如何在派生类中隐藏基类公共属性

Question

I want to hide the base public property(a data member) in my derived class:

class Program
{
    static void Main(string[] args)
    {
        b obj = new b();
        obj.item1 = 4;// should show an error but it doent ???
    }
}

class a
{
    public int item1 {get; set;}
    public int item2 { get; set; }
}

class b : a
{
    new private int item1;
}

class c : a
{

}

i have member as public because i want the member to be inherited in c class , but want to hide the member in b class , how can i do this ?

dont i have an option to selectively inherite the variable i want in my base class ??? thats really bad , i think ms should provide us with an option (may be a modifier) to perform this

---

Edit:

I found the answer myself (i heard lots of them telling this is not possible in c#, but you can kind of do it)

I am including the code in case it is useful

class Program
{
    static void Main(string[] args)
    {
        b obj = new b();
        obj.item1 = 4; // shows an error  : )
    }
}

class a
{
    public int item1 { get; set; }
    public int item2 { get; set; }
}

class b : a
{
    new public static int item1
    {
        get;
        private set;
    }
}

Answer 1

I'm going to attempt to explain with examples why this is a bad idea, rather than using cryptic terms.

Your proposal would be to have code that looks like this:

public class Base
{
    public int Item1 { get; set; }
    public int Item2 { get; set; }
}


public class WithHidden : Base
{
    hide Item1; // Assuming some new feature "hide" in C#
}

public class WithoutHidden : Base { }

This would then make the following code invalid:

WithHidden a = new WithHidden();
a.Item1 = 10; // Invalid - cannot access property Item1
int i = a.Item1; // Invalid - cannot access property Item1

And that would be just what you wanted. However, suppose we now have the following code:

Base withHidden = new WithHidden();
Base withoutHidden = new WithoutHidden();

SetItem1(withHidden);
SetItem1(withoutHidden);

public void SetItem1(Base base)
{
    base.Item1 = 10;
}

The compiler doesn't know what runtime type the argument base in SetItem1 will be, only that it is at least of type Base (or some type derived from Base, but it can't tell which -- it may be obvious looking at the code snippet, but more complex scenarios make it practically impossible).

So the compiler will not, in a large percentage of the cases, be able to give a compiler error that Item1 is in fact inaccessible. So that leaves the possibility of a runtime check. When you try and set Item1 on an object which is in fact of type WithHidden it would throw an exception.

Now accessing any member, any property on any non-sealed class (which is most of them) may throw an exception because it was actually a derived class which hid the member. Any library which exposes any non-sealed types would have to write defensive code when accessing any member just because someone may have hidden it.

A potential solution to this is to write the feature such that only members which declare themselves hideable can be hidden. The compiler would then disallow any access to the hidden member on variables of that type (compile time), and also include runtime checks so that a FieldAccessException is thrown if it is cast to the base type and tried to be accessed from that (runtime).

But even if the C# developers did go to the huge trouble and expense of this feature (remember, features are expensive , especially in language design) defensive code still has to be written to avoid the problems of potential FieldAccessExceptions being thrown, so what advantage over reorganising your inheritance hierarchy have you gained? With the new member hiding feature there would be a huge number of potential places for bugs to creep into your application and libraries, increasing development and testing time.

Answer 2

What you want to do goes directly against the grain of OO, you can't 'unpublish' members as this violates the substitution principle. You have to refactor this into something else.

Answer 3

Vadim's response reminded me of how MS achieve this in the Framework in certain places. The general strategy is to hide the member from Intellisense using the EditorBrowsable attribute . (NB This only hides it if it is in another assembly) Whilst it does not stop anyone from using the attribute, and they can see it if they cast to the base type (see my previous explination) it makes it far less discoverable as it doesn't appear in Intellisense and keeps the interface of the class clean.

It should be used sparingly though, only when other options like restructuring the inheritance hierarchy would make it a lot more complex. It's a last resort rather than the first solution to think of.

Answer 4

If you use an interface instead of a base class for defining the property, you could implement the property explicitly. The would require an explicit cast to the interface to use the property.

public interface IMyInterface
{
    string Name { get; set; }
}

public class MyClass : IMyInterface
{

    string IMyInterface.Name { get; set; }

}

You can find more out here .

Answer 5

The only thing I can think of is to make item1 virtual in class a:

class a
{
    public virtual int item1 { get; set; }
    public int item2 { get; set; }

}

and then override it in class b but throw an exception in getter and setter. Also if this property is used in a visual designer you can use Browsable attribute to not display.

class b : a
{
    [Browsable(false)]
    public override int item1
    {
        get
        {
            throw new NotSupportedException();
        }
        set
        {
            throw new NotSupportedException();
        }
    }
}

Answer 6

First of all this is not good idea if you using some methods, that operates base class.
You can try to use obsolete argument to make users twice think to use this property.

[System.Obsolete("Do not use this property",true)]  
public override YourType YourProperty { get; set; }

Answer 7

You can override it and then Add a [Browsable(false)] tag to prevent showing it in designer.

Simple:

public class a:TextBox
{
        [Browsable(false)]
        public override string Text
        {
            get { return ""; }
            set { }
        }
}

Answer 8

您所描述的内容类似于 C++ 中的“私有继承”，并且在 C# 中不可用。

Answer 9

You cant do it directly, but you could override the properties in the child class and make them readonly eg

class Program
{
    static void Main(string[] args)
    {
        b obj = new b();
        obj.item1 = 4;// should show an error but it doent ???
    }
}

class a
{
    public virtual int item1 {get; set;}
    public virtual int item2 { get; set; }

}

class b : a
{
    public override int item1
    { 
         get { return base.item1; }
         set { }
    }
}

    class c : a
{

}

Answer 10

You could use interfaces to hide the property. The child class would implemented an interface that didn't have the property then it wouldn't appear.

You would need two interfaces for when you want the property and when you don't, thus making it a horrible hack.

Answer 11

Changing the accessibility of a virtual member is an inheriting class is specifically prohibited by the C# language spec:

The override declaration and the overridden base method have the same declared accessibility. In other words, an override declaration cannot change the accessibility of the virtual method. However, if the overridden base method is protected internal and it is declared in a different assembly than the assembly containing the override method then the override method's declared accessibility must be protected.

From section 10.6.4 Override methods

The same rules which apply to overriding method also apply to properties, so going from public to private by inheriting from the base class can't be done in C#.

Answer 12

If you wanna hide a member from base class then you will need to add a new base class let's call it baseA and your code should be as follows:

class Program
{
    static void Main(string[] args)
    {
        b obj = new b();
        obj.item1 = 4;// should show an error but it doent ???
    }
}

class baseA
{
    public int item2 { get; set; }
}
class a:baseA
{
    public int item1 { get; set; }        
}

class b : baseA { }

class c : a { }

Answer 13

What you actually need are interfaces:

    public interface ProvidesItem1
    {
        int item1 { get; set; }
    }

    public interface ProvidesItem2
    {
        int item2 { get; set; }
    }

    class a : ProvidesItem1, ProvidesItem2
    {
        public int item1 { get; set; }
        public int item2 { get; set; }
    }

    class b : ProvidesItem1
    {
        public int item1 { get; set; }
    }

Then just pass the interfaces around. If the classes should use a common implementation, put that in a third class and let them derive from that class aswell as implement their respective interface.

Answer 14

Yes, it is possible. What say you on the delegation. I will try to give an idea of what is called "delegation" in OOP with a piece of code:

public class ClassA
{
    // public
    public virtual int MyProperty { get; set; }

    // protected
    protected virtual int MyProperty2 { get; set; }
}

public class ClassB
{
    protected ClassC MyClassC;

    public ClassB()
    {
        MyClassC = new ClassC();
    }

    protected int MyProperty2
    {
        get { return MyClassC.MyProperty2; }
        set { MyClassC.MyProperty2 = value; }
    }

    protected int MyProperty
    {
        get { return MyClassC.MyProperty; }
        set { MyClassC.MyProperty = value; }
    }

    protected class ClassC : ClassA
    {
        public new int MyProperty2
        {
            get { return base.MyProperty2; }
            set { base.MyProperty2 = value; }
        }

        public override int MyProperty
        {
            get { return base.MyProperty; }
            set { base.MyProperty = value; }
        }
    }
}

Answer 15

namespace PropertyTest    
{    
    class a
    {    
        int nVal;

        public virtual int PropVal
        {
            get
            {
                return nVal;
            }
            set
            {
                nVal = value;
            }
        }
    }

    class b : a
    {
        public new int PropVal
        {
            get
            {
                return base.PropVal;
            }
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            a objA = new a();
            objA.PropVal = 1;

            Console.WriteLine(objA.PropVal);

            b objB = new b();
            objB.PropVal = 10; // ERROR! Can't set PropVal using B class obj. 
            Console.Read();
        }
    }
}

Answer 16

You can user new modifer.

Sample;

public class Duck
{
    public string Color{get; set;}
    public void Swim() { }
}
public class DonaldDuck : Duck
{
    new public void Swim() 
    { 
        /*you could call in DonaldDuck.Swim  only here but not public for DonaldDuck client.*/ 
    }
}