interfaces c# what the point?

Just been reading a chapter on Interfaces; what's the point of adding an interface to your class? Considering all i get for using them is a bunch of methods which i have to implement myself.

So what's the purpose? what can they be used for? as per usual the book only gives pointless code examples and no real explanation of why you would use them.

thanks.

An interface is a contract. It says that any user of your code and expect the following methods. That interface might be used to read data and you might have written a class to get that data from a text file. Later you might recode that class into to pull data from a database or a remote procedure call. Later again you might decide to completely rewrite the code to make it faster and more robust.

As long as you use the same interface, the users of your code don't need to know about what you are doing because no matter what's going on behind the scenes, you expose the same methods via that initial interface.

It gives the users of your code an assurance that they wont have to completely recode their programs due to changes you make.

thanks for reply, so it's only to ensure the way the methods used do not change? Is this used a lot in the industry, i mean using the language Interface ability?

More importantly it can give multiple different classes the same type of operations. A (naive) way of supporting multiple types of printers could be done like this:

Code:

---

public class FilePrinter
{
  public void PrintToFile();
}

public class NetworkPrinter
{
  public PrintOverNetwork();
}

public void Print(object printer)
{
  if(printer is FilePrinter)
  {
    var file_printer = printer as FilePrinter;
    file_printer.PrintToFile();
  }
  else if(printer is NetworkPrinter)
  {
    var network_printer = printer as NetworkPrinter;
    network_printer.PrintOverNetwork();
  }
}

---

A better way would be to give them a common printing interface:

Code:

---

public interface IPrinter
{
  void Print();
}

public class FilePrinter : IPrinter
{
  public void PrintToFile();
  public void Print() { PrintToFile(); }
}

public class NetworkPrinter : IPrinter
{
  public PrintOverNetwork();
  public void Print() { PrintOverNetwork(); }
}

public void Print(IPrinter printer)
{
  printer.Print();
}

---

A cleaner solution, less cluttering if-elses and much easier to extend with more Printers later (maybe an USBPrinter?).

Only time I ever used interfaces was when I was playing around with .net plugins. Having an interface of methods and variables is really cool if you want to add expandability to your application through .net dll plugins and so on.

Why no abstract class?

Quote:

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Originally Posted by audinue

Why no abstract class?

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What exactly do you mean by that question? For example, are you asking why an interface should be used instead of an abstract class, or do you think that C# does not have abstract classes as a language feature?

Quote:

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Originally Posted by audinue

Why no abstract class?

---

Well

Ok, why use an interface instead of an abstract class?

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