Interfacecasting

[Hendrik Debedts]

What's the meaning of casting from a class to an interface?

[Guffa]

It means that you tell the compiler that instead of the reference to a specific class that you have, you want a reference to the same object but the type of the reference should be one of the interfaces that the class implements.

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[Hendrik Debedts]

I know, but what is the need of doing this? Why should someone do this?

[Guffa]

Hendrik Debedts wrote:

I know

So that's why you asked? ;)

Hendrik Debedts wrote:

but what is the need of doing this? Why should someone do this?

To get a reference to an interface. If you for example use the Array.Sort method with an IComparer, you need a reference to an IComparer: Array.Sort(SomeArrayOfMine, MyOwnComparer); Here, the reference to the MyOwnComparer object is implicitly casted to IComparer.

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[Hendrik Debedts]

Yes ok, but if you can cast an object to an interface, it means that the class of the object implements the interface and when the class implements the interface you don't have firstly cast the object to the interface

[PIEBALDconsult]

You wouldn't cast a class to an interface, it's unnecessary. But you may need to go the other way. If you have two classes (A and B) that implement an interface (I), you may have a method that takes a parameter of type I public void F ( I i ) then inside the method you may need to cast the parameter to it's actually type (however, this may be poor style) { if ( i is A ) { (A) i = blah blah blah } else if ( i is B ) { (B) i = blah blah blah } else throw something perhaps } but when calling F you needn't cast your instance to I A a = new A() ; F ( (I) a ) ; // legal, but needless F ( a ) ; // prefered

[Scott Dorman]

PIEBALDconsult wrote:

You wouldn't cast a class to an interface, it's unnecessary

This is sometimes necessary. If you have a class that implements two interfaces, each with a method of the same name you will need to explictly cast to the appropriate interface in order to access the correct method.

public interface IList {
public void Add(object x);
}

public interface IDictionary {
public void Add(object x);
}

public class Test : IDictionary, IList {
}

Test t = new Test();
((IList)t).Add(x);
((IDictionary)t).Add(x);

Without the explicit cast the compiler will not know which Add method you intended to call.

----------------------------- In just two days, tomorrow will be yesterday.

[PIEBALDconsult]

Ew. I sit corrected. I should have put in more qualifications like "usually unnecessary". But then you're probably getting into the reason C# only supports single inheritance.

[Scott Dorman]

PIEBALDconsult wrote:

But then you're probably getting into the reason C# only supports single inheritance.

Yep. I was approaching this from a C# viewpoint, which does support single inheritance for the base class (you can inherit from as many interfaces as you want).

----------------------------- In just two days, tomorrow will be yesterday.

[Guffa]

Hendrik Debedts wrote:

when the class implements the interface you don't have firstly cast the object to the interface

Yes, sometimes you have to. You can't call a method using parameters of the wrong type. If a method expects an interface reference, you have to use an interface reference. You can't use the object reference, as it's not the correct type.

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[Hendrik Debedts]

Oh yes you can, this code compiles without any problem ;) Here you pass an object of a class who implements the System.Collections.ICollection interface (Person) to a method that has a System.Collections.ICollection attribute (static void aMethod(System.Collections.ICollection i)) namespace ConsoleApplication1 { class Program { static void Main(string[] args) { Person oPerson = new Person("Hendrik Debedts"); aMethod(oPerson); Console.ReadLine(); } static void aMethod(System.Collections.ICollection i) { Console.WriteLine(i); } } public class Person : System.Collections.ICollection { public string msNaam; public Person(string sNaam) { this.msNaam = sNaam; } public override string ToString() { return this.msNaam; } public void CopyTo(System.Array array, int index) { } public int Count { get { int anInteger = 0; return anInteger; } } public bool IsSynchronized { get { bool aBoolean = false; return aBoolean; } } public object SyncRoot { get { object anObject = new object(); return anObject; } } public System.Collections.IEnumerator GetEnumerator() { System.Collections.ArrayList arl = new System.Collections.ArrayList(); return arl.GetEnumerator(); } } }

[Guffa]

Hendrik Debedts wrote:

Here you pass an object of a class who implements the System.Collections.ICollection interface (Person) to a method that has a System.Collections.ICollection attribute (static void aMethod(System.Collections.ICollection i))

...Thereby causing an implicit cast of the reference. Exactly as in the code I showed earlier in the thread[^]. Just because you don't explicitly write the code for the casting, doesn't mean that the code you write doesn't do a cast.

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[PIEBALDconsult]

I would only use the term "cast" when it's explicit, not when it's implicit.

[Dave Kreskowiak]

Then what would you call it?

Dave Kreskowiak Microsoft MVP - Visual Basic