Why is it that the following is legal C#:
public interface ISomeInterface
{
int SomeProperty
{
get;
}
}
public class SomeClassImplementingInterface : ISomeInterface
{
public int SomeProperty
{
get { return 32; }
protected set {}
}
}
but this is not:
public abstract class SomeAbstractClass
{
public abstract int SomeProperty
{
get;
}
}
public class SomeClassExtendingAbstractClass : SomeAbstractClass
{
public override int SomeProperty
{
get { return 32; }
protected set {}
}
}
The latter results in the following compile-time error:
'InterfaceAbstractTest.SomeClassExtendingAbstractClass.SomeProperty.set':
cannot override because
'InterfaceAbstractTest.SomeAbstractClass.SomeProperty' does not have
an overridable set accessor InterfaceAbstractTest
What is the reasoning for not disallowing the latter whilst allowing the former?
Because a caller using the interface only cares that an implementer of the interface at least implements the interface's definition, as #davisoa states, whereas SomeAbstractClass in your example defines a public contract which states exactly the type, accessibility, and (for properties) readability/writability of members.
If you use reflection to get the PropertyInfo of SomeProperty (from either the base or child class), it needs to resolve that information from somewhere. Allowing the child class to change the readability/writability would be as much of a contract violation as a change in return type or argument list.
Imagine for instance:
SomeAbstractClass sc = new SomeClassExtendingAbstractClass();
PropertyInfo pi = sc.GetType().GetProperty("SomeProperty");
Console.Out.WriteLine(pi.CanWrite); // What should be printed here?
This is because the Interface implementation is making a promise that there will be a property SomeProperty that you can "Get".
The abstract class implementation is making a promise that it's child classes will provide an implementation of a property SomeProperty with a public get method.
In the end, the base class is defining something that must be overridden, whereas the interface is defining a contract.
This is by design. I am quoting from the C# language specs:
An overriding property declaration must specify the exact same
accessibility modifiers, types and name as the inherited property, if
the inherited property has only a single accessor (i.e.,... ready only
or write-only), the overriding property must include only that
accessor.
The reason behind that decesion could be because the interfaces are more flexibly type of contracts than abstract classes. Interfaces cares only about the least common denominator rather than the whole implementation. I think there are good reasons to choose one design over the other.
You're trying to override a set operator that doesn't exist. Either define a set portion of the property in the abstract class, or don't try to define one in the concrete class. Since you have the set as protected in the concrete class, my guess is what you want to do is make a protected set operator in the abstract definition.
What is necessary is to both override the existing property and shadow it with a new read-write one. Unfortunately, .net does not provide any means of both overriding and shadowing a member within a single class. The best one can do is probably to have the abstract base class define a concrete non-virtual read-only property whose getter calls an abstract function. A derived class can then shadow the property with a non-virtual read-write function which calls the same function in its getter, and a new abstract or virtual function in its setter.
Related
Okay, I have two namespaces. One contains my interface and one contains the implementing class. Like this:
namespace Project.DataAccess.Interfaces
{
public interface IAccount
{
string SomeMethod();
}
}
namespace Project.DataAccess.Concrete
{
class Account : Project.DataAccess.Interfaces.IAccount
{
string SomeMethod()
{
return "Test";
}
}
}
With this code I get an error:
'Project.DataAccess.Concrete.Account' does not implement interface member 'Project.DataAccess.Interfaces.IAccount.SomeMethod'. 'Project.DataAccess.Concrete.Account.SomeMethod' cannot implement an interface member because it is not public
If I make the class and method public it works fine. But if I instead qualify the method name with the interface name, that fixes it too. Why? Ex:
namespace Project.DataAccess.Concrete
{
class Account : Project.DataAccess.Interfaces.IAccount
{
string IAccount.SomeMethod()
{
return "Test";
}
}
}
Why does this fix it? And why does it have to be public if I don't do that?
To be clear
I am well aware that making it public fixes it. Making the method signature look like this WITHOUT making anything public fixes it:
string IAccount.SomeMethod()
Why?
Interface implementations need to be public or explicit:
Public:
class Account : IAccount
{
public string SomeMethod()
{
return "Test";
}
}
Explicit:
class Account : IAccount
{
string IAccount.SomeMethod()
{
return "Test";
}
}
The default access modifier in C# is private if you do not specify the access modifier.
You have 2 basic options when implementing intefaces: implicit or explicit. Implicit implementation takes on the form of a public method or property, while explicit is in the form of a method or property prefaced with the IFoo. modifier that is otherwise not public.
interface IFoo
{
void Bar();
}
class FooA : IFoo
{
public void Bar() { }
}
class FooB : IFoo
{
void IFoo.Bar() { }
}
The difference here is that in the case of FooA, void Bar is part of the publicly visible API of the class. Code can call Bar via the instance of the class.
FooA foo = new FooA();
foo.Bar(); // legal
In the case of FooB, void Bar is not part of the publicly visible API of the class. The method can still be called, but it must explicitly be called via the interface.
FooB foo = new FooB();
foo.Bar(); // not legal
IFoo myFoo = foo;
myFoo.Bar(); // legal
Your code does not compile because it walks in the uncharted waters between an implicit and explicit interface implementation. You saw that with your change, you had legally defined an explicit implementation, which is why that particular version compiles. Otherwise, you've also found that the public modifer is needed to make the non-explicit version compile.
Methods implementing interface needs to be public. In your later case, you are declaring it explicitly. This is what specification says about explicit interface implementation.
Explicit interface member
implementations have different
accessibility characteristics than
other members. Because explicit
interface member implementations are
never accessible through their fully
qualified name in a method invocation
or a property access, they are in a
sense private. However, since they can
be accessed through an interface
instance, they are in a sense also
public.
'Project.DataAccess.Concrete.Account.SomeMethod' cannot implement an interface member because it is not public
namespace Project.DataAccess.Concrete
{
public class Account : IAccount
{
public string IAccount.SomeMethod()
{
return "Test";
}
}
}
You're not mentioning in your class declaration that you'll be implementing IAccount.
Your class declaration should look like this:
class Account : IAccount
{
//Implementation here.
}
Also, what's happening is that you're using the "default" protection level for Account, and that protection level isn't "public", but an Interface (IAccount) defines public methods by default.
So, when you preface both the Class and Method names with public you're actually implementing the interface. Likewise, when you declare SomeMethod as
IAccount.SomeMethod()
{
//Implementation Here
}
what you're doing is Explicitly Implementing the interface.
I think declaring it explicitly with string IAccount.SomeMethod() works because .NET knows that this implementation of the method can only be accessed through the interface, and so it carries over the public visibility from the interface. In other words, since you are explicitly saying it is an interface member, the compiler can imply that it has to be public, so you don't have to re-state the obvious.
This is also discussed here: http://msdn.microsoft.com/en-us/library/aa664591%28v=vs.71%29.aspx
It is a compile-time error for an
explicit interface member
implementation to include access
modifiers, and it is a compile-time
error to include the modifiers
abstract, virtual, override, or
static.
Explicit interface member
implementations have different
accessibility characteristics than
other members. Because explicit
interface member implementations are
never accessible through their fully
qualified name in a method invocation
or a property access, they are in a
sense private. However, since they can
be accessed through an interface
instance, they are in a sense also
public.
class Account : IAccount
{
public string SomeMethod()
{
return "Test";
}
}
Tell your class to implement the interface. Using the Interface.Method name explicitly implements the method (but i'm not sure why or how) and you need to make them public.
You want to use Interface.Method to explicitly implement an interface requirement when the class already has a member of the same name
class MyClass : IAccount
{
public int SomeMethod() {}
public string IAccount.SomeMethod() {}
}
Greetings everyone...
If I have the following interface:
interface IMyInterface
{
int property { get; set; }
}
And the following implementation:
class MyClass : IMyInterface
{
// anything
}
How can I hide the set method of property from the instances of MyClass... In other words, I don't want the set method of property to be public, is that possible?
It would be easy to do with abstract class:
abstract class IMyInterface
{
int property { get; protected set; }
}
Then I could only set the property within the class that implements the abstract class above...
Don't have the set in the interface to begin with. You can still implement it as private.
You can't "hide" it, it's part of the contract. If you don't want it to be part of the contract, don't define it.
If you use the following interface the set method will be unavailable when classes are manipulated via the interface:
interface IMyInterface
{
int property { get; }
}
You could then implement the class like this:
class MyClass : IMyInterface
{
int property { get; protected set; }
}
If some implementations will only implement some parts of an interface, it may be a good idea to subdivide the interface into the parts which each implementation will either implement completely or not at all, and then define interfaces which inherit all the common combinations of them. Adapting your example:
interface IMyReadableInterface
{
int property { get; }
}
interface IMyFullInterface : IMyReadableInterface
{
new int property { get; set; }
}
Classes which want to support read-write access should implement IMyFullInterface; those which want to only support read access should only implement IMyReadableInterface. This segregation will not require any extra work for implementations of either interface which are written in C# and implement property implicitly. Code which implements property in VB, or explicitly implements property in C#, will have to define two implementations of property--a read-only one and a read-write one, but such is life. Note that while one could define an IMyWritableInterface which just had a setter, and have IMyFullInterface inherit both IMyReadableInterface and IMyWritableInterface, IMyFullInterface would still have to define a read-write property of its own, and when using explicit implementation one would then have to define three properties (I really don't understand why C# can't use a read-only and write-only property together as thought they were a read-write property, but it can't).
Assuming you need the setter to be part of the interface but for some reason it does not make sense for it to be used on a particular implementer (in this case MyClass) you can always throw an exception in the setter (such as an InvalidOperationException). This will not protect you at compile time, only at run time. It is a bit strange though, as code that operates on the interface has no idea whether calling the setter is allowed.
There are certainly cases where you want the interface to have a set and then hide it in some concrete class.
I believe the code below shows what we want to accomplish. I.e. the implementation hides the setter, but any IMyInterface aware component will have access to it.
public static void Main()
{
var myClass = new MyClass();
myClass.Property = 123; // Error
((IMyInterface)myClass).Property = 123; // OK
}
It's basically the same pattern you often see for IDisposable.Dispose() where you have an Explicit Interface Implementation. Here's an example for completeness.
public interface IMyInterface
{
int Property { get; set; }
}
public class MyClass : IMyInterface, IDisposable
{
public int Property { get; private set; }
int IMyInterface.Property
{
get => Property;
set => Property = value;
}
void IDisposable.Dispose() {}
}
Too much typing :(
C# doesn't help us much here. Ideally, it would be possible to have an explicit interface implementation for the setter:
// In C# 10 maybe we can do this instead:
public class MyFutureClass : IMyInterface
{
public int Property { get; IMyInterface.set; }
}
See C# feature proposal here.
There is no protected or private in interface, everything is public. Either you don't define any set or use it as public.
For example, suppose I want an ICar interface and that all implementations will contain the field Year. Does this mean that every implementation has to separately declare Year? Wouldn't it be nicer to simply define this in the interface?
Though many of the other answers are correct at the semantic level, I find it interesting to also approach these sorts of questions from the implementation details level.
An interface can be thought of as a collection of slots, which contain methods. When a class implements an interface, the class is required to tell the runtime how to fill in all the required slots. When you say
interface IFoo { void M(); }
class Foo : IFoo { public void M() { ... } }
the class says "when you create an instance of me, stuff a reference to Foo.M in the slot for IFoo.M.
Then when you do a call:
IFoo ifoo = new Foo();
ifoo.M();
the compiler generates code that says "ask the object what method is in the slot for IFoo.M, and call that method.
If an interface is a collection of slots that contain methods, then some of those slots can also contain the get and set methods of a property, the get and set methods of an indexer, and the add and remove methods of an event. But a field is not a method. There's no "slot" associated with a field that you can then "fill in" with a reference to the field location. And therefore, interfaces can define methods, properties, indexers and events, but not fields.
Interfaces in C# are intended to define the contract that a class will adhere to - not a particular implementation.
In that spirit, C# interfaces do allow properties to be defined - which the caller must supply an implementation for:
interface ICar
{
int Year { get; set; }
}
Implementing classes can use auto-properties to simplify implementation, if there's no special logic associated with the property:
class Automobile : ICar
{
public int Year { get; set; } // automatically implemented
}
Declare it as a property:
interface ICar {
int Year { get; set; }
}
Eric Lippert nailed it, I'll use a different way to say what he said. All of the members of an interface are virtual and they all need to be overridden by a class that inherits the interface. You don't explicitly write the virtual keyword in the interface declaration, nor use the override keyword in the class, they are implied.
The virtual keyword is implemented in .NET with methods and a so-called v-table, an array of method pointers. The override keyword fills the v-table slot with a different method pointer, overwriting the one produced by the base class. Properties, events and indexers are implemented as methods under the hood. But fields are not. Interfaces can therefore not contain fields.
Why not just have a Year property, which is perfectly fine?
Interfaces don't contain fields because fields represent a specific implementation of data representation, and exposing them would break encapsulation. Thus having an interface with a field would effectively be coding to an implementation instead of an interface, which is a curious paradox for an interface to have!
For instance, part of your Year specification might require that it be invalid for ICar implementers to allow assignment to a Year which is later than the current year + 1 or before 1900. There's no way to say that if you had exposed Year fields -- far better to use properties instead to do the work here.
The short answer is yes, every implementing type will have to create its own backing variable. This is because an interface is analogous to a contract. All it can do is specify particular publicly accessible pieces of code that an implementing type must make available; it cannot contain any code itself.
Consider this scenario using what you suggest:
public interface InterfaceOne
{
int myBackingVariable;
int MyProperty { get { return myBackingVariable; } }
}
public interface InterfaceTwo
{
int myBackingVariable;
int MyProperty { get { return myBackingVariable; } }
}
public class MyClass : InterfaceOne, InterfaceTwo { }
We have a couple of problems here:
Because all members of an interface are--by definition--public, our backing variable is now exposed to anyone using the interface
Which myBackingVariable will MyClass use?
The most common approach taken is to declare the interface and a barebones abstract class that implements it. This allows you the flexibility of either inheriting from the abstract class and getting the implementation for free, or explicitly implementing the interface and being allowed to inherit from another class. It works something like this:
public interface IMyInterface
{
int MyProperty { get; set; }
}
public abstract class MyInterfaceBase : IMyInterface
{
int myProperty;
public int MyProperty
{
get { return myProperty; }
set { myProperty = value; }
}
}
Others have given the 'Why', so I'll just add that your interface can define a Control; if you wrap it in a property:
public interface IView {
Control Year { get; }
}
public Form : IView {
public Control Year { get { return uxYear; } } //numeric text box or whatever
}
A lot has been said already, but to make it simple, here's my take.
Interfaces are intended to have method contracts to be implemented by the consumers or classes and not to have fields to store values.
You may argue that then why properties are allowed? So the simple answer is - properties are internally defined as methods only.
Interfaces do not contain any implementation.
Define an interface with a property.
Further you can implement that interface in any class and use this class going forward.
If required you can have this property defined as virtual in the class so that you can modify its behaviour.
Beginning with C# 8.0, an interface may define a default implementation for members, including properties. Defining a default implementation for a property in an interface is rare because interfaces may not define instance data fields.
https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/interface-properties
interface IEmployee
{
string Name
{
get;
set;
}
int Counter
{
get;
}
}
public class Employee : IEmployee
{
public static int numberOfEmployees;
private string _name;
public string Name // read-write instance property
{
get => _name;
set => _name = value;
}
private int _counter;
public int Counter // read-only instance property
{
get => _counter;
}
// constructor
public Employee() => _counter = ++numberOfEmployees;
}
For this you can have a Car base class that implement the year field, and all other implementations can inheritance from it.
An interface defines public instance properties and methods. Fields are typically private, or at the most protected, internal or protected internal (the term "field" is typically not used for anything public).
As stated by other replies you can define a base class and define a protected property which will be accessible by all inheritors.
One oddity is that an interface can in fact be defined as internal but it limits the usefulness of the interface, and it is typically used to define internal functionality that is not used by other external code.
I have an abstract class with an abstract property that is set to have both Get and Set. I know I'll always want to be able to get this property from derived classes but there are some cases where it doesn't make sense to set this property in certain types of derived classes.
I can't just omit the Set accessor in a derived class (see code example below). I could override the set accessor in a derived classes to do nothing with the values passed by the user. But is there another way that actually make the property in a specific derived class read only? Ultimately I'm displaying these properties in a property grid and I don't want the user to be entering values into a field that is going to do nothing. Maybe I just attribute the property as read only in specific derived classes?
Also I'd really really rather not mess with any of the type descriptor stuff to get properties to display correctly in a property grid, such as overriding ICustomTypeDescriptor.
public abstract class MyClass
{
public abstract string MyProperty
{
get;
set;
}
}
public abstract class MyDerivedClass
{
public override string MyProperty
{
//VS complains that the Set accessor is missing
get;
}
}
You should not do this. What you are saying by defining your getter and setter in the abstract class is "you must implement this if you want to inherit from me." Then you are asking, "how can I made a derived class ignore this rule."
The answer is that if you have a situation that every derived class needs a getter, put that in the abstract class and let the derived class decide if they will implement a setter or not by leaving it out of the abstract class.
Or alternatively, you can create two more classes that derive from the initial abstract class, one that implement the setter and one that does not and then have your derived class generalize the one of those that makes sense, but that is overkill I think.
look like you searching for
[ReadOnly(true)] attribute
this will show to property grid your property, as readonly.
but in your class you can use it as usual property (with read and write possibilities)
You should use abstract not override:
public abstract class MyClass
{
public abstract string MyProperty
{
get;
set;
}
}
public abstract class MyDerivedClass
{
public abstract string MyProperty
{
get;
}
}
but like #JP wrote, you shouldn't do this.
In MSDN, it is mentioned,
http://msdn.microsoft.com/en-us/library/9fkccyh4(VS.80).aspx
I am confused what does this item mean "A virtual inherited property can be overridden in a derived class by including a property declaration that uses the override modifier."?
(this is the 2nd differences between virtual and abstract)
thanks in advance,
George
The only difference between virtual and abstract, is that an abstract method or propery has no implementation in the class where it has been defined (the abstract class), and that it must be overriden in a subclass; whereas a virtual method or property has an implementation in the class where it has been defined, and so it is not mandatory to override it in a subclass.
public abstract AbstractClass
{
// This class cannot be instantiated, since it is
// abstract, and the class is abstract because
// it has an abstract member
public abstract MyProperty {get; set; }
}
In a class where you derive from AbstractClass (the above AbstractClass is just for explanation purposes; since it has no methods / properies that have an implementation, you could create an interface instead of an abstract class), you will have to provide an implementation of MyProperty. Otherwise, it won't compile.
You do this by 'overriding' the MyProperty, you don't want to introduce a new member, but just provide an implementation for a property that has been defined previously.
public class ConcreteClass : AbstractClass
{
public override MyProperty {
get
{
return _someValue;
}
set
{
if( _someValue != value ) _someValue = value;
}
}
I can understand the confusion. I have been there before, so I'll share how I keep the basic differences straight...
virtual vs. abstract:
If a class method (or property) is
marked virtual, then it may be
overridden using the override
keyword, if you choose to inherit
(aka derive) from that class. The virtual keyword is intended to evoke the idea that the method may or may not be the actual method called. Therefore, I always think of virtual members as default implementations, meaning that it represents functionality that can be generalized, such as an Eat() method on a Human class, which might involve eating with one's hands. However, a ChineseHuman class might override the default implementation of Eat(), in order to allow for an implementation that uses chop sticks instead. Finally, because virtual methods and properties are default implementations, the class that defines that member must provide a complete implementation of the method or property. All Human objects must know how to Eat(). An object-oriented way of thinking might declare that virtual members represent instincts. To Eat() is an instinct of a Human class object. A ChineseHuman may learn to Eat() with chop sticks.
If a class method (or property) is
marked abstract, then it must
be overridden using the override
keyword, if you choose to inherit
from that class.The abstract keyword is intended to evoke the idea that the class only supports the capability represented by the member, and that there is not any common logic that can be generalized for that feature. In other words, abstract members are only conceptual, and therefore they lack an implementation. It is a little confusing that C# asks us to override abstract members when we implement an inheritance relationship, but in this case it really means that we are overriding the empty concept with a concrete implementation. An example of an abstract member of a Human class might be Speak(). There would not be a common way of speaking for all Human objects, nor is it instinctual, because it requires language to express. Note: Some might argue that Speak() belongs on an interface instead.An object-oriented way of thinking might declare that abstract members represent behavior (methods) to be learned and knowledge or beliefs (properties) to be acquired. To Speak() is a learned behavior of a Human class object. A ChineseHuman may learn to Speak() differently than an EnglishHuman and neither knows how to Speak() just because they are both Human.
Nuances:
virtual methods do NOT need to be overridden.
There is no such thing as a virtual class.
abstract members can only appear on abstract classes. In the above examples, having an abstract method on the Human class implies that Human is an abstract class, and that, therefore, a Human cannot be instantiated using the phrase var baby = new Human();. Instead, the BabyHuman class should inherit from Human, and it should be instantiated as var baby = new BabyHuman();. Because a BabyHuman() is a Human and EnglishHuman and ChineseHuman both also inherit from Human, EnglishHuman could inherit from BabyHuman instead of Human. Being Human is abstract because we are all something more than simply Human.
abstract members cannot be hidden, only their override implementations may be (further up the inheritance chain). For example, BabyHuman must implement the abstract Speak() method as an override. If EnglishHuman inherits from BabyHuman, it may then hide the BabyHuman implementation of Speak() with its own implementation by using the new keyword (see "Method Hiding in C#" reference below).
abstract classes can have virtual members. That's a main distinction between an interface and an abstract class. In that sense, an abstract class can define both a contract and a template of the class' behavior, whereas an interface only defines a contract.
Code Reference:
Abstract Class versus Interface
By Rahman Mahmoodi
http://www.codeproject.com/KB/cs/abstractsvsinterfaces.aspx
Method Hiding in C#
By Chetan Kudalkar
http://www.codeproject.com/KB/cs/cs_methodhiding.aspx
Can you explain what's confusing about it? Properties can be overridden like any other method.
public class Base {
public virtual int Prop1 { get { ... } set { ... } }
}
public class Derived : Base {
public override int Prop1 { get { ... } set { ... } }
If you declare a method virtual in your base class you can override it in your derived class.
Example
class MyBaseClass
{
public virtual void MyOverridableMethod()
{
...
}
}
class MyDerivedClass : MyBaseClass
{
public override void MyOverridableMethod()
{
...
}
}
Notice the override modifier in MyDerivedClass.
Okay, let's say you have a base class, and that that base class is, itself, derived from another class.
public class Bar : Foo
{
virtual public int SomeProperty { get; set; }
}
What the virtual keyword means is that in a class derived from Bar, you can override SomeProperty to change its behavior:
public class Baz : Bar
{
private int thisInt;
override public int SomeProperty
{
get { return thisInt; }
set
{
if(value < 0)
{
throw new ArgumentException("Value must be greater than or equal to zero.");
}
thisInt = 0;
}
}
}
Clarification: When an object of type Baz is used, its version of SomeProperty is invoked, unless the type is cast to Bar. If you define Baz's SomeProperty as virtual, classes derived from Baz can also override it (in fact, that may be required--can't recall right off the top of my head).
Further Clarification: An abstract method has no implementation; when you add one to your class, you must also mark the class as abstract, and you cannot instantiate new instances of it, like this:
MyAbstractType m = new MyAbstractType();
Virtual members, on the other hand, can have an implementation (like SomeProperty, above), so you don't have to mark the class abstract, and you can instantiate them.