Comments on: NotImplementedException and the Interface Segregation Principle

By: bogardj

bogardj — Tue, 19 May 2009 18:40:04 +0000

@etro

This isn’t necessarily tied to the .NET version of an interface. It applies to abstract classes as well.

If an interface has a bunch of methods that I don’t want to implement, doesn’t that mean that the interface doesn’t apply to me?

By: etro

etro — Tue, 19 May 2009 18:26:22 +0000

So the real compliant here is the way interfaces work in the first place? You want to implement an interface, not implement all of it, not tell anyone you are not implementing it, and then expect every consumer to understand that…

I suppose an argument could be made that the default behaivor of an non-fully-implemented interface could be to “softly” back out of the call and make everyone play nice… but isn’t this sort of the POINT of interfaces? So that everyone is assured that they know all the calls to get something done, just not the implementation?

By: Jimmy Bogard

Jimmy Bogard — Fri, 17 Oct 2008 15:25:42 +0000

@Ramon

That still doesn’t solve the ISP problem. If I have to throw exceptions to ensure certain features aren’t used, by definition, it’s an ISP violation.

By: Ramon Smits

Ramon Smits — Fri, 17 Oct 2008 14:15:34 +0000

That’s why you should use NotSupportedException instead of NotImplementedException.

You start with NotImplemented and when you decide that a certain method/feature is not needed then make the decision to not support the feature at all thus throwing NotSupportedException together with the reason for that.

By: Jimmy Bogard

Jimmy Bogard — Mon, 13 Oct 2008 02:44:52 +0000

@Derick

Yeah I hate that. I want it to fail because the assertion fails, not because of some garbage in the SUT.

By: Derick Bailey

Derick Bailey — Mon, 13 Oct 2008 00:53:04 +0000

“The NotImplementedExceptions are noisy, and I’d rather never see one in code (even temporarily in a test). ”

amen to that! I can’t stand the default “throw NotImplementedException” that Resharper puts into generated code. The first thing I do after generating any method or property is get rid of this and put in code that I know will run, but fail.

By: smaclell

smaclell — Sun, 12 Oct 2008 19:49:28 +0000

It always makes me chuckle but .NET actually provides an exception specifically intended for this case, where you want to implement something but don’t want to support specific functionality. System.NotSupportedException.