Could this `if` statement generate a Null-Reference Exception ever?

I think my point is clear, under any condition, could this throw null-reference exception? i.e. objecting that e.Result is null while trying to access its Count property.

if (e.Result == null || e.Result.Count == 0)
    return;

EDIT: To gain the most of this question (for later-coming askers), how will 开发者_开发问答we prevent that exception from happening, can we use lock statement? how?

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That code can throw a NullReferenceException only if the method is not thread safe. Meaning, if another thread decides to assign null to e.Result at coincidentally the exact moment after your e.Result == null check and right before your e.Result.Count == 0 check.

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It will only do so if e is null.

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e == null

OR

Multithreading & race condition ?

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It might throw "Null-Reference Exception" if Result has its own code behind it and that code is failing with that error.

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No. - e.Result ==null will be evaluated first, if it is true then return will be called immediately and the second case will not be evaluated. It may be have differently in threading

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            EventArgs args = e;
            if (args.Result == null || args.Result.Count == 0)
            {
                return;
            }

The idea is that you first copy the event args ( e ) into your own local element and use this for the if conditional check. Be careful to ensure that the copy is deep enough to copy the Result also ( otherwise you still end up running into the multithreading case when another thread has just nulled the result midway through the if check ). A local copy is always threadsafe.

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Aside from the multithreading/race-condition issue that others have already pointed out, the topic you might be looking for is short-circuit evaluation. In C#, the || operator uses short-circuiting (as opposed to |, which is eager), so if e.Result == null evaluates to true, the e.Result.Count == 0 won't be evaluated.

Others have answered already but I wanted to give you the name for this type of behavior and some further reading.