I have the following c# classes:
class A : Object
{
foo() {}
}
class B : Object
{
foo() {}
}
I want to write a generic method that applies to both:
void bar<T>(T t)
{
t.foo();
}
this does not compile complaining the foo() is not a member of T. I can add a constraint for T to derive from one of the classes:
void bar<T>(T t) where T : A
but how can I have it for both?
Simply put you can't. There are a couple of approaches to work around this problem though.
The most common is to use an interface. Say IMyType
interface IMyType { void foo(); }
class A : IMyType ...
class B : IMyType ...
void bar<T>(T t) where T : IMyType {
t.Foo();
}
This is a bit heavy weight though is it requires a metadata change for a solution. A cheaper approach is to provide a lambda expression which calls the appropriate function.
void bar<T>(T t, Action doFoo) {
...
doFoo();
}
var v1 = new A();
var v2 = new B();
bar(v1, () => v1.Foo());
bar(v2, () => v2.Foo());
You should define an interface:
interface IFoo
{
void foo();
}
class A : IFoo
{
public void foo() {}
}
class B : IFoo
{
public void foo() {}
}
And your generic method:
void bar<T>(T t) where T:IFoo
{
t.foo();
}
Define an interface that contains the foo method, and have classes A & B implement that interface. Then define an interface constraint on your generic type.
You can't do that unless you:
Derive from a base class
Derive from an interface
I prefer the interface because it doesn't force you to share behavior:
public interface IHasFoo
{
void foo();
}
public class B : IHasFoo // you don't need to explicitly subclass object
{
public void foo()
{
}
}
public class A : IHasFoo // you don't need to explicitly subclass object
{
public void foo()
{
}
}
void bar<T>(T t) where T : IHasFoo
{
t.foo(); // works
}
This is possible in .NET 4.0 using dynamic.
void bar(dynamic t)
{
t.foo();
}
Why are you making this generic? Just overload the method.
void bar(A a) { a.Foo(); }
void bar(B b) { b.Foo(); }
Generics are there so that you can make potentially infinite bar() methods.
Related
This question already has answers here:
Can a C# class call an interface's default interface method from its own implementation?
(3 answers)
Closed 8 months ago.
When this code is run (I'm using .NET 6.0), it recurses infinitely, and never gets to DoSomething() in IInterface, instead of returning the Class instance from the interface.
It seems because of the return type of the method in the class being the same as in the interface, the compiler seems to think the interface's method is being reimplemented in the class, and the method calls itself.
If the method's return type is changed to the concrete class, it works without a problem. Why is it?
using System;
public class Program
{
public static void Main()
{
var obj = new Class();
var ret = obj.DoSomething();
Console.WriteLine("Finished");
}
}
interface IInterface {
IInterface DoSomething() {
return new Class();
}
}
class Class : IInterface {
// Infinite recursion
public IInterface DoSomething() => ((IInterface)this).DoSomething();
// Works
//public Class DoSomething() => (Class)((IInterface)this).DoSomething();
}
If you mark your interface method as sealed it will prevent recursion, but you won't be able to re-implement the method in another class
interface IInterface {
sealed IInterface DoSomething() {
return new Class();
}
}
class Class : IInterface {
public IInterface DoSomething() => ((IInterface)this).DoSomething();
}
Alternatively, you could make the class implementation private, and access it from another method. This will allow you to re-implement the interface in another class.
internal class Class : IInterface
{
private IInterface DoSomething() => ((IInterface)this).DoSomething();
public IInterface DoSomethingPublic() => DoSomething();
}
Unfortunately, there just isn't support for what you want to do currently. There was a section in the original Default Interface Methods proposal about the possibility of using base() to explicitly call an inherited interface, but that was cut.
I admit that I am not very familiar with default interface methods, but I suspect that the default behavior is to override the implementation, and by marking the interface as sealed you prevent this from happening.
If someone has a better explanation, please correct me!
Some reading material:
Default Interface Methods Proposal
A similar question with hacky workarounds
A. Base class
Classic base class implementation:
class BaseClass {
protected void DoSomething() {
Console.WriteLine("Hello from BaseClass!");
}
}
class Class : BaseClass {
public new void DoSomething() // new or virtual + override
{
Console.WriteLine("Hello from Class");
base.DoSomething();
}
}
B. Helper method
static class Helper {
public static void DoSomething() => Console.WriteLine("Do something!");
}
interface IInterface {
void DoSomething() => Helper.DoSomething();
}
class Class : IInterface {
public void DoSomething() { Console.WriteLine("Hello from Class"); Helper.DoSomething();}
}
C. Static interface method
I would say that this seems to be a case for a base class not an interface, but one way to 'reuse' the interface method is to make the interface method static.
For example:
var obj = new Class();
obj.DoSomething();
Console.WriteLine("Finished");
interface IInterface {
static void DoSomething() {
Console.WriteLine("Hello from IInterface!");
}
}
class Class : IInterface {
public void DoSomething() { Console.WriteLine("Hello from Class"); IInterface.DoSomething();}
}
This prints:
Hello from Class
Hello from IInterface!
Finished
I have a couple of classes which realize one method:
class ClassA : BaseClass
{
void Copy(ClassA a) {}
}
class ClassB : BaseClass
{
void Copy(ClassB b) {}
}
I want to describe these methods in the interface. Is it possible?
you can make use of Generic interface , for example as below
interface ICopy<T>
{
void Copy<T>(T t)
}
Class A: ICopy<A>,BaseClass//(if you need baseclass)
{
public void Copy(A a)
{}
}
Class B: ICopy<B>,BaseClass//(if you need baseclass)
{
public void Copy(B b)
{}
}
you can also try ICloneable inbuilt interface , if you just want to make clone of you class,
This is just suggestion
class Rock : ICloneable
{
int _weight;
bool _round;
bool _mossy;
public Rock(int weight, bool round, bool mossy)
{
this._weight = weight;
this._round = round;
this._mossy = mossy;
}
public object Clone()
{
return new Rock(this._weight, this._round, this._mossy);
}
}
Use a generic interface. Using where you can constrain the type parameter T to BaseClass and its derived types.
interface Interface<T> where T : BaseClass
{
void Copy<T>(T t);
}
class ClassA : BaseClass, Interface<ClassA>
{
public void Copy(ClassA b) {}
}
class ClassB : BaseClass, Interface<ClassB>
{
public void Copy(ClassB b) {}
}
I'd like to constrain a delegate to a member of a type. For example...
class Foo {
public virtual void Bar() {}
}
class Baz : Foo {
public override void Bar() {
this.BarAndMore(base.Bar);
}
}
delegate void FooMemberDelegate(); // Can I restrict this so `FooMemberDelegate`s can only be assigned to members of Foo
static class Extensions {
public static void BarAndMore(this Foo foo, FooMemberDelegate fooMemberDelegate) {
// Other stuff
fooMemberDelegate();
// More stuff
}
}
EDIT: Seems to be a duplicate of How can I constrain a method that takes an Action<T> to only allow delegates from a specific class
In C#, is it possible to create a struct that does something like this?
I'm trying to vary the field size inside PrimeStruct without declaring _myField as an interface, just deferring from closing the IData declaration until the PrimeStruct is created.
I know WHY this doesn't work, but trying to find if someone has syntactic sugar for deferring type declaration of the contents of PrimeStruct to a contained generic type.
(Like PrimeStruct<T>)
(no surprise: this does not compile)
interface IBehavior { void Foo(); }
interface IData<T> where T : IBehavior { }
struct MyStruct1 : IBehavior
{
public void Foo() { }
}
struct MyStruct2 : IBehavior
{
public void Foo() { }
}
//specifying an open type <T> here doesn't compile
public struct PrimeStruct : IData<T>, IBehavior
{
T _myField;
internal void SetData<T>(T value) where T : IBehavior
{
_myField = value;
}
public void Foo()
{
_myField.Foo();
}
}
public class Runner
{
public static void Main(string[] args)
{
PrimeStruct p = new PrimeStruct();
p.SetData(new MyStruct1());
p.Foo();
}
}
Or does this fall under the umbrella of the type not being known at compile time? I'm trying to avoid boxing the _myField struct value to an interface or resorting to HAllocGlobal.
PrimeStruct also needs a generic parameter for this to work.
public struct PrimeStruct<T> : IData<T>, IBehavior
Since IData<T> has no members, it's unclear what you're wanting the generic parameter for. If the intention is that IData<T> should constrain T to IBehavior, and provide a means of getting and setting it, I would suggest that you have IData<T> inherit from a non-generic IData with a member of type IBehavior; that method may provide a property getter, and a method to set the property, but the documentation for the method should make clear that most implementations of non-generic IData will only accept certain implementations of IBehavior.
Like supercat said, it is not clear from this code what IData<T> is for. If you take out the requirement that PrimeStruct implement IData<T>, and change _myField to an IBehavior, then this will compile.
interface IBehavior { void Foo(); }
interface IData<T> where T : IBehavior { }
struct MyStruct1 : IBehavior
{
public void Foo() { }
}
struct MyStruct2 : IBehavior
{
public void Foo() { }
}
//specifying an open type <T> here doesn't compile
public struct PrimeStruct : IBehavior
{
IBehavior _myField;
internal void SetData<T>(T value) where T : IBehavior
{
_myField = value;
}
public void Foo()
{
_myField.Foo();
}
}
public class Runner
{
public static void Main(string[] args)
{
PrimeStruct p = new PrimeStruct();
p.SetData(new MyStruct1());
p.Foo();
}
}
As Preston Guillot said, you cannot have an non-generic class that inherits from an open generic (i.e. IData<T>).
PrimeStruct : IData<int> // Non-generic inherits from closed generic is OK
PrimeStruct<T> : IData<T> // Open generic inherits from open generic is OK
PrimeStruct : IData<T> // Non-generic inherits from open generic will not compile
I have a theoretical question concerning how to deal with the following scenario in a language which does not allow multiple inheritance.
Imagine I have a base class Foo and from it I am wishing to create three sub-classes:
Class Bar inherits Foo and implements functionality "A"
Class Baz inherits Foo and implements functionality "B"
Class Qux inherits Foo and implements functionalities "A" and "B"
Imagine that the code to implement functionalities "A" and "B" is always the same. Is there a way to write the code for "A" and "B" only once, and then have the appropriate classes apply (or "inherit") it?
Well the only way I can see you achieving this in C#/Java is by composition. Consider this:
class Foo {
}
interface A {
public void a();
}
interface B {
public void b();
}
class ImplA implements A {
#Override
public void a() {
System.out.println("a");
}
}
class ImplB implements B {
#Override
public void b() {
System.out.println("b");
}
}
class Bar extends Foo {
A a = new ImplA();
public void a() {
a.a();
}
}
class Baz extends Foo {
B b = new ImplB();
public void b() {
b.b();
}
}
class Qux extends Foo {
A a = new ImplA();
B b = new ImplB();
public void b() {
b.b();
}
public void a() {
a.a();
}
}
Now Qux has both the functionality of Foo via normal inheritance but also the implementations of A and B by composition.
The more general term for this is a Mixin. Some languages provide support out of the box, such as Scala and D. There are various ways to achieve the same results in other languages though.
One way you can create a pseudo-mixin in C# is to use empty interfaces and provide the methods with extension methods.
interface A { }
static class AMixin {
public static void aFunc(this A inst) {
... //implementation to work for all A.
}
}
interface B { }
static class BMixin {
public static void bFunc(this B inst) {
...
}
}
class Qux : Foo, A, B {
...
}
This is achievable in languages providing traits (here: scala):
class Foo {
def fooM() {}
}
trait A {
def aFunc() {}
}
trait B {
def bFunc() {}
}
class Bar extends Foo with A {}
class Baz extends Foo with B {}
class Qux extends Foo with A with B {}
Because Scala runs on top of Java (having neither multiple inheritance nor traits) it is translated into something like this (simplified) - which might be a hint how to implement it in Java/C# manually:
class Foo {
}
interface A {
void aFunc();
}
interface B {
void bFunc();
}
class Bar extends Foo implements A {
public void aFunc() {
$A.aFunc();
}
}
class Baz extends Foo implements B {
public void bFunc() {
$B.bFunc();
}
}
class Qux extends Foo implements A, B {
public void aFunc() {
$A.aFunc();
}
public void bFunc() {
$B.bFunc();
}
}
class $A {
public static void aFunc() {}
}
class $B {
public static void bFunc() {}
}
There are several ways to do something like this. More specifically, if we abandon the inheritance aspect for a moment, there are ways to introduce the same unit of functionality to different classes, while writing the unit only once.
Okay, I love AOP Frameworks, and they exist for many languages (C# and Java having several). AOP Frameworks basically allow you add self-contained functionality into different classes throughout your inheritance structure.
For C#, you have PostSharp and for Java you have AspectJ, among many others.
Many AOP frameworks allow 'hijacking' or 'overriding' method calls without using inheritance.