality in there? Can what I'm asking even be done?
Trying to hide a public method like this when used by a base class is problematic. You're purposely trying to violate the Liskov substitution principle.
You can't do what you are wanting to do here; C# does not allow negative variance in inherited members. (almost no languages truly do, actually)
It may be that you don't want an inherited class here at all, though; what you may really want is an interface. Or... your two classes here may not have the correct relationship; perhaps they should both instead be common siblings of a third class, which is their parent.
You can use explicit interface implementation to hide this method from the inheritor. But you will need to add an interface of course and you will need to cast your type to the interface to call your method:
public interface MyInterface
{
void Copy(MyClass classToCopy)
}
public class MyClass : MyInterface
{
void MyInterface.Copy(MyClass classToCopy)
{
//copy code
}
}
This is not possible. An inherited class inherits all public and protected members, methods and properties. Using the sealed modifier with make it non-overridable, but still accessible to your inherited class.
What everyone else said, but if I am inferring your goal correctly, it is to make sure that InheritedClass users never use the MyClass method. In that case, exclude it from MyClass and make two classes that inherit it.
Make MyBaseClass abstract if it should not be instantiated (most likely).
(Edited -- you probably would want to include copy code for anything that's part of the base class in the base class)
public abstract class MyBaseClass
{
public MyClass()
{
//constructor code
}
protected void Copy(MyBaseClass classToCopy)
{
//copy code
}
// other methods that all inherited classes can use
}
public class MyClass: MyBaseClass
{
public MyClass():base()
{
//constructor code
}
public void Copy(MyClass myClassToCopy)
{
base.Copy(myClassToCopy);
//specific copy code for this extensions in this class
}
}
public class InheritedClass : MyBaseClass
{
public InheritedClass():base()
{
//constructor code
}
public void Copy(InheritedClass inheritedClassToCopy)
{
base.Copy(myClassToCopy);
//specific copy code for this extensions in this class
}
}
I've got a (poorly written) base class that I want to wrap in a proxy object. The base class resembles the following:
public class BaseClass : SomeOtherBase
{
public BaseClass() {}
public BaseClass(int someValue) {}
//...more code, not important here
}
and, my proxy resembles:
public BaseClassProxy : BaseClass
{
public BaseClassProxy(bool fakeOut){}
}
Without the "fakeOut" constructor, the base constructor is expected to be called. However, with it, I expected it to not be called. Either way, I either need a way to not call any base class constructors, or some other way to effectively proxy this (evil) class.
There is a way to create an object without calling any instance constructors.
Before you proceed, be very sure you want to do it this way. 99% of the time this is the wrong solution.
This is how you do it:
FormatterServices.GetUninitializedObject(typeof(MyClass));
Call it in place of the object's constructor. It will create and return you an instance without calling any constructors or field initializers.
When you deserialize an object in WCF, it uses this method to create the object. When this happens, constructors and even field initializers are not run.
If you do not explicitly call any constructor in the base class, the parameterless constructor will be called implicitly. There's no way around it, you cannot instantiate a class without a constructor being called.
At least 1 ctor has to be called. The only way around it I see is containment. Have the class inside or referencing the other class.
I don't believe you can get around calling the constructor. But you could do something like this:
public class BaseClass : SomeOtherBase
{
public BaseClass() {}
protected virtual void Setup()
{
}
}
public BaseClassProxy : BaseClass
{
bool _fakeOut;
protected BaseClassProxy(bool fakeOut)
{
_fakeOut = fakeOut;
Setup();
}
public override void Setup()
{
if(_fakeOut)
{
base.Setup();
}
//Your other constructor code
}
}
If what you want is to not call either of the two base class constructors, this cannot be done.
C# class constructors must call base class constructors. If you don't call one explicitly, base( ) is implied. In your example, if you do not specify which base class constructor to call, it is the same as:
public BaseClassProxy : BaseClass
{
public BaseClassProxy() : base() { }
}
If you prefer to use the other base class constructor, you can use:
public BaseClassProxy : BaseClass
{
public BaseClassProxy() : base(someIntValue) { }
}
Either way, one of the two will be called, explicitly or implicitly.
When you create a BaseClassProxy object it NEEDS to create a instance of it's base class, so you need to call the base class constructor, what you can doo is choose wich one to call, like:
public BaseClassProxy (bool fakeOut) : base (10) {}
To call the second constructor instead of the first one
I am affraid that not base calling constructor isn't option.
I ended up doing something like this:
public class BaseClassProxy : BaseClass
{
public BaseClass BaseClass { get; private set; }
public virtual int MethodINeedToOverride(){}
public virtual string PropertyINeedToOverride() { get; protected set; }
}
This got me around some of the bad practices of the base class.
constructors are public by nature. do not use a constructor and use another for construction and make it private.so you would create an instance with no paramtersand call that function for constructing your object instance.
All right, here is an ugly solution to the problem of one class inheriting the constructors of another class that I didn't want to allow some of them to work. I was hoping to avoid using this in my class but here it is:
Here is my class constructor:
public MyClass();
{
throw new Exception("Error: Must call constructor with parameters.");
}
OK now you were warned that it was ugly. No complaints please!
I wanted to force at least the minimal parameters from my main constructor without it allowing the inherited base constructor with no parameters.
I also believe that if you create a constructor and do not put the : base() after it that it will not call the base class constructor. And if you create constructors for all of the ones in the base class and provide the same exact parameters for them in the main class, that it will not pass through. But this can be tedious if you have a lot of constructors in the base class!
It is possible to create an object without calling the parameterless constructor (see answer above). But I use code like this to create a base class and an inherited class, in which I can choose whether to execute the base class's init.
public class MyClass_Base
{
public MyClass_Base()
{
/// Don't call the InitClass() when the object is inherited
/// !!! CAUTION: The inherited constructor must call InitClass() itself when init is needed !!!
if (this.GetType().IsSubclassOf(typeof(MyClass_Base)) == false)
{
this.InitClass();
}
}
protected void InitClass()
{
// The init stuff
}
}
public class MyClass : MyClass_Base
{
public MyClass(bool callBaseClassInit)
{
if(callBaseClassInit == true)
base.InitClass();
}
}
Here is my solution to the problem
using System;
public class Program
{
public static void Main()
{
Console.WriteLine(new Child().Test);
}
public class Child : Parent {
public Child() : base(false) {
//No Parent Constructor called
}
}
public class Parent {
public int Test {get;set;}
public Parent()
{
Test = 5;
}
public Parent(bool NoBase){
//Don't do anything
}
}
}
A simple elegant solution. You can change it according to your need.
Another simple solution from me:
class parent
{
public parent()
{
//code for all children
if (this.GetType() == typeof(child1))
{
//code only for objects of class "child1"
}
else
{
//code for objects of other child classes
}
}
}
class child1 : parent
{
public child1()
{}
}
// class child2: parent ... child3 : parent ... e.t.c