I need a method that creates an empty clone of an object in a base class? For instance:
public class ChildClass : ParentClass
{
public ChildClass()
{
}
}
public class ParentClass
{
public SomeMethod()
{
// I want to create an instance of the ChildClass here
}
}
Up until now, we have an abstract method defined in the parent class. And, all of the child classes implement them. But, the implementation is the same for all, just a different type.
public class ChildClass : ParentClass
{
public ChildClass()
{
}
public ParentClass CreateEmpty()
{
return new ChildClass();
}
}
public class ParentClass
{
public SomeMethod()
{
// I want to create an instance of the ChildClass here
ParentClass empty = CreateEmpty();
}
public abstract ParentClass CreateEmpty();
}
Is there any way to do this from the parent class so that I don't have to keep implementing the same logic for each different child class? Note that there may be more levels of inheritance (i.e. ChildChildClass : ChildClass : ParentClass).
If using reflection isn't a problem to you, you could do it using Activator class:
//In parent class
public ParentClass CreateEmpty()
{
return (ParentClass)Activator.CreateInstance(this.GetType());
}
This will return empty object of the type you want. Notice that this method does not need to be virtual.
On the other hand, I think that your current approach is perfectly fine, few more lines of code aren't so bad.
You can make a deep clone of the object using the binary serializer.
EDIT: Just noticed the word "empty" next to clone (which I thought was an oxymoron). Leaving this response up anyhow hoping it will help others that find this question because they are looking to do a regular clone.
This is somewhat experimental. I don't know whether this will lead to a cyclic dependency. Haven't touched C# for some months.
public class ParentClass<T> where T : ParentClass<T>, new() { // fixed
public ParentClass() {
var x = new T(); // fixed, was T.new()
}
}
public class ChildClass : ParentClass<ChildClass> {
public ChildClass() { }
}
Otherwise go for the ReflectionCode by Ravadre.
I'm using the following pattern.
Pros:
This pattern secure the type-safety of cloning in private and public sides of classes.
The output class will be always correct.
You never forgot override the "clone" method. The "MyDerivedClass" never returns another class than the "MyDerivedClass".
Cons:
For one class, you need create one interface and two classes (prototype and final)
Sample:
// Common interface for cloneable classes.
public interface IPrototype : ICloneable {
new IPrototype Clone();
}
// Generic interface for cloneable classes.
// The 'TFinal' is finaly class (type) which should be cloned.
public interface IPrototype<TFinal> where TFinal : IPrototype<TFinal> {
new TFinal Clone();
}
// Base class for cloneable classes.
// The 'TFinal' is finaly class (type) which should be cloned.
public abstract class PrototypeBase<TFinal> : IPrototype<TFinal> where TFinal : PrototypeBase<TFinal> {
public TFinal Clone() {
TFinal ret = this.CreateCloneInstance();
if ( null == ret ) {
throw new InvalidOperationException( "Clone instance was not created." );
}
this.FillCloneInstance( ret );
return ret;
}
// If overriden, creates new cloned instance
protected abstract TFinal CreateCloneInstance();
// If overriden, fill clone instance with correct values.
protected abstract void FillCloneInstance( TFinal clone );
IPrototype IPrototype.Clone() { return this.Clone(); }
object ICloneable.Clone() { return this.Clone(); }
}
// Common interface for standalone class.
public interface IMyStandaloneClass : IPrototype<IMyStandaloneClass> {
string SomeText{get;set;}
string SomeNumber{get;set;}
}
// The prototype class contains all functionality exception the clone instance creation.
public abstract class MyStandaloneClassPrototype<TFinal> : PrototypeBase<TFinal>, IMyStandaloneClass where TFinal : MyStandaloneClassPrototype<TFinal> {
public string SomeText {get; set;}
public int SomeNumber {get; set}
protected override FillCloneInstance( TFinal clone ) {
// Now fill clone with values
clone.SomeText = this.SomeText;
clone.SomeNumber = this.SomeNumber;
}
}
// The sealed clas contains only functionality for clone instance creation.
public sealed class MyStandaloneClass : MyStandaloneClassPrototype<MyStandaloneClass> {
protected override MyStandaloneClass CreateCloneInstance() {
return new MyStandaloneClass();
}
}
public interface IMyExtendedStandaloneClass : IMyStandaloneClass, IPrototype<IMyExtendedStandaloneClass> {
DateTime SomeTime {get; set;}
}
// The extended prototype of MyStandaloneClassPrototype<TFinal>.
public abstract class MyExtendedStandaloneClassPrototype<TFinal> : MyStandaloneClassPrototype<TFinal> where TFinal : MyExtendedStandaloneClassPrototype<TFinal> {
public DateTime SomeTime {get; set;}
protected override FillCloneInstance( TFinal clone ) {
// at first, fill the base class members
base.FillCloneInstance( clone );
// Now fill clone with values
clone.SomeTime = this.SomeTime;
}
}
public sealed class MyExtendedStandaloneClass : MyExtendedStandaloneClassPrototype<TFinal> {
protected override MyExtendedStandaloneClass CreateCloneInstance() {
return new MyExtendedStandaloneClass
}
}
Related
I have a bunch of derived classes that inherit from a base class. Each derived class still has unique members. One of these derived classes needs to be able to access all the other derived class's members. My aim with this problem is to write as little code as possible.
Would it be sloppy or bad practice to initialize all the derived classes' members in the base class so that one of the derived classes can access those members, instead of initializing them in said derived class?
Proposed approach:
public class BaseClass {
public BaseClass() {
...
der1Initializer = Der1Initializer(new Factory1());
der2Initializer = Der1Initializer(new Factory2());
List initializers = new List(){ der1Initializer , der2Initializer };
der3Initializer = Der3Initializer(initializers);
}
Der1Initializer der1Initializer;
Der2Initializer der2Initializer;
Der3Initializer der3Initializer;
}
public class DerivedClass1 : BaseClass {
public SomeFunction {
der1Initializer.init();
}
}
public class DerivedClass2 : BaseClass {
public SomeFunction {
der2Initializer.init();
}
}
public class DerivedClass3 : BaseClass {
...
}
So that:
public class Der3Initializer {
public GroupInitializationFunction {
initializers[0].init(); //der1Initializer
initializers[1].init(); //der2Initializer
}
}
Instead of:
public class BaseClass {
public BaseClass() {
...
}
public class DerivedClass1 : BaseClass {
public DerivedClass1 {
der1Initializer = Der1Initializer(new Factory1());
}
public SomeFunction {
der1Initializer.init();
}
Der1Initializer der1Initializer;
}
public class DerivedClass2 : BaseClass {
public DerivedClass2 {
der2Initializer = Der2Initializer(new Factory2());
}
public SomeFunction {
der2Initializer.init();
}
Der2Initializer der2Initializer;
}
public class DerivedClass3 : BaseClass {
public DerivedClass3 {
List initializers = new List()
{
Der1Initializer(new Factory1()),
Der2Initializer(new Factory2());
};
der3Initializer = Der3Initializer(initializers);
}
...
Der3Initializer der3Initializer;
}
...
public class Der3Initializer {
public GroupInitializationFunction {
initializers[0].init(); //der1Initializer
initializers[1].init(); //der2Initializer
}
}
This is a vast oversimplification of the "problem". The purpose of this code is to reduce duplicate code and the need to reinitialize members that can be shared and to optimize performance.
I am aware that it isn't necessarily a good idea to give other classes access to members they aren't using.
I just thought this is an interesting problem regarding code separation vs. duplicate code.
I would not recommend initializing childs on the parent class as
Violates Open Closed Principle as the base class has the responsibility of initializing its children and there will always be the need to add new child classes and consequently modifying base class .
Violates Single Responsibility Principle as this is not the role for the base class (to initialize its childs )
Violates Inversion of Control as base class is tightly coupled to child class see this link
I have 2 classes that are both derived from a base class X. The base class contains a property of a class T. Both subclasses of X should contain a property that's derived from T.
What I would like to achieve is to declare the a property of T in the base class X and have it used in several methods, while using the same property of T in the derived classes of X but have it recognized as a derived class from T, so that I won't have to cast it every time I want to use it.
One of my solutions would be just remove the property from the base class and copy the methods for each derived class of X, but that would defeat the purpose of using inheritance.
Is anything like that achievable?
internal class TestObject
{
public string ID;
public XObject obj;
//....
}
internal class TestDocument : TestObject
{
public XDocument obj; //<--- want to be able to call obj in my methods
//as if it were an XDocument type but relate to the obj property in the base class
//....
}
internal class XObject
{
//....
}
internal class XDocument : XObject
{
//....
}
Generics should work for you:
class Base<T> where T: MyType
{
T MyProperty { get; set; }
public void DoSomething()
{
// do something with MyProperty
}
}
with MyType being the base-class of the property within Base.
Then in your derived class you can define the generic constraint, e.g.
class Derived : Base<DerivedType>
{
}
Now an instance of Derived has the property MyProperty of type DerivedType instead of MyType.
So in your case TestObject should be similar to this:
internal class TestObject<T> where T: XObject
{
public string ID;
public T obj;
//....
}
internal class TestDocument : TestObject<XDocument>
{
// obj is here of type XDocument
}
Make the type of the property a generic parameter of your base class:
class PropertyTypeBase { }
class PropertyTypeA : PropertyTypeBase { }
class Base<T> where T : PropertyTypeBase
{
public T Property { get; }
}
class Foo : Base<PropertyTypeA>
{
public Foo()
{
PropertyTypeBase x = Property;
PropertyTypeA a = Property;
}
}
The simplest way would be to make the base class generic, and constrain the generic parameter to be derived form a certain class:
class BaseProp { }
class DerivedPropA: BaseProp { }
class DerivedPropB : BaseProp { }
abstract class X<T>
where T: BaseProp
{
public T Value { get; set; }
public abstract void With(T value);
}
class A : X<DerivedPropA>
{
public override void With(DerivedPropA value)
{
this.Value = value;
}
}
class B : X<DerivedPropB>
{
public override void With(DerivedPropB value)
{
this.Value = value;
}
}
This is possible by using generics.
First, let me explain the example classes. Let's say these are your properties:
public class BaseHead {}
public class OrganicHead : BaseHead {}
public class CyborgHead : BaseHead {}
And you now want to implement these heads on your person classes:
public class BaseCreature {}
public class OrganicCreature : BaseCreature {}
public class CyborgCreature : BaseCreature {}
The solution:
public class BaseCreature<THead> where THead : BaseHead
{
public THead Head { get; set; }
public BaseCreature(THead head)
{
this.Head = head;
}
}
We make the BaseCreature generic
We limit the THead type to only allow types that either are BaseHead or are derived from BaseHead
However, we also want to ensure that the right creature (organic/cyborg) only uses the correct head (organic/cyborg). This can be done by deriving from a BaseCreature with a specific generic type:
public class OrganicCreature : BaseCreature<OrganicHead>
{
public OrganicCreature(OrganicHead head) : base(head)
{
}
}
CyborgCreature is analogous.
Suppose you wanted to make it possible that every creature can use every type of head. If that's what you want, then you need to keep the generic parameter generic:
public class OrganicCreature<THead> : BaseCreature<THead> where THead : BaseHead
{
public OrganicCreature(THead head) : base(head)
{
}
}
CyborgCreature is analogous.
I have a desing question. Lets say I have a base class, and many derived classes.
class MyBase
{
MyBase()
{
Record();
}
void Record()
{
/// this line is able to handle every object
Recorder.Process(this);
}
}
class DerivedA : MyBase
{
public int SpecialPropertyOfA { get; set; }
DerivedA(int specialPropertyOfA)
{
SpecialPropertyOfA = specialPropertyOfA;
}
}
class DerivedB : MyBase
{
public string SpecialPropertyOfB { get; set; }
DerivedA(string specialPropertyOfB)
{
SpecialPropertyOfB = specialPropertyOfB;
}
}
As you may deduce, I want to record derived classes with all their properties set. But the above example cannot record the special properties of derived classes since they are not set when the Record method called.
One way to overcome this is to make Record method protected and call it as the last line of derived classes constructors. But this violates my base principals(every derived class should be recorded right after initialization). I want to make this mandotory by design.
Any ways to handle this?
Take a look again at what you're trying to do.
Remember that base class constructor is called before the derived class's constructor.
MyBase()
{
Record();
}
void Record()
{
Recorder.Process(this); // <-- What is "this" ?
}
What would be the value of this at this point?
The instance has not been constructed yet, the derived class's constructor hasn't event been called so there's no reference to this, hence, you can't use it here.
You'll have to call this method after the instance has been fully constructed.
A way to overcome this is using a factory method:
public static class MyBaseFactory
{
public static MyBase CreateDerivedA()
{
int specialPropertyOfA = // ...
MyBase instance = new DerivedA(specialPropertyOfA);
Recorder.Process(instance);
return instance;
}
public static MyBase CreateDerivedB()
{
string specialPropertyOfB = // ...
MyBase instance = new DerivedB(specialPropertyOfA);
Recorder.Process(instance);
return instance;
}
}
Is there a neat way to specify that a class must contain a factory method that returns the same kind of object as the class that overrides the abstract method? (Edit: Or as Johnathon Sullinger more eloquently puts it, [...] have a base class enforce a child class to implement a method that returns an instance of the child class itself, and not allow returning an instance of any other Type that inherits from the base class.)
For example, if I've got two classes, SimpleFoo : BaseFoo and FancyFoo : BaseFoo, can I define an abstract factory method public TFoo WithSomeProp(SomeProp prop) where TFoo is a type parameter that is somehow fixed by the abstract method definition to the particular class that overrides it?
I had hopes of compile-time guarantees that either
a concrete WithSomeProp method definition in SomeFoo : BaseFoo will only be able to produce SomeFoos. If static abstract method definitions were legal, perhaps the following (pseudo-syntax) method extension best expresses this need:
public static abstract TFoo WithSomeProp<TFoo>(this TFoo source, SomeProp prop)
where TFoo : BaseFoo;
I don't think this is possible in C#.
or at least some way to parameterize the return type in an abstract method, e.g.
public abstract TFoo WithSomeProp<TFoo>(SomeProp prop)
where TFoo : BaseFoo;
This wouldn't prevent FancyFoo.WithSomeProp from returning SimpleFoos, but ok.
This abstract method itself seems to work, but my concrete definition then fails:
public override SimpleFoo WithSomeProp(SomeProp prop)
{
return new SimpleFoo(this.SomeOtherProp, ..., prop);
}
with the warning
no suitable method found to override
It appears to me that specifying type parameters in an abstract method does not allow fixing them in the overrides of those definitions, but rather it specifies that "A method with a type parameter should exist".
For now I simply have public abstract BaseFoo WithSomeProp(SomeProp prop);.
It sounds like what you want to do, is have a base class enforce a child class to implement a method that returns an instance of the child class itself, and not allow returning an instance of any other Type that inherits from the base class. Unfortunately, to the best of my knowledge, that is not something you can do.
You can however force the child-class to specify what it's Type is to the base class, so that the base class can then enforce that the return value must be the Type specified by the child-class.
For instance, given a base class called BaseFactory, and BaseFactory<T>, we can create an abstract class that requires children to specify to the parent, what type the creation method returns. We include a BaseFactory class so we can constrain T to only being children classes of BaseFactory.
EDIT
I'll leave the original answer below in the event that it helps, but after some thought, I think I've got a better solution for you.
You'll still need the base class to take a generic argument that defines what the child Type is. The difference now however is that the base class has a static creation method instead of instance methods. You can use this creation method to create a new instance of the child class, and optionally invoke a callback for configuring the property values on the new instance before you return it.
public abstract class BaseFactory { }
public abstract class BaseFactory<TImpl> : BaseFactory where TImpl : BaseFactory, new()
{
public static TImpl Create(Action<TImpl> itemConfiguration = null)
{
var child = new TImpl();
itemConfiguration?.Invoke(child);
return child;
}
}
You then just create your children classes normally, without worrying about overriding any methods.
public class Foo : BaseFactory<Foo>
{
public bool IsCompleted { get; set; }
public int Percentage { get; set; }
public string Data { get; set; }
}
public class Bar : BaseFactory<Bar>
{
public string Username { get; set; }
}
Then you would use the factory as-such.
class Program
{
static void Main(string[] args)
{
// Both work
Bar bar1 = Bar.Create();
Foo foo1 = Foo.Create();
// Won't compile because of different Types.
Bar bar2 = Foo.Create();
// Allows for configuring the properties
Bar bar3 = Bar.Create(instanceBar => instanceBar.Username = "Jane Done");
Foo foo2 = Foo.Create(instanceFoo =>
{
instanceFoo.IsCompleted = true;
instanceFoo.Percentage = 100;
instanceFoo.Data = "My work here is done.";
});
}
Original Answer
The BaseFactory<T> will be reponsible for creating a new instance of TImpl and giving it back.
public abstract class BaseFactory { }
public abstract class BaseFactory<TImpl> : BaseFactory where TImpl : BaseFactory
{
public abstract TImpl WithSomeProp();
}
Now, your child class can be created, and inherit from BaseFactory<T>, telling the base class that T represents itself. This means the child can only ever return itself.
public class Foo : BaseFactory<Foo>
{
public override Foo WithSomeProp()
{
return new Foo();
}
}
public class Bar : BaseFactory<Bar>
{
public override Bar WithSomeProp()
{
return new Bar();
}
}
Then you would use it like:
class Program
{
static void Main(string[] args)
{
var obj1 = new Bar();
// Works
Bar obj2 = obj1.WithSomeProp();
// Won't compile because obj1 returns Bar.
Foo obj3 = obj1.WithSomeProp();
}
}
If you really want to make sure that the generic specified is the same as the owning Type, you could instead make WithSomeProp a protected method, so that children classes can only see it. Then, you create a public method on the base class that can do type checking.
public abstract class BaseFactory { }
public abstract class BaseFactory<TImpl> : BaseFactory where TImpl : BaseFactory
{
protected abstract TImpl WithSomeProp();
public TImpl Create()
{
Type myType = this.GetType();
if (typeof(TImpl) != myType)
{
throw new InvalidOperationException($"{myType.Name} can not create instances of itself because the generic argument it provided to the factory is of a different Type.");
}
return this.WithSomeProp();
}
}
public class Foo : BaseFactory<Foo>
{
protected override Foo WithSomeProp()
{
return new Foo();
}
}
public class Bar : BaseFactory<Bar>
{
protected override Bar WithSomeProp()
{
return new Bar();
}
}
class Program
{
static void Main(string[] args)
{
var obj1 = new Bar();
// Works
Bar obj2 = obj1.Create();
// Won't compile because obj1 returns Bar.
Foo obj3 = obj1.Create();
}
}
Now, if you create a child class that passes a different Type as T, the base class will catch it and throw an exception.
// Throws exception when BaseFactory.Create() is called, even though this compiles fine.
public class Bar : BaseFactory<Foo>
{
protected override Foo WithSomeProp()
{
return new Foo();
}
}
Not sure if this gets you what you wanted at least, but I think this will probably be the closest thing you can get.
Inspired by Johnathon Sullinger's fine answer, here is the code I ended with. (I added a theme.)
I passed the type parameter T along with the class definition and constrained that T : Base<T>.
BaseHyperLink.cs:
public abstract class BaseHyperLink<THyperLink> : Entity<int>
where THyperLink : BaseHyperLink<THyperLink>
{
protected BaseHyperLink(int? id, Uri hyperLink, ContentType contentType, DocumentType documentType)
: base(id)
{
this.HyperLink = hyperLink;
this.ContentType = contentType;
this.DocumentType = documentType;
}
public Uri HyperLink { get; }
public ContentType ContentType { get; }
public DocumentType DocumentType { get; }
public abstract THyperLink WithContentType(ContentType contentType);
}
SharedHyperLink.cs:
public sealed class SharedHyperLink : BaseHyperLink<SharedHyperLink>
{
public SharedHyperLink(int? id, Uri hyperLink, ContentType contentType, DocumentType documentType)
: base(id, hyperLink, contentType, documentType)
{
}
public override SharedHyperLink WithContentType(ContentType contentType)
{
return new SharedHyperLink(this.Id, contentType, this.DocumentType);
}
}
MarkedHyperLink.cs:
public sealed class MarkedHyperLink : BaseHyperLink<MarkedHyperLink>
{
public MarkedHyperLink(int? id, Uri hyperLink, ContentType contentType, DocumentType documentType, Mark mark)
: base(id, hyperLink, contentType, documentType)
{
this.Mark = mark;
}
public Mark Mark { get; }
public override MarkedHyperLink WithContentType(ContentType contentType)
{
return new MarkedHyperLink(this.Id, contentType, this.DocumentType, this.Mark);
}
}
During some research, I ran into an inheritance pattern using generics I have not seen before.
http://thwadi.blogspot.ca/2013/07/using-protobuff-net-with-inheritance.html
public abstract class BaseClass<TClass> where TClass : BaseClass<TClass>
{
//...
}
public class DerivedClass : BaseClass<DerivedClass>
{
//...
}
Usage:
static void Main(string[] args)
{
DerivedClass derivedReference = new DerivedClass();
//this looks odd...
BaseClass<DerivedClass> baseReference = derivedReference;
//this doesn't work
//BaseClass baseClass = derivedReference;
}
I was surprised that this even worked, I had to test it myself. I still can't understand why you would want to do this.
The only thing I could come up with, is preventing different derived classes from being stored in a collection together as their base class. This may be the reason, I guess I'm just curious to the application.
It is called the Curiously recurring template pattern it is often used to allow methods in the class to use the type of the derived class as a passed in or returned parameter.
For example, this is Clone method implemented so that only each layer needs to add it's own properties to the method as it goes down the chain.
public abstract class BaseClass<TClass> where TClass : BaseClass<TClass>, new()
{
public int Foo {get;set;}
public virtual TClass Clone()
{
var clone = new TClass();
clone.Foo = this.Foo;
return clone;
}
}
public class DerivedClass : BaseClass<DerivedClass>
{
public int Bar {get;set;}
public override DerivedClass Clone()
{
var clone = base.Clone();
clone.Bar = this.Bar;
return clone;
}
}
Usage:
static void Main(string[] args)
{
DerivedClass derivedReference = new DerivedClass();
DerivedClass clone = derivedReference.Clone();
}
As an example of usage, suppose you want to implement some chainable builder methods on the base type and derived type like this:
var d = new DerivedClass();
d.SetPropertyA("some value").SetPropertyB(1);
While SetPropertyA belongs to base class and SetPropertyB belongs to derived class.
By implementing classes like below, when chaining methods, after calling SetPropertyA because the return value is of type DerivedClass you can call SetPropertyB:
public abstract class BaseClass<TClass> where TClass : BaseClass<TClass>
{
public string A {get ; set; }
public TClass SetPropertyA(string value)
{
this.A=value;
return this as TClass;
}
}
public class DerivedClass : BaseClass<DerivedClass>
{
public int B {get ; set; }
public DerivedClass SetPropertyB(int value)
{
this.B=value;
return this;
}
}
Then if you have some other derived classes, each of them can use the base SetPropertyA knowing the return value is of type of itself.