cast object to interface

[mr.peteryu]

Hi,

Can someone explain the idea behind casting to an interface?

For example:
-> I have an IInterface that contains a Read() method.
-> I have an object "obj" that implements IInterface.

Why would someone do the following and what does it mean?

Object obj = new Object();
IInterface var = (IInterface) obj
var.Read()
Thanks

[Nicholas Paldino [.NET/C# MVP]]

You can't do this. Object doesn't implement IInterface. The type that
you create has to implement that interface (agree to the contract, in a
sense), in order for the cast to succeed.

You can't just cast any type to any other arbitrary type successfully.

Hope this helps.
--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard. caspershouse.co m

<mr********@gma il.com> wrote in message
news:11******** **************@ f14g2000cwb.goo glegroups.com.. .

Hi,

Can someone explain the idea behind casting to an interface?

For example:
-> I have an IInterface that contains a Read() method.
-> I have an object "obj" that implements IInterface.

Why would someone do the following and what does it mean?

Object obj = new Object();
IInterface var = (IInterface) obj
var.Read()
Thanks

[chris martin]

> Hi,

Can someone explain the idea behind casting to an interface?

For example:
-> I have an IInterface that contains a Read() method.
-> I have an object "obj" that implements IInterface.
Why would someone do the following and what does it mean?

Object obj = new Object();
IInterface var = (IInterface) obj
var.Read()
Thanks

Assuming that "Object obj = new Object();" really means "SomeObjectThat ImplemetsIInter face
obj = new SomeObjectThatI mplemetsIInterf ace();", there really is no point,
that I can think of, in casting to IInterface. If it is true that SomeObjectThatI mplemetsIInterf ace
implements IInterface then you c(w)ould just call obj.Read();

My guess is that the code was written by an inexperienced programmer. Please
correct me if I'm wrong.

Chris

[jeraldp]

Normally, if you have objects that support interface "IInterface ", your
member variable will be defined as type "IInterface " also. However,
you will most likely use reflection (either directly or via a class
factory) to create your IInterface object, and reflection returns
references to type object because it cannot pass back a specific type
reference. At some point, that object reference must be cast to the
interface. Reflection allows you to programmaticall y create types that
you may not even know about at compile time.

[Jeff Louie]

Base classes are useful for polymorphic behavior and as a transport
mechanism.
For polymorphic behavior one might create a collection of references of
type
IMyInterface. For transport one might pass a reference (variable) of
type object.

Regards,
Jeff

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[Stefan]

Hi,

Check out my article, it might be helpful.

http://www.developersdex.com/gurus/articles/739.asp

Happy Coding,

Stefan
C# GURU
www.DotNETovation.com

"You always have to look beyond the horizon and can never be complacent
-- God forbid we become complacent."

Jozef Straus

*** Sent via Developersdex http://www.developersdex.com ***

[methodios]

Sorry if I was a little vague. Here's the code from the book:

interface IStorable
{
void Read();
void Write(object);
int Status {get; set; }
}
public class Document : IStorable
{
public void Read() {...}
public void Write (object obj) {...}

public int Status {
get { return status;}
set { status = value; }
}
.....
}

-------

Then here is my question:

Document doc = new Document("Test Document");
doc.status = -1;
doc.Read();
IStorable isDoc = (IStorable) doc;
isDoc.Status = 0;
isDoc.Read();

Why would someone cast an object to an interface? Instead of
isDoc.Status, can't we just use doc.Status ? This is all very confusing
to me because I'm not quite sure what the purpose is for interfaces. I
understand interfaces are contracts and contains no implementations .
But why would someone want do the casting stated above? I'm kind of
stuck right now in the book because I'm worried I might be missing a
very important concept here which might be used later on; therefore I
would like to get all this cleared up now.

Thanks for all your help!
-----------

[methodios]

Hi Chris,

That's what I was thinking. What is the need for var.Read() when you
can just call obj.Read().

[Joanna Carter [TeamB]]

"methodios" <mr********@gma il.com> a écrit dans le message de news:
11************* *********@g14g2 00...legr oups.com...

| Why would someone cast an object to an interface? Instead of
| isDoc.Status, can't we just use doc.Status ?

In the given example, yes you could do just that, but interfaces are
designed, amongst other things, to allow commoin behaviour across
hierarchies. IOW, you dont have to derive all your classes from one base
class to get a common behaviour. It also allows you to "derive from"
(implement) more than one interface.

So, you can not only have Document implement IStorable, but any other class
that you desire to have the Read, Write and Status behaviour. Thus I can
then have a list of IStorable items and treat them all the same, regardless
of what their real class is.

Also, I can implement the interface explicitly, thereby setting the
properties and methods to be essentially private to clients of the class.

public class Document : IStorable
{
void IStorable.Read( ) {...}
void IStorable.Write (object obj) {...}

public int IStorable.Statu s
{
get { return status;}
set { status = value; }
}
...
}

Joanna

--
Joanna Carter [TeamB]
Consultant Software Engineer

[Joanna Carter [TeamB]]

"Joanna Carter [TeamB]" <jo****@not.for .spam> a écrit dans le message de
news: %2************* ***@TK2MSFTNGP1 0.phx.gbl...

Sorry, slight change required :

| public int IStorable.Statu s
| {
| get { return status;}
| set { status = value; }
| }

should be :

int IStorable.Statu s
{
get { return status;}
set { status = value; }
}

Joanna

--
Joanna Carter [TeamB]
Consultant Software Engineer