I know this may be basic but I cannot seem to add a member variable to an interface.
I tried inheriting the interface to an abstract class and add member variable to the abstract class but it still does not work. Here is my code:
public interface IBase {
void AddData();
void DeleteData();
}
public abstract class AbstractBase : IBase {
string ErrorMessage;
public abstract void AddData();
public abstract void DeleteData();
}
public class AClass : AbstractBase {
public override void AddData();
public override void DeleteData();
}
updated base on comment of Robert Fraser
You cannot add fields to an interface.Interface can only contain methods , so only methods , properties , events can be declared inside an interface decleration.In place of field you can use a property.
public interface IBase {
string ErrorMessage {get;set;}
void AddData();
void DeleteData();
}
public interface IBase {
void AddData();
void DeleteData();
}
public abstract class AbstractBase : IBase {
string ErrorMessage;
public abstract void AddData();
public abstract void DeleteData();
}
Workd for me. You were missing the "public" and "void" on the abstract class methods.
Since interfaces only declare non-implementation details, you cannot declare a member variable in an interface.
However, you can define a property on your interface, and implement the property in your class.
If it is something implementing classes need to implement then use a property definition i.e.
That said if it is something which needs to be private then that it not something which should be in an interface anyway.
Did you mean a property instead of a field?
public interface IBase {
string ErrorMessage { get; set; }
void AddData();
void DeleteData();
}
public abstract class AbstractBase : IBase {
abstract string ErrorMessage { get; set; }
// also, you need to declare a return type for the methods
abstract void AddData();
abstract void DeleteData();
}
Related
I have a class that inherits an abstract class which inherits another abstract class. I can define the most-super class as being generic just fine, but I don't know how to define the middle class as being generic before I express the actual type I want in the final ListResults class.
internal abstract class LowerCommand<T> {
public abstract void Execute();
public abstract List<T> ExecuteList();
}
// currently gives error for `T`
internal abstract class HigherCommand : Lowercommand<T> {
// ... defines other stuff, nothing to do with
// already instantiated methods or T ...
}
class ListResults : HigherCommand<Results>
{
public override void Execute() {...}
public override List<Results> ExecuteList() {...}
}
Thanks for your help.
You still need to define the generic type parameter T on the definition of HigherCommand so it can in turn properly define LowerCommand.
internal abstract class LowerCommand<T> {
public abstract void Execute();
public abstract List<T> ExecuteList();
}
// Note that HigherCommand require a declaration of `T`
internal abstract class HigherCommand<T> : LowerCommand<T> {
// ... defines other stuff, nothing to do with
// already instantiated methods or T ...
}
class ListResults : HigherCommand<Results>
{
public override void Execute() {...}
public override List<Results> ExecuteList() {...}
}
Say I have a base class like this:
public abstract class MyBaseClass
{
protected void MyMethod(string myVariable)
{
//...
}
}
Then I inherit this class in a separate assembly:
public abstract class MyDerivedClass : MyBaseClass
{
static readonly string MyConstantString = "Hello";
protected void MyMethod()
{
MyMethod(MyConstantString);
}
}
I now want to make sure that any other class that inherits from MyDerivedClass does not have access to the MyBaseClass.MyMethod() method. (To clarify, I still want to be able to call MyDerivedClass.MyMethod() with no parameters)
I tried using protected internal but that didn't work.
Update: I'm trying to do this because the application I'm working on has a bunch of separate programs that use a base class. There are 5 different "types" of programs, each performs a specific, separate function but they all have some common code that I am trying to abstract into this base class. The common code is 99% the same, differing only slightly depending on what type of program is calling it. This is why I have my base class taking a string parameter in the example which then disappears in the derived base class, as that class knows it performs role x so it tells its base class accordingly.
Then I would instead of inheritance use composition in the MyDerivedClass. So all derived classes from this class does not know the methods from MyBaseClass. The class MyBaseClass would i make package visible so it is not possible to use it.
abstract class MyBaseClass
{
void MyMethod(string myVariable)
{
//...
}
}
abstract class MyDerivedClass
{
static readonly string MyConstantString = "Hello";
private MyBaseClass baseClass;
MyDerivedClass(MyBaseClass baseClass)
{
this.baseClass = baseClass;
}
protected void MyMethod()
{
baseClass.MyMethod(MyConstantString);
}
}
The class names should be changed of course.
This is not quite possible. And it may be a sign that your object design might be in trouble, but that's not a question for SO.
You can try a bit more underhanded approach, though:
public abstract class MyBaseClass
{
protected abstract string MyConstantString { get; }
protected void MyMethod()
{
//...
}
}
public abstract class MyDerivedClass : MyBaseClass
{
protected override sealed string MyConstantString => "Hello";
}
Or, more typically, just use the constructor to pass the required argument:
public abstract class MyBaseClass
{
private readonly string myString;
protected MyBaseClass(string myString)
{
this.myString = myString;
}
protected void MyMethod()
{
//...
}
}
public abstract class MyDerivedClass : MyBaseClass
{
protected MyBaseClass() : base("Hello") {}
}
Classes derived from MyDerivedClass have no way to change the argument in either case, the second approach is a bit nicer from inheritance perspective (basically, the type is a closure over an argument of its ancestor type).
You cannot stop inheriting classes from calling this method - you have made it protected so your intent is for it to be accessible to classes that inherit from it, whether directly, or via another sub-class.
If you want to keep the inheritance, the best you can do is to throw an error if the sub-class calls it in MyDerivedClass:
public abstract class MyBaseClass
{
protected void MyMethod(string myVariable)
{
Console.WriteLine(myVariable);
}
}
public abstract class MyDerivedClass : MyBaseClass
{
static readonly string MyConstantString = "Hello";
protected void MyMethod()
{
base.MyMethod(MyConstantString);
}
protected new void MyMethod(string myVariable)
{
throw new Exception("Not allowed");
}
}
public class SubDerivedClass : MyDerivedClass
{
static readonly string MyConstantString = "Hello";
public void Foo()
{
MyMethod(MyConstantString);
}
}
When Foo() is called in SubDerivedClass, it will call MyMethod in DerivedClass, which will throw the Exception.
I have two abstract classes AbstractClass1 and AbstractClass2 that both should have the same method names Meth1.
Now what if my AbstractClass1 inherits from AbstractClass2. Currently it is throwing me a compile time error as "Hides Inherited Abstract Member". Below is my code.
abstract class AbstractClass1 : AbstractClass2
{
public abstract string Meth1();
}
abstract class AbstractClass2
{
public abstract string Meth1();
}
You can use abstract override on derived class method to avoid compile time error.
abstract class BaseAbstract
{
public abstract string Meth1();
}
abstract class ChildAbstract : BaseAbstract
{
public abstract override string Meth1();
}
BUT please note that this is not quite right place to use abstract override.
Usually abstract override is used when deriving an abstract class from non abstract class. to ensure that derived class implements the method. (see this)
Take a look at below code
class BaseNonAbstract
{
public virtual string Meth1()
{
// do something
return string.Empty;
}
}
abstract class ChildAbstract : BaseNonAbstract
{
public abstract override string Meth1();
}
Try this
abstract class AbstractClass1 : AbstractClass2
{
}
abstract class AbstractClass2
{
public abstract string Meth1();
}
I have a base class that has some abstract methods on it and there are 21 classes that are inheriting from this base class. Now for one of those abstract methods I want to implement it with a common implementation for 6 of the 21 classes so I thought about creating another base class that would do this.
I am open to suggestions but my main purpose of creating another base class between the current base class and the 21 classes is to keep from repeating the same code in 6 of the 21 classes if I didn't have to.
Here is a sample of code to illustrate the situation:
public abstract class FooBase
{
public abstract string Bar();
public abstract string SomeMethod();
public virtual string OtherMethod()
{
return this.SomeMethod();
}
}
public abstract class AnotherBase : FooBase
{
public abstract string Bar();
public abstract string SomeMethod();
public override OtherMethod()
{
//this is the common method used by 6 of the classes
return "special string for the 6 classes";
}
}
public class Foo1 : FooBase
{
public override string Bar()
{
//do something specific for the Foo1 class here
return "Foo1 special string";
}
public override string SomeMethod()
{
//do something specific for the Foo1 class here
return "Foo1 special string";
}
}
public class Another2 : AnotherBase
{
public override string Bar()
{
//do something specific for the Another2 class here
return "Another special string";
}
public override string SomeMethod()
{
//do something specific for the Another2 class here
return "Another2 special string";
}
}
Yes, you can derive an abstract class from another abstract class
public abstract class FooBase
{
//Base class content
}
public abstract class AnotherBase : FooBase
{
//it is "optional" to make the definition of the abstract methods of the Parent class in here
}
When we say it is optional to define the abstract methods of the parent class inside of the child class, it is mandatory that the child class should be abstract.
public abstract class FooBase
{
public abstract string Bar();
public abstract string SomeMethod();
public abstract string OtherMethod();
}
public abstract class AnotherBase : FooBase
{
public override string OtherMethod()
{
//common method that you wanted to use for 6 of your classes
return "special string for the 6 classes";
}
}
//child class that inherits FooBase where none of the method is defined
public class Foo1 : FooBase
{
public override string Bar()
{
//definition
}
public override string SomeMethod()
{
//definition
}
public override string OtherMethod()
{
//definition
}
}
//child class that inherits AnotheBase that defines OtherMethod
public class Another2 : AnotherBase
{
public override string Bar()
{
//definition
}
public override string SomeMethod()
{
//definition
}
}
So I'm guessing that there will be 5 more classes like Another2 which inherits from AnotherBase that will have a common definition for OtherMethod
Yes, that is entirely possible and frequently done. There is no rule that says that you can have only one base class at the bottommost level of your class hierarchy; subclasses of that class can just as well be abstract and thereby become (somewhat more specialized) base classes for one group of classes indirectly derived from your general base class.
You should specify what exactly those classes do, but.. given the information you provided:
This is the exact problem that the Strategy pattern aims to solve, as shown in the example given in the Head First Design Patterns book.
You have an abstract Duck class, from which other ducks (e.g., RedheadDuck, MallardDuck) derive. The Duck class has a Quack method, that simply displays the string "quack" on the screen.
Now you are told to add a RubberDuck. This guy doesn't quack! So what do you do? Make Quack abstract and let the subclasses decide how to implement this? No, that'll lead to duplicated code.
Instead, you define an IQuackBehaviour interface with a Quack method. From there, you derive two classes, QuackBehaviour and SqueakBehaviour.
public class SqueakBehaviour: IQuackBehaviour
{
public void Quack(){
Console.WriteLine("squeak");
}
}
public class QuackBehaviour: IQuackBehaviour
{
public void Quack(){
Console.WriteLine("quack");
}
}
Now, you compose your ducks with this behaviour as appropriate:
public class MallardDuck : Duck
{
private IQuackBehaviour quackBehaviour = new QuackBehaviour();
public override void Quack()
{
quackBehaviour.Quack();
}
}
public class RubberDuck : Duck
{
private IQuackBehaviour quackBehaviour = new SqueakBehaviour();
public override void Quack()
{
quackBehaviour.Quack();
}
}
You can even inject an instance of IQuackBehaviour through a property if you want the ducks to change their behaviour at runtime.
I'm a Java developer who's trying to move into C#, and I'm trying to find a nice equivalent to some Java code. In Java, I can do this:
public interface MyInterface
{
public void theMethod();
}
public abstract class MyAbstractClass implements MyInterface
{
/* No interface implementation, because it's abstract */
}
public class MyClass extends MyAbstractClass
{
public void theMethod()
{
/* Implement missing interface methods in this class. */
}
}
What would be a C# equivalent to this? The best solutions using abstract/new/override etc all seem to result in 'theMethod' being declared with a body of some form or another in the abstract class. How can I go about removing reference to this method in the abstract class where it doesn't belong, whilst enforcing it's implementation in the concrete class?
You cannot, you would have to do it like this:
public interface MyInterface
{
void theMethod();
}
public abstract class MyAbstractClass : MyInterface
{
public abstract void theMethod();
}
public class MyClass : MyAbstractClass
{
public override void theMethod()
{
/* Implement missing interface methods in this class. */
}
}
No you would have to still have the method signature in the abstract class, but implement it in the derived class.
e.g.
public interface MyInterface
{
void theMethod();
}
public abstract class MyAbstractClass: MyInterface
{
public abstract void theMethod();
}
public class MyClass: MyAbstractClass
{
public override void theMethod()
{
/* implementation */
}
}