I have this base class having the following interface:
abstract class Base
{
abstract public object Val
{
get;
}
}
For any derived classes, Val's value must be specified at object creation time.
The question is: How can I make a derived class do this (hopefully at compile time)?
I tried adding a constructor:
abstract class Base
{
public Base(object value)
{
val = value;
}
private object val;
...
}
But as you can see then I had to declare a private field to store value in it (because Value is read-only).
The problem arises because I want to add some kind of effect to derived classes using the Decorator/Wrapper pattern introduced in GoF Design Patterns. But because I have declared the field inside Base class, the decorators keep saving a copy of the same data and I end up wasting memory.
Try this instead:
abstract class Base
{
public Base(object val)
{
this.Val = val;
}
public object Val { get; private set; }
}
That way, your derived class doesn't need its own field:
public class Derived : Base
{
public Derived(object val) : base(val) { }
}
If it is a decorator, then don't have a field:
public override object Val {
// add any decoration effects here if needed
get { return tail.Val; }
}
Where tail is the thing you are decorating.
However, it sounds like you mean inheritance (not decoration) - if so:
abstract class BaseClass {
protected BaseClass(object val) {...}
}
class ConcreteType : BaseClass {
public ConcreteType(object val)
: base(val) { }
}
Here the base class could even handle the storage etc.
Related
I have a property A in all subclasses of base class Base.
How can I generate an abstract property definition of property A into base class Base?
I know ReSharper's refactoring Pull Members Up, but that moves the property to base class.
I need an abstract property in base class and a overriding properties in all sub classes. Is there a refactoring in Visual Studio or in ReSharper that can do it automatically for me?
There is a checkbox "Make abstract" for that in ReSharper Pull Members Up dialog :
I'm not sure Resharper can move up and create an abstraction as you want automatically, but you can atleast define it manually like this
In abstract class:
public abstract double A
{
get;
}
In Sub class:
public override double A
{
get
{
return 3.141;
}
}
It might be a clearner design to define a new Interface (or use an existing one) and define the property in the interface. That way, your existing subclasses won't have to use override.
public interface IInterface {
string MyProperty { get; }
}
public class Class : IInterface {
public string MyProperty { get; set; }
}
public abstract class AbstractClass {
public abstract string Value { get; }
}
public class ConcreteClass : AbstractClass {
private string m_Value;
public override string Value {
get { return m_Value; }
}
public void SetValue(string value) {
m_Value = value;
}
}
I hope this will be helpful to you.
I have a base class that takes a single generic argument. I then have several classes that inherit from this base class. Is there a simple way for the child classes to inherent a factory from the base class?
Example
class BaseClass<T>
{
T Value {get; set;}
string Name {get; set;}
public static BaseClass<T> Factory(T Value)
{
return new BaseClass<T>(Value);
}
}
class ChildClass : BaseClass<int>
{
public void Test()
{
// I want this below to work
// but Factory() returns a BaseClass
ChildClass bs = ChildClass.Factory(10);
}
}
I've noted in the code what I want to work. I can think of one way to overcome this, by adding an implicit operator to either BaseClass or SubClass that converts from BaseClass to ChildClass.
I can also just explicitly add the Factory to ChildClass but that defeats the point of inheritance.
Is there a better, more standardized way of doing this?
I would do something like this:
class BaseClass<T, K> where K : BaseClass<T, K>, new()
{
T Value { get; set; }
string Name { get; set; }
public static K Factory(T value)
{
return new K { Value = value };
}
}
class ChildClass : BaseClass<int, ChildClass>
{
public void Test()
{
ChildClass cs = Factory(10);
}
}
It's a bit hard to answer your question since you have described what you are trying to do, but not why. Hence I got to try to guess what you want.
I would not put the factory method in the same class as in the other answer or your question. How would you handle inheritance for once? It works for the two levels that you have. But what if you want to extend ChildClass?
Instead I would create a generic factory used for the object creation. Implement it has a singleton wrapped around a factory interface to be able to easy extend it or swap the implementation.
class MyFactory
{
private static IMyFactory _instance;
public static void Assign(IMyFactory factory) { _instance = factory; }
public static T Create<T>() { return _instance.Create<T>(); }
}
interface IMyFactory
{
T Create<T>();
}
class MyFactoryImp : IMyFactory
{
//do whatever needed in here
public T Create<T>(){ return new T(); }
}
class BaseClass<T>
{
T Value {get; set;}
string Name {get; set;}
}
class ChildClass : BaseClass<int>
{
public void Test()
{
ChildClass bs = MyFactory.Create<ChildClass>(10);
}
}
// start with this, you can easily switch implementation
MyFactory.Assign(new MyFactoryImp());
The other obvious answer would be to start using a Inversion Of Control container, for example autofac.
I have the following classes
class GridBase
{
public object DataSource { get; set; }
}
class GenericGrid<T> : GridBase
{
public new T DataSource { get; set; }
}
Both GridBase and Generic Grid classes can be instantiated and one can descend from either as well.
Is this considered the correct/accepted way to implement such a hierarchy?
Or should you go the extra mile and implement it like the following
class GridBase
{
protected object dataSource;
public object DataSource { get { return dataSource; } set { dataSource = value; } }
}
class GenericGrid<T> : GridBase
{
public new T DataSource { get { return (T)dataSource; } set { dataSource = value; } }
}
The same applies to non generic classes when a property is re-introduced in a descendant, I'm just using a generic example here.
Another case and question
abstract class SomeBase
{
protected abstract void DoSomething();
}
class Child : SomeBase
{
protected override void DoSomething()
{
/* Some implementation here */
}
}
The situation here is that framework "X" declares SomeBase allowing you to define your own descendants. The classes they create (at run time) then descend from your class (Child in the this case). However, they don't call your DoSomething() method, from their implementation of DoSomething().
On their part, they can't blindly call base.Dosomething() either because the typical case is that the class they generate normally descends from SomeBase and since the method is abstract that's not valid. (Personally, I don't like this behavior in C#).
But anyway, is that good or accepted design, that is not calling base.xxx(), especially when the the "intent" seems to contradict?
EDIT From a framework design perspective. Is it ok/acceptable that it does this? If not how would it be designed so as to either prevent such a case or better impart their intent (in both cases).
I would prefer something like this:
interface IGrid {
object DataSource { get; }
}
interface IGrid<T> {
T DataSource { get; }
}
public Grid : IGrid {
public object DataSource { get; private set; }
// details elided
}
public Grid<T> : IGrid<T> {
public T DataSource { get; private set; }
object IGrid.DataSource { get { return this.DataSource; } }
// details elided
}
Note that I am NOT inheriting from Grid.
For the DataSource question I prefer the following pattern
abstract class GridBase {
public abstract object DataSource { get; }
}
class GenericGrid<T> : GridBase {
private T m_data;
public override object DataSource {
get { return m_data; }
}
public T DataSourceTyped {
get { return m_data; }
set { m_data = value; }
}
}
Reasons
Having the GridBase.DataSource member be writable is type unsafe. It allows me to break the contract of GenericGrid<T> by setting the value to a non-T instance
This is more of a matter of opinion but I dislike the use of new because it often confuses users. I prefer the suffix ~Type" for this scenario
This only requires the data be stored once
Doesn't require any unsafe casting.
EDIT OP corrected that GridBase and GenericGrid are both usable types
In that case I would say you need to reconsider your design a bit. Having them both as usable types opens you up to very easy to expose type errors.
GenericGrid<int> grid = new GenericGrid<int>();
GridBase baseGrid = grid;
baseGrid.DataSource = "bad";
Console.Write(grid.DataSource); // Error!!!
The design will be a lot more reliable if separate the storage from the access of the values in a manner like my original sample. You could extend it further with the following code to have a usable non-generic container
class Grid : GridBase {
private objecm m_data;
public override object DataSource {
get { return m_data; }
}
public object DataSourceTyped {
get { return m_data; }
set { m_data = value; }
}
}
The second form of the generic inheritance (casting the base class' attribute) is more correct as it does not violate Liskov Substitution Principle. It is conceivable that an instance of the generic class is cast into base class and accessing Data through the base class points to a different property. You will need to keep both in sync in order for the derived class to be substitutable for the base class.
Alternatively, you can implement some sort of a strategy pattern where the base class asks for the Data property from the derived class, in order to avoid awkward downcasting. This is what I had in mind:
public class Base {
private readonly object m_Data; //immutable data, as per JaredPar suggestion that base class shouldn't be able to change it
publlic Base(object data) {
m_Data = data;
}
protected virtual object GetData() {return m_Data;}
public Object DataSource {get {return GetData();}}
}
public class Derived<T> : Base {
private T m_Data;
public Derived():base(null){}
protected override object GetData() {return m_Data;}
protected new T Data {return m_Data;}
}
With regards to the second question, I am note sure I understand the question. Sound like the problem you are having is to with the framework not calling the abstract method when it generates a proxy at runtime, which is always legal in abstract classes, as the only way for that code to execute is through a derived class which must override the abstract method.
I want to force subclasses to define a constant value.
Like
const string SomeConstantEverySubclassMustDefine = "abc";
I need that because I need to have it tied to the Type, rather than to the instance and you can't override static Methods/Properties iirc.
I'd really like to have a compile-time check for those constants.
Let me explain in more detail:
Some classes in our Domain-Model are special, you can take certain actions for them, depending on the type. Thus the logic is tied to the type. The action to be taken requires a string tied to the type. I sure could create an instance everytime as a workaround and declare an abstract property, but that's not what I want. I want to enforce the declaration of the string at compile-time, just to be sure.
No, you can't. I would suggest you make your base class abstract, with an abstract property which you can fetch when you want. Each child class can then implement the property just by returning a constant if it wants. The downside is that you can't use this within static methods in the base class - but those aren't associated with the child classes anyway.
(It also allows child classes to customise the property per instance as well, if necessary... but that's rarely an actual problem.)
If this doesn't do enough for you, you might want to consider a parallel type hierarchy. Basically polymorphism simply doesn't happen in a type-specific way in .NET; only in an instance-specific way.
If you still want to do this and fetch it with reflection, I suggest you just write unit tests to ensure that the relevant constants are defined. When you get beyond what the type system can describe, that's often the best you can do.
Make an abstract property with only a get. That's what I think you could do to enforce a class has a value. Then you can just return a constant in the property.
Example:
Base class:
public abstract string MyConst { get; }
Derived class:
public override string MyConst {
get { return "constant"; }
}
Here is how I made mine work. I used Attribute as others have suggested.
public class ObjectAttribute : Attribute
{
public int ObjectSize { get; set; }
public ObjectAttribute(int objectSize)
{
this.ObjectSize = objectSize;
}
}
public abstract class BaseObject
{
public static int GetObjectSize<T>() where T : IPacket
{
ObjectAttribute[] attributes = (ObjectAttribute[])typeof(T).GetCustomAttributes(typeof(ObjectAttribute), false);
return attributes.Length > 0 ? attributes[0].ObjectSize : 0;
}
}
[ObjectAttribute(15)]
public class AObject : BaseObject
{
public string Code { get; set; }
public int Height { get; set; }
}
[ObjectAttribute(25)]
public class BObject : BaseObject
{
public string Code { get; set; }
public int Weight { get; set; }
}
If you would like instance access to the attribute just add it to the base abstract class.
public abstract class BaseObject
{
public static int GetObjectSize<T>() where T : IPacket
{
ObjectAttribute[] attributes = (ObjectAttribute[])typeof(T).GetCustomAttributes(typeof(ObjectAttribute), false);
return attributes.Length > 0 ? attributes[0].ObjectSize : 0;
}
public int ObjectSize
{
get
{
ObjectAttribute[] attributes = (ObjectAttribute[])GetType().GetCustomAttributes(typeof(ObjectAttribute), false);
return attributes.Length > 0 ? attributes[0].ObjectSize : 0;
}
}
}
Usage of the constants
int constantValueA = AObject.GetObjectSize<AObject>();
int constantValueB = BObject.GetObjectSize<BObject>();
AObject aInstance = new AObject();
int instanceValueA = aInstance.ObjectSize;
New idea
Here's a sort of weird idea: instead of using inheritance directly, you create a separate class to provide a constant value for every type deriving from some type T. The constructor for this type uses reflection to verify that every derived type has indeed been supplied a value.
public abstract class Constant<T, TConstant>
{
private Dictionary<Type, TConstant> _constants;
protected Constant()
{
_constants = new Dictionary<Type, TConstant>();
// Here any class deriving from Constant<T, TConstant>
// should put a value in the dictionary for every type
// deriving from T, using the DefineConstant method below.
DefineConstants();
EnsureConstantsDefinedForAllTypes();
}
protected abstract void DefineConstants();
protected void DefineConstant<U>(TConstant constant) where U : T
{
_constants[typeof(U)] = constant;
}
private void EnsureConstantsDefinedForAllTypes()
{
Type baseType = typeof(T);
// Here we discover all types deriving from T
// and verify that each has a key present in the
// dictionary.
var appDomain = AppDomain.CurrentDomain;
var assemblies = appDomain.GetAssemblies();
var types = assemblies
.SelectMany(a => a.GetTypes())
.Where(t => baseType.IsAssignableFrom(t));
foreach (Type t in types)
{
if (!_constants.ContainsKey(t))
{
throw new Exception(
string.Format("No constant defined for type '{0}'.", t)
);
}
}
}
public TConstant GetValue<U>() where U : T
{
return _constants[typeof(U)];
}
}
Basic example:
public class BaseType
{
public static Constant<BaseType, string> Description { get; private set; }
static BaseType()
{
Description = new BaseTypeDescription();
}
}
public class DerivedType : BaseType
{ }
internal sealed class BaseTypeDescription : Constant<BaseType, string>
{
public BaseTypeDescription() : base()
{ }
protected override DefineConstants()
{
DefineConstant<BaseType>("A base type");
DefineConstant<DerivedType>("A derived type");
}
}
Now I have code that allows me to do this:
var description = BaseType.Description;
// returns "A base type"
string baseTypeDescription = description.GetValue<BaseType>();
// returns "A derived type"
string derivedTypeDescription = description.GetValue<DerivedType>();
Original answer
You may not like it, but the closest way to accomplish this is by declaring an abstract read-only (no set) property.
If you've got an instance of your subclass, then this can work just as well as a constant, even though it is technically instance-level (it will just be the same for all instances of the given class).
Consider, for instance, IList.IsReadOnly. In most cases this is actually a property that tells you about the underlying class implementation, as opposed to any state specific to a particular instance. (It may be an interface member as opposed to an abstract class member, but it's the same idea.)
If you are trying to access it statically, well... then you're out of luck. But in this case I fail to see how you'd obtain the value without using reflection anyway. Maybe that's your intention; I don't know.
You could have a static method in the base class called, for instance "Register", that is passed a Type and a constant value, with the intention being that it is called by the class constructors of the subtypes. Then, add a check in all of your base class constructors that the object being constructed is of a registered type.
abstract class Base
{
private static Dictionary<Type, string> _registry = new Dictionary<Type, string>();
protected static void Register(Type t, string constVal)
{
_registry.Add(t, constVal);
}
protected Base()
{
if(!_registry.ContainsKey(this.GetType()))
throw new NotSupportedException("Type must have a registered constant");
}
public string TypeConstant
{
get
{
return _registry[this.GetType()];
}
}
}
class GoodSubtype : Base
{
static GoodSubtype()
{
Base.Register(typeof(GoodSubtype), "Good");
}
public GoodSubtype()
: base()
{
}
}
class Badsubtype : Base
{
public Badsubtype()
: base()
{
}
}
And then elsewhere, you can construct GoodSubtype instances, but trying to construct a Badsubtype gets an exception. I think a runtime error at construction is the soonest you can get an error with this type of scheme.
(You'd want to use ConcurrentDictionary for your registry if threading is involved)
There's one other method that hasn't been covered and it uses the new modifier to hide consts values in the base class. In a way, it's similar to Nap's solution, but doesn't allow per-instance access and therefore doesn't allow for polymorphic access within the base class. This solution is only useful if you want to have constant value defined but wish to have the option of changing it to different values in different subclasses.
static void Main(string[] args)
{
Console.WriteLine("BaseClass.MyConst = {0}, ClassA.MyConst = {1}, ClassB.MyConst = {2}", BaseClass.MyConst, ClassA.MyConst, ClassB.MyConst);
Console.ReadKey();
}
class BaseClass
{
public const int MyConst = 1;
}
class ClassA : BaseClass
{
public new const int MyConst = 2;
}
class ClassB : BaseClass
{
}
I have an abstract class that defines a get, but not set, because as far as that abstract class is concerned, it needs only a get.
public abstract BaseClass
{
public abstract double MyPop
{get;}
}
However, in some of the derive class, I need a set property, so I am looking at this implementation
public class DClass: BaseClass
{
public override double MyPop
{get;set;}
}
The problem is, I got a compilation error, saying that
*.set: cannot override because *. does not have an overridable set accessor.
Even though I think that the above syntax is perfectly legitimate.
Any idea on this? Workaround, or why this is so?
Edit: The only approach I can think of is to put both get and set as in the abstract class, and let the subclass throws a NotImplementedException if set is called and it's not necessary. That's something I don't like, along with a special setter method .
One possible answer would be to override the getter, and then to implement a separate setter method. If you don't want the property setter to be defined in the base, you don't have many other options.
public override double MyPop
{
get { return _myPop; }
}
public void SetMyPop(double value)
{
_myPop = value;
}
New in C# 6.0:
If you are only calling the setter within your constructor, you can resolve this problem using read-only properties.
void Main()
{
BaseClass demo = new DClass(3.6);
}
public abstract class BaseClass
{
public abstract double MyPop{ get; }
}
public class DClass : BaseClass
{
public override double MyPop { get; }
public DClass(double myPop) { MyPop = myPop;}
}
It is not possible to do what you want. You have to define the setter in the abstract property, otherwise you won't be able to override it properly.
The only case I know where a getter is defined and a getter/setter are implemented is by using an interface:
public interface IBaseInterface
{
double MyPop { get; }
}
public class DClass : IBaseInterface
{
public double MyPop { get; set; }
}
If BaseClass is in your own codebase, then you can do:
abstract public class BaseClass
{
abstract public double MyPop { get; protected set; }
}
public class DClass : BaseClass
{
private double _myProp;
public override double MyProp
{
get { return _myProp; }
protected set { _myProp = value; }
}
}
EDIT: You can then go make a public method in DClass SetMyProp(double myProp) or the like. The class design for your domain model should be clear about or speak for itself why you can't set the property directly in the base class and why you can do so in the derived one.
Are you sure that doing what you are trying to do would be a good design if you found a way to do it?
It would allow objects of the subclass to make state changes that objects of the parent class can not make. Wouldn't that violate the Liskov Substitution Principle?
You could do something like this:
abstract class TestBase
{
public abstract int Int { get; }
}
class TestDerivedHelper : TestBase
{
private int _Int;
public override int Int
{
get
{
return _Int;
}
}
protected void SetInt(int value)
{
this._Int = value;
}
}
class TestDerived : TestDerivedHelper
{
public new int Int
{
get { return base.Int; }
set { base.SetInt(value); }
}
}
Using TestDerived will have the functionality you're looking for. The only drawback I can see from this method is that you have to implement every abstract method in TestDerivedHelper, but it gives you more control later.
Hope this helps. ;)
The reason that this is not possible is due to the way parameters are "Magicked" into existence by C#. When you define a parameter, C# creates a private field that the implicit getter and setter manipulate. If there is no setter in the base class, it's impossible to change this variable from a method written in a sub class (as the private flag prohibits even sub classes from accessing it). What usually happens is it uses the implicit setter of the base class instead.
I wouldn't advise putting the set in the base class if not all sub classes can do it, because this goes against the whole principle of polymorphic programming (any abstract method defined in the abstract class must be implemented by a subclass). Creating a special setter method, as described in other answers is probably the best way go.
Siege
abstract class TestBase
{
public abstract int Int { get; }
}
class TestDerivedHelper : TestBase
{
private int _Int;
public override int Int
{
get
{
return _Int;
}
}
protected void SetInt(int value)
{
this._Int = value;
}
}
class TestDerived : TestDerivedHelper
{
public new int Int
{
get { return base.Int; }
set { base.SetInt(value); }
}
}
Using TestDerived will have the functionality you're looking for. The
only drawback I can see from this method is that you have to implement
every abstract method in TestDerivedHelper, but it gives you more
control later.
I use this approach and works very well for me.
Also, I made my "TestDerivedHelper" class abstract too, then all the methods must be implemented on "TestDerived" class.
Even though this thread is old I'm positing my solution, in case it helps someone. It is not my own but is based off answers in other SO topics.
public abstract BaseClass
{
public double MyPoP { get { return GetMyPoP; } }
protected abstract double GetMyPoP { get; }
}
public class DClass: BaseClass
{
public new double MyPoP { get; set; }
protected override double GetMyPop { get { return MyPoP; } }
}
This solution adds an extra line of code for each such property that needs accessor modified. However, there is no change to external visibility and provides needed functionality.
public abstract class BaseClass
{
public abstract double MyPop { get; }
}
public class DClass: BaseClass
{
private double _myPop = 0;
public override double MyPop
{
get { return _myPop; }
}
// some other methods here that use the _myPop field
}
If you need to set the property from outside DClass then maybe it would be better to put the setter into the base class.
EDIT:
OK I may have been hasty with this response, but I've given it some more thought now.
Do you have to use an abstract base class? If it's not required, try this:
public interface ISomeRelevantName
{
double MyPop { get; }
}
public class DClass : ISomeRelevantName
{
public double MyPop { get; set; }
}
Why not just have a property in the base class that has a private setter, then in your subclass that needs the setter, override it and make it public.
You cannot override the set accessor since the base class has no set accessor defined.
What you can do is use the new keyword to hide the base classes implementation, but that may not be what you want.