An interface is sort of like a contract which when it's implemented within a class you are guaranteed that this class has a set of specific public methods that you can call.
I was wondering how I could use this principle for attributes ; i.e. having a interface which when a class implements it I'm guaranteed that this class has a set of specific attributes.
I am looking for this feature because I am using Protobuf and this library requires the classes you want to serialize have a specific attribute (XmlType for instance, but other similar attributes work too). I would like to manipulate a set of objects whatever their type is as long as they have this specific attribute.
I don't see how you can do that with interfaces.
However, you can do it with an abstract class which has methods with the attributes you want.
These methods will call abstracts methods which then must be implemented.
For example:
public abstract class A
{
[XmlType]
public void F()
{
InnerF();
}
protected abstract InnerF();
}
public class B extends A
{
protected void InnerF()
{
}
}
Another option ofcourse, is running a post build tool that will validate your code for rules you define.
Take a look here
Related
What I have is a non generic interface for the purpose of having a common contact that I can call functions. The interface returns objects which implement other interfaces. For example:
public interface ISearchAdvancedInputController
{
ISearchAdvancedInput GetAdvancedInput();
void LoadFromModel(ISearchAdvancedInput advancedInput);
}
I then currently have an abstract generic class which implements the interface but imposes requirements of the type. The types of the abstract class must implement the same interfaces as the interface's properties and functions demand. I cast the generic type to the implemented type when necessary so that I can satisfy the requirements of the implemented non abstract interface. This way, I can extend this abstract class and it will enforce type requirements across a larger class w/ many different types used across it. For example:
public abstract class ISearchAdvancedInputControllerBase<standardInput, advancedInputType> : ISearchAdvancedInputController
where advancedInputType : ISearchAdvancedInput
{
protected abstract advancedInputType GetAdvancedInput();
ISearchAdvancedInput ISearchAdvancedInputController.GetAdvancedInput()
{
return GetAdvancedInput();
}
void ISearchAdvancedInputController.LoadFromModel(ISearchAdvancedInput advancedInput)
{
LoadFromModel((advancedInputType)advancedInput);
}
public abstract void LoadFromModel(advancedInputType advancedInput);
}
This works really well in general however it falls short because I'm having to use an abstract CLASS in order to perform this overriding. As such when I want to actually make use of it for more concrete examples, I encounter the error that I can only extend a single class.
So to get around this I extend the "other" class in the previous base abstract class. However this is not ideal because if I wind up creating another concrete implementation I need to redefine all of the type translations that I'm doing which is NOT related to the concrete classes implementation.
What I'd like is to not have an abstract class but instead some sort of abstract interface. If I had this I'd be able to implement concrete classes more succinctly. I've looked at other instances of this question and have tried what seems to be the main suggestion which is to make the initial interface generic and have the type extend the resulting interface type and then extend that interface with the more abstract interface as such:
interface TestGenericInterface<a> where a:TestClassInterfaceA
{
TestClassInterfaceA testGeneric { get; }
}
interface TestGenericComplexInterface<a> : TestGenericInterface<a>
where a:TestClassInterfaceA
{
new a testGeneric { get; }
}
However the concrete class seems to suffer from the same issue that's shown when you start from a non generic interface where each function / property of the base interface needs overwritten.
public class TestClass : TestGenericComplexInterface<TestGC>
{
public TestGC testGeneric => I want to complete this because its return is the type that I'm wanting to use for this concrete implementation
TestClassInterfaceA TestGenericInterface<TestGC>.testGeneric => I don't want to have to complete this because this function is already handled by the previous function in a round about sense.
}
public class TestGC : TestClassInterfaceA { }
I do see a note that I could provide default implementation of functions if I use c# v8.0 or greater, so I must be on a version prior to that but I figure this should be possible w/o that, but maybe in a different way. Hope ya'll can assist.
I'm creating a modular type system were one application(host) loads other modules in to a common UI interface, I have also created an API for this and is available for other users to use.
The API in some instances uses interfaces and abstract classes to force the client using the API to implement the specific methods.
I cannot use interfaces for everything as there are some things were I require putting in my own body so the end user does not need to implement all the unnecessary events and methods himself. EG: So I handle the size changing events myself then pass him the size to a function he implements from called SizeChanged were he can handle his program from the size change.
Abstract classes are what I really want to use but I cannot because the controls the user has may need the x:Name specified in XAML and will recieve the error:
The type ... cannot have a Name attribute. Values types and types without a default constructor can be used as items within ResourceDictionary.
The error occurs because an instance of it needs to be created:
Inherited Window can not have a name?
The only solution available to me that I know of is to use a regular class with virtual methods that can be overridden and that does work fine but it does not force the user to implement my methods that are required.
Is there anything I can do cleanly such as any public implementable or something?
This is the structure:
API:
IContext -> ContextControl -> (Abstract methods)
Module DLL
ContextControl(API) -> AppContextControl(Override methods)
Host Application pull's AppContextControl
I know I can tell the module dev to implement an interface as well as this ContextControl that constrains them to implement the interfaces but it would be much more professional for the compiler to tell them.
In the dev module if I instead of inheriting from ContextControl I implement IContext then I lose all the default bodys and the module dev gotta implement a lot lot more.
Cheers.
The two ways you've described - interfaces and abstract classes - are (as I'm sure you know) the recommended ways to do what you're describing, at least, if you want to enforce the implementation at compile-time.
The only other way I'm aware of is to provide a default implementation that throws a NotImplementedException(). That'll give you a run-time error, but nothing at compile-time, unfortunately.
This may or may not be applicable in your specific situation, but another possible way to get around your limitation is to use a Strategy pattern.
Instead of having the user override members, have them pass in parameters containing the functionality to the constructor. This could either be as delegates or as specific classes you create for the purpose.
Can't you use a combination of inheritance and interfaces? In other words, create virtual methods for the methods that can be overridden and use interfaces to implement methods that must be overridden?
I recently have a similar problem (dnt know it will help in your case). where i have to generate errors at compile time instead of Runtime, with generic functionality. the approach i used was combination of Generics and Interfaces. Examples are...
1) Enum Ex:(C# String enums)
Problem was to set things so that i dnt have to implement every code throughout the project and force Constructor.
public abstract class EnumEx<T> where T : EnumEx<T>
{
private readonly string _displayValue;
private readonly string _value;
protected static ReadOnlyCollection<T> EnumExs;
protected EnumEx(string displayValue, string value)
{
_displayValue = displayValue;
_value = value;
}
public string DisplayValue
{
get { return _displayValue; }
}
public static T FromString(string option)
{
foreach (var enumEx in EnumExs.Where(enumEx => enumEx._value == option))
{
return enumEx;
}
Debug.WriteLine(string.Format("Exception in EnumEX FromString({0})", option));
return null;
}
public override string ToString()
{
return _value ?? string.Empty;
}
}
2) Deep Copy(Generic method to create deep copy of all elements in a collection) + Editable Implementation IEditableObject over custom List
public abstract class CloneAbleBase<T> : ObservableObjectEx, ICloneable, IEditableObject
where T : DataBase<T>, new()
{
protected T CurrentData = new T();
private T _backupData;
private bool _isInEditMode;
public object Clone()
{
var dataObject = (CloneAbleBase<T>) MemberwiseClone();
dataObject.CurrentData = CurrentData.Copy();
return dataObject;
}
public void BeginEdit()
{
if (_isInEditMode) return;
_backupData = CurrentData.Copy();
_isInEditMode = true;
}
public void EndEdit()
{
if (!_isInEditMode) return;
_backupData = default(T);
_isInEditMode = false;
RaisePropertyChanged(string.Empty);
}
public void CancelEdit()
{
if (!_isInEditMode) return;
CurrentData = _backupData;
RaisePropertyChanged(string.Empty);
}
}
In similar way you can create base class for your Window or any control, where you need functionality some sort of generic functionality..
An alternative i can see in your very specific situation is to use interfaces (which, as you said, won't let you inject your own code) and then inherit from the actual type the end user provides and passed in as an interface and inject your own code there at runtime
For example you load all classes implementing IMyPlugin defined as such
public interface IMyPlugin{
void MyEndUserMethod();
void YourMethod();
}
User implements a class that inherits from it
public class UserClass:IMyPlugin{
....
}
You want to force your own code instead of YourMethod, then generate a class at runtime that inherits from UserClass and in YourMethod create your own code and call (or not, depending on your need) base on it. Call base on all other method where the user is to provide the implementation.
This is a bit more work for you but it hides all the uglyness from the end user, simply forcing him to implement the interface. For less uglyness make that interface into 2 interface
public interface IPlugin : IUserPlugin,ICreatorPlugin
And be clear to your users that while the classes must implement IPlugin anything from ICreatorPlugin can be left empty.
An even better solution would be to only expose IUserPlugin and do even more work on your side (your runtime class inherits from the user class AND ICreatorPlugin, and then use duck typing to make it into IPlugin).
for our asp.net web application v 4.0, we are in process of defining a class that contains methods that are common across the application. to achieve this there are 2 Suggestions within our team.. one to create a base class, define the methods in that and derive all the other classes from that base class.. the other one is to create a seperate class (not a base class) and instantiate that common class in other classes when required to access the common methods. Please guide me in identifying the best approach..
I'd only go the base class route if there is a real is-a relationship between the base class and the derived classes. One reason is that a class can only inherit from a single base class. I'd use this relationship in a sensible way. Just sharing some helper methods is not a scenario worth blocking this relationship.
If you want to use some helper methods in several classes, composition is the better way as you describe in the 2nd approach. Instead of creating the objects in the classes, you should think about whether you can inject the instances into the classes (see this link for details on dependency injection), e.g.:
public class HelperClass
{
public virtual void HelperMethod()
{
// ...
}
}
public class ClassThatUsesHelper
{
private readonly HelperClass _helper;
public ClassThatUsesHelper(HelperClass helper)
{
_helper = helper;
}
public void DoSomething()
{
_helper.HelperMethod();
}
}
By injecting the helper class you decouple the classes so that you can substitute the helper class by a different implementation that shares the same interface. ClassThatUsesHelper works with any class that is derived from HelperClass (or HelperClass itself of course). So if you need to decorate the helper method or need a special implementation in some cases, this is possible without any problem.
Using composition also enables you to test the helper methods separately.
However, if it is about very basic helper methods, you might also think about having a static class with static helper methods. Please note that you introduce a strong dependency between the classes and that you cannot adjust the implementation easily.
public static class HelperClass
{
public static void HelperMethod()
{
// ...
}
}
public class ClassThatUsesHelper
{
public void DoSomething()
{
HelperClass.HelperMethod();
}
}
Your question is vague, but if you need a method which all objects in your program will need to have access to, that uses their member variables, then I wold recommend creating an abstract class upon which your objects are based.
If you need a means of performing some sort of calculation from anywhere in your code, just create a public static method in a class meant for the purpose. MyMathClass.InterestingFourierTransform(), for example.
Is it possible to make a C# base class accessible only within the library assembly it's compiled into, while making other subclasses that inherit from it public?
For example:
using System.IO;
class BaseOutput: Stream // Hidden base class
{
protected BaseOutput(Stream o)
{ ... }
...lots of common methods...
}
public class MyOutput: BaseOutput // Public subclass
{
public BaseOutput(Stream o):
base(o)
{ ... }
public override int Write(int b)
{ ... }
}
Here I'd like the BaseOutput class to be inaccessible to clients of my library, but allow the subclass MyOutput to be completely public. I know that C# does not allow base classes to have more restrictive access than subclasses, but is there some other legal way of achieving the same effect?
UPDATE
My solution for this particular library is to make the base class public and abstract, and to document it with "Do not use this base class directly". I also make the constructor of the base class internal, which effectively prevents outside clients from using or inheriting the class.
(It's a shame, because other O-O languages let me have hidden base classes.)
Unfortunately not. You can't derive a public class from an internal or private class.
You need to either expose the base class, or you need to declare all the methods for all of your similar classes. If you go the route where you declare all methods again, it's probably useful to create a helper class, which has the actual implementation of them. Still it's quite a bit of boilerplate.
Consider a pattern such as a Facade. That's what they're there for. I don't think you can achieve what you require with straight inheritance.
Depending on what "lot of common methods" are doing you may achieve some of it with internal extension methods:
internal static class MyStreamExtensions
{
internal static int UsefulOne(this Stream stream)
{
return 42;
}
}
Another approach is to make constructor internal to prevent unintentional derivation from that class:
public class BaseOutput: Stream
{
internal BaseOutput(Stream o)
{ ... }
...lots of common methods...
}
This will make code more understandable compared to "not-really-visible" intermediate class in hierarchy.
I was looking at some code in an abstract class:
public virtual void CountX(){}
public virtual void DoCalculation() { ...code}
Why should I declare an empty virtual method in an abstract class if it is not mandatory to override it in derived types?
Because if the default behaviour is to do nothing, but derived classes might want to do something. It's a perfectly valid structure.
It allows your base code to call it. You tend to see similar designs when there is "BeforeXXX" and "AfterXXX" code, at the base class this code is empty, but the method needs to be there to compile. In derived classes, this code is optional, but needs to be virtual to be overridden.
The fact that it is in an abstract class shouldn't confuse its behaviour.
An example:
abstract class Base
{
public void ProcessMessages(IMessage[] messages)
{
PreProcess(messages);
// Process.
PostProcess(messages);
}
public virtual void PreProcess(IMessage[] messages)
{
// Base class does nothing.
}
public virtual void PostProcess(IMessage[] messages)
{
// Base class does nothing.
}
}
class Derived : Base
{
public override void PostProcess(IMessage[] messages)
{
// Do something, log or whatever.
}
// Don't want to bother with pre-process.
}
If these methods (Pre, Post) were abstract, then all derived classes would need to implement them (likely as empty methods) - code litter that can be removed using empty virtual methods at the base.
As #Adam told you, there are many cases in which it makes sense. When you create an abstract class, it's because you want to create a common interface for all classes deriving from that one; however, at that level of inheritance you won't have enough information to be able to create working code for that method.
For example, if you create the class Figure, with the getArea() method, you won't be able to write code that is going to correctly calculate the area for all figures. You'll have to wait to write the code for Rectangle, or Circle (both deriving from Figure), in order to be able to write working code for them.
If it is MANDATORY to override and no default logics could be written in base class, than virtuality is wrong and method should be abstract. If the default action is to do nothing, than as Adam mentioned, making empty virtual method in base class is perfectly valid structure
When you declare the method as abstract, the inherited class has to override that method (provide an implementation). It is mandatory.
When the method is declared as virtual, the inheritor can override the method and provide an implementation other than the default.
From a design perspective this smells bad and indicates that the implementation of the design is in an immature state. If a method is not required by every class that derives a particular base class then by definition it does not belong in the base class. You will usually discover that this method is used by particular derivations of the base class and that indicates a new interface or layer of abstraction in your inheritance hierarchy.