What happens to an object I'm returning from a Using statement?

Question

5.00/5 (2 votes)

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Consider this code snippet

C#

using(MemoryStream ms = new MemoryStream())
{
     //code
     return ms;
}

Will ms get disposed? And if it does, when?

Posted 25-Sep-14 10:25am

Jörgen Andersson

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Comments

PhilLenoir 25-Sep-14 16:47pm

My bad, I didn't spot that you were returning a reference. I'd never use a Using block in this case, so I'm not certain that the returned object would be valid. You can easily test this yourself by checking the object in the calling code.

Sergey Alexandrovich Kryukov 25-Sep-14 16:49pm

Returning a reference from this point is a wrong thing. On a closing '}', ms.Dispose() call is generated.
—SA

Jörgen Andersson 25-Sep-14 17:19pm

I did test and Piebald is spot on, but what I'm curious about isn't just this simple example, but rather might happen in fringe cases where IDisposable is badly implemented.

2 solutions

Solution 2

This code

C#

using(MemoryStream ms = new MemoryStream())
{
     //code
     //return ms; I commented it out because it makes no sense at all
}

is strictly equivalent to:

C#

MemoryStream ms = new MemoryStream();
try
{
     //code
}
finally
{
    ms.Dispose();
}

In other words, this is an automatic way of calling IDisposable.Dispose(). A note for the author of Solution 1: In general case, disposal have nothing to do with garbage collection and reclaiming memory. In fact, IDisposable often used in reclaiming unmanaged memory, but there are many other uses.

Please see:
http://msdn.microsoft.com/en-us/library/yh598w02.aspx[^],
Understanding the 'using' statement in C#[^],
http://msdn.microsoft.com/en-us/library/system.idisposable%28v=vs.110%29.aspx[^].

—SA

Posted 25-Sep-14 10:45am

Sergey Alexandrovich Kryukov

Updated 25-Sep-14 10:48am

v2

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PIEBALDconsult · Accepted Answer · 2014-09-25T10:54:00

Solution 3

Dispose will be called before the item is returned, therefore the returned item is unlikely to be useful, but it depends on the implementation of the actual class. In most cases, any access to the item should result in an ObjectDisposedException.

Don't return or otherwise make the item available outside the confines of the using block.

C#

public X x ;

using ( X y = new X() ) { x = y } ;

Is just as bad.

Posted 25-Sep-14 10:54am

PIEBALDconsult

Comments

Jörgen Andersson 25-Sep-14 17:11pm

You say most cases, in which cases don't you get an ObjectDisposedException?

PIEBALDconsult 25-Sep-14 17:30pm

As you said "cases where IDisposable is badly implemented". I'd say "try it", but really you shouldn't.