Understanding Interfaces

Understanding Interfaces
Bhushan Mulmule
bhushan.mulmule@gmail.com
www.dotnetvideotutorial.com

Interfaces are simplest construct of C#. But understanding
when and why to use them can take years…


In this session let us focus on understanding interface as a C# construct.
We will discuss one of its usability in next session “Dependency Injection”
where we will see how we can create loosely coupled systems using interfaces

An interface contains only the signatures of
methods, properties, events or indexers.
A class or struct that implements the interface must implement the
members of the interface that are specified in the interface
definition.

Let us try to understand using simple example.
Let us create simple interface IDemoInterface with two methods
defined in it…


By default every member of interface is public so interface
doesn„t allow use of access modifier.

Interface doesn't have any implementation.
We have to implement members defined by interface
in class that we will drive from interface.

That‟s why we never say that
“we are deriving class from interface”
Instead we say that
“we are implementing interface in class”

Let us implement IDemoInterface in DemoClass


Interface implementation syntax is same as class inheritance.
We need to implement all methods of IDemoInterface in DemoClass

Now we have two ways to create object of DemoClass.

Normal Instantiation: Using reference of DemoClass only

Or


Upcasting: Using reference of IDemoInterface

And in both the cases output will be same...

Now let us add little complexity to our example.

Assume that there is one more interface ISampleInterface
with two methods…


This interface have two methods fun2() and fun3().
We have intentionally kept fun2() in both the interfaces .

C# allows multiple interface implementation.
Let us implement both the interfaces in DemoClass

Imp Note: Multiple class inheritance is not allowed in C#. i.e. we can‟t derive
class from two or more classes

Now we have three ways to create object of DemoClass


First let us have normal instantiation…

Output:


But in this case we have assumed that fun2() of both the interfaces
has common implementation.
Go back and check in DemoClass we have single implementation of
fun2()

What if we wanted to provide separate implementations for fun2()
of IDemoInterface and ISampleInterface?


We can do it using
Explicit Interface Implementation

Now fun2() is explicitly implemented and have two separate
implementations.

Also observe that explicitly implemented methods can‟t have
access modifier in DemoClass as they will be always public

We can’t call explicitly implemented methods using
reference of class.


We can call only fun1() and fun3() using reference of DemoClass. Compiler will not able to
resolve method call to fun2() as there are two implementation on fun2() in DemoClass.

Million dollar question

How to call fun2()?

We can call explicitly implemented methods using
reference of Interface.

So to call fun2() of IDemoInterface we can create object of
DemoClass using reference of IDemoInterface


Interface reference has access to all its methods.

Same way to call fun2() of ISampleInterface we can create object of
DemoClass using reference of ISampleIneterface

Note that Interface reference has access to its
own methods only. Reference of one interface
can‟t call method of other interface.

Is there any way out to call explicitly implemented
method using reference of class?

Yes!
We can typecast reference of class to interface


Here we are typecasting reference of DemoClass ; first to IDemoInterface
and then to ISampleInterface.

We can also derive one interface from other..


Here IDerivedInterface is derived from IBaseInterface

So we have to implement methods of both the interfaces in
class which implements IDerivedInterface

In next session we will have look on how to use interfaces to
create loosely coupled systems.


For video visit

Bhushan Mulmule
bhushan.mulmule@dotnetvideotutorial.com

Understanding Interfaces

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