whether an interface can be instantiated

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I have Read An interface cannot be instantiated directly in MSDN

but there only while giving an example they are instantiating it like this

SampleClass obj = new SampleClass();
//obj.Paint();  // Compiler error.
IControl c = (IControl)obj;
c.Paint();  // Calls IControl.Paint on SampleClass.
ISurface s = (ISurface)obj;
s.Paint(); // Calls ISurface.Paint on SampleClass.

where IControl and ISurface are interfaces and paint is method which is defined in SampleClass which inherits these two interface

My question is

IControl c = (IControl)obj;

here also they are creating an instance of IControl
so wheter interface can be instantiated or not??
please guide me..

Posted 21-Jan-10 0:03am

nishuthegreat

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4 solutions

Solution 2

IControl c is not really an instance of IControl, but an instance of SampleClass casted to IControl.

Posted 21-Jan-10 2:16am

Toli Cuturicu

Updated 21-Jan-10 2:18am

v2

Solution 3

nishuthegreat wrote:
My question is

IControl c = (IControl)obj;

here also they are creating an instance of IControl
so wheter interface can be instantiated or not??

No, they create a reference to an object implementing the interface.
:)

Posted 21-Jan-10 2:30am

CPallini

Solution 4

In the MSDN example, they have declared a variable of type IControl and assigned it an instance of the concrete class that implements IControl.

Posted 12-Oct-15 4:18am

Vamsi Rageer

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CHill60 12-Oct-15 10:59am

Post was answered over 5 years ago! You have added nothing to the existing solutions

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AspDotNetDev · Accepted Answer · 2010-01-21T00:13:00

Not really sure what your question is, but that bottom line shows the SampleClass instance being casted as an IControl and then assigned to an IControl variable. Actually, the compiler is smart enough to know when a class implements an interface, so the explicit cast is not necessary. You could just write:

C#

IControl c = obj;

It is correct to say interfaces cannot be directly instantiated, as they are not really classes (they are just interfaces that some class must implement). You can then create an instance of a class that implements the interface, then assign that instance to a variable of the interface type. For example, you could have also done this:

C#

IControl c = new SampleClass();
c.Paint();

Now, if you wanted to assign that to an ISurface variable, you would have to explicitly cast it:

C#

ISurface s = (ISurface)c;

I recommend you do some reading (a book or some tutorials online) about object oriented programming, perhaps specifically as it relates to C#.