Will Dispose() be called in a using statement with a null object?

Question

Is it safe to use the using statement on a (potentially) null object?
Consider the following example:

class Test {
    IDisposable GetObject(string name) {
        // returns null if not found
    }

    void DoSomething() {
        using (IDisposable x = GetObject("invalid name")) {
            if (x != null) {
                 // etc...
            }
        }
    }
}

Is it guaranteed that Dispose will be called only if the object is not null, and I will not get a NullReferenceException?

To answer your last question directly [from the language spec](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/statements#1314-the-using-statement): `If a null resource is acquired, then no call to Dispose is made, and no exception is thrown.` — Eric Mutta, May 24 '23 at 16:47

score 194 · Accepted Answer · edited Jan 04 '16 at 18:40

194

Yes, Dispose() is only called on non-null objects:

http://msdn.microsoft.com/en-us/library/yh598w02.aspx

edited Jan 04 '16 at 18:40

BartoszKP

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answered Mar 26 '10 at 11:26

reko_t

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41

Note that even if your variable is null, the using block is executed, and if you reference your variable inside the using block without first null-checking it, you WILL get NullReferenceException. To prevent misinterpretation, this answer should state: "Yes, `Dispose()` is only called on non-null objects". – surfen Nov 28 '11 at 09:05

score 46 · Answer 2 · edited Apr 22 '13 at 17:22

46

The expansion for using checks that the object is not null before calling Dispose on it, so yes, it's safe.

In your case you would get something like:

IDisposable x = GetObject("invalid name");
try
{
    // etc...
}
finally
{
    if(x != null)
    {
        x.Dispose();
    }
}

edited Apr 22 '13 at 17:22

Andriy Volkov

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answered Mar 26 '10 at 11:25

João Angelo

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score 22 · Answer 3 · answered Mar 26 '10 at 11:25

22

You should be ok with it:

using ((IDisposable)null) { }

No exception thrown here.

Side note: don't mistake this with foreach and IEnumerable where an exception will be thrown.

answered Mar 26 '10 at 11:25

Darin Dimitrov

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score 1 · Answer 4 · answered Mar 26 '10 at 11:27

1

Yes, before Disposing the reference will be null-checked. You can examine yourself by viewing your code in Reflector.

answered Mar 26 '10 at 11:27

oli

84
1

score -3 · Answer 5 · answered Mar 26 '10 at 11:26

-3

You will not get null reference exception as per my experience. It will be simply ignored.

answered Mar 26 '10 at 11:26

malay

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Will Dispose() be called in a using statement with a null object?

5 Answers5

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