What does “throw new NotImplementedException();” do exactly?

Question

I have a class \'b\' that inherits from class \'a\'. In class \'a\' there is some code that performs an action if an event is not null. I need that code to fire in class \'b\'

Answer 1

NotImplementedException, I believe, has a little special meaning: "this code is still in development and needs to be changed". Developers put this exception to make some code compileable, but not executable (to avoid data damage).

Information about this exception type can be found in documentation, it explains the meaning very detailed: https://docs.microsoft.com/en-us/dotnet/api/system.notimplementedexception?view=netcore-3.1

Some development tools, like Resharper, for example, generates new members with NotImplementedException inside, thus protecting you from execution the code, which is not ready. Same time they highlight this exceptions same way as "//todo:" comment

For other situations, for example, when you don't need to implement an interface or virtual member, or may be, you don't implement some paths in switch/case, if/else etc statements, you probably will use NotSupportedException, OutOfRangeException, ArgumentNullException, InvalidOperationException etc.

At the end, consider this situation to understand the purpose of NotImplementedException: We are writing banking application, and, at the moment, implementing money transferring feature, we have:

public void Transfer(sourceAccount, destAccount, decimal sum)
{
   sourceAccount.Credit(sum);
   destAccount.Debit(sum);
}

Here we are calling two methods which do not exist yet. We are generating both with default NotImplementedException, and going to first one (Credit) to implement it. Lets say, implementation took some time, we have written test for it, and even have done several manual tests. We completely forgot about second method "Debit" and deploying our application to beta, or even to production. Testers or users start using our application and soon they are coming to money transfer functionality. They are trying to call Transfer, which shows them a general message "We are so sorry, we got some errors", same time we, as a developer team, receive notification about NotImplementedException happened with the stack trace pointing us to the method "Debit". Now we are implementing it, and releasing new version, which can do money transfers. Now imagine, what would happen, if there was not an exception there: users would spend lot of money trying to do that transfers, trying several times, each time throwing money in to a hole. Depending on the purposes of the application it can be bigger or smaller problem: may be button on your calculator does not work, or may be that was scientific calculator and we just missed some important math while calculating vaccine code against some aggressive virus.