I want to understand AND (&&) run time exception

Question

Hello everyone today I was working to make API.I got confused when I saw different behavior of C# compiler. let me explain:

I get exception:

If(userLoginModel.UserId>0)
{
//do something
}
else{
//do something
}

Exception: {"Object reference not set to an instance of an object."}

Yes it is null; But if write :

if (userLoginReturnModel != null && userLoginReturnModel.UserId >0)
{
//do something
}
else
{
//do something
}

This code did not throw any Exception.

Note: My question is why compiler is not throwing any exception on 2nd code because there is && condition in if so when compiler will check that userLoginReturnModel.userId > 0 then this will also need to throw exception because object is null. And as we know that both condition will be check in if. Any answer with good explanation.

If I ask you to retrieve my book from the bookcase, but I don't have a bookcase, that's a problem - right? Same thing with null. If you try and access an instance of `userLoginReturnModel` but you don't have one - that's a problem. `&&` short-circuits the if statement so it never executes the second half. — ProgrammingLlama, Jan 10 '19 at 07:53
@John the question was actually why only the first condition is checked in second example. — Piotr Wojsa, Jan 10 '19 at 07:56
@John, the question is about the behaviour of `&&` and how it *avoids* a `NullReferenceException`, not how to fix one, thus not a possible dupe of that question — Rob, Jan 10 '19 at 07:56
@Wajidkhan if it answers your question you can mark it as correct answer to close this topic. — Piotr Wojsa, Jan 10 '19 at 08:01
To people like Me - John's comment answered the situation well. But may be others need more detailed answer. — Prateek Shrivastava, Jan 10 '19 at 08:04
Suggest you edit your question. Your first example references userLoginModel you null check references userLoginReturnModel — Matt Evans, Jan 10 '19 at 08:11
@Wajidkhan: see also https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/conditional-and-operator — Dennis, Jan 10 '19 at 08:14

score 7 · Accepted Answer · answered Jan 10 '19 at 07:52

7

In case of:

if (userLoginReturnModel != null && userLoginReturnModel.UserId >0)

if the first condition is false then next is not checked (because there is no need to do so - false && anything gives false).

Similiar with || - if first is true then next are not checked (no need - true || anything gives true).

answered Jan 10 '19 at 07:52

Piotr Wojsa

938
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Maybe as a appendix: the expection would occur, when only concating the bool confitions with one "&". Check: https://stackoverflow.com/questions/5537607/usage-of-versus – Malior Jan 10 '19 at 07:58

score 2 · Answer 2 · answered Jan 10 '19 at 07:58

2

in this case if userLoginReturnModel is null you will have:

if ( false && ...

do you need to check conditions after false? False && true is still false. so rest of conditions doesn't matter and will not be checked.

answered Jan 10 '19 at 07:58

Pablo notPicasso

3,031
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I want to understand AND (&&) run time exception

2 Answers2