What does backward compatibility of interface mean?

Suraj Rai

Hi,
We have an 7 year old application which is in the maintainance phase.
I have recently joined the project and I keep hearing "backward compatibility of inteface".
Can someone explain to me with an example what does this means?
Thank you.
Regards

6 Replies

Know the answer? Post it — somebody with the same question will find it here.

Suraj Rai14y ago
Thanks to all of you.
Vulpes14y ago
Here's a simple example of the point I was making in my previous post:

using System;

interface IFace
{
void MyMethod();
}

interface IFace2 : IFace
{
void MyMethod2();
}

class MyClass : IFace
{
public void MyMethod()
{
Console.WriteLine("Hello from MyClass.MyMethod");
}
}

class MyClass2 : IFace2
{
public void MyMethod()
{
Console.WriteLine("Hello from MyClass2.MyMethod");
}

public void MyMethod2()
{
Console.WriteLine("Hello from MyClass2.MyMethod2");
}
}

class Test
{
static void Main()
{
IFace mc = new MyClass(); // still works OK
mc.MyMethod();
IFace2 mc2 = new MyClass2();
mc2.MyMethod();
mc2.MyMethod2();
IFace mc22 = mc2; // MyClass2 also implements IFace automatically
mc22.MyMethod();
Console.ReadKey();
}
}
Vulpes14y ago
The only way to do that is to leave the existing interface unchanged. If you try to change it (by adding new members etc), then existing classes which implement that interface will no longer work.

If you do want to add new members, then I'd create a new interface which inherits the old one.

Any new classes you write can then implement the new interface (and therefore the old one as well) and the existing classes which implement the old interface will continue to work without the need for any changes.
Suraj Rai14y ago
Hi,
Thanks a lot for the replies.
I got the meaning of backward compatibility.
But I want to know is there a way in c# to write interfaces in such a way that it will be backward
compatible and wont break the code ?
Thanks again.
Chintan Rathod14y ago
Backward compatibility is related to versions of application or device.

Def:
A device or technology is said to be backward or downward compatible if it can work with input generated by an older device.

Meaning:
If we are producing any new version, say N, of any application or device, then it should be compatible with older version, say N-1,N-2, of the same.
For example,
First C language come, then C++, but you can see that though C++ is totally different then C language, it is capable to run C programs also.
Same like newer devices. Taking android phone versions. If device is of version 3.0, it is still capable to handle applications of 1.0, 1.5, 2.2 etc.
We can give another example like Microsoft Word (2007,2003).

Reverse of backward is forward. Forward Compatible means,taking example, Microsoft 2003 should handle Microsoft 2007 file.
Mamta M14y ago
It means support for older versions of the software or dependent software elements. For example, if we say Visual Studio 2008 is backward compatible, it means that it supports applications created in Vs 2005 etc. It may not be fully supported, as in, the application may need to be converted by the newer IDE but at least the application will OPEN. If there was no backward compatibility in VS 2008, the applications created in VS 2005 won't even open or wont even be converted to newer version. The software will simply reject the open operation by saying something like "It was created in an older version of the software and cannot be opened".

So if your software needs to be backward compatible, it must include support for at least a few older versions. This is important so that you dont lose out on existing customers who dont want to upgrade to new versions.

For example, though today the market has Windows 7, there are many who are still using Windows XP. Such customers cannot be forced to upgrade to Windows 7, it should be their choice. But MS Office 2010 works on Windows XP. That's the power of backward compatibility.