Should I Dispose() DataSet and DataTable?

Question

DataSet and DataTable both implement IDisposable, so, by conventional best practices, I should call their Dispose() methods.

However, from what I've read so far, DataSet and DataTable don't actually have any unmanaged resources, so Dispose() doesn't actually do much.

Plus, I can't just use using(DataSet myDataSet...) because DataSet has a collection of DataTables.

So, to be safe, I'd need to iterate through myDataSet.Tables, dispose of each of the DataTables, then dispose of the DataSet.

So, is it worth the hassle to call Dispose() on all of my DataSets and DataTables?

Addendum:

For those of you who think that DataSet should be disposed: In general, the pattern for disposing is to use using or try..finally, because you want to guarantee that Dispose() will be called.

However, this gets ugly real fast for a collection. For example, what do you do if one of the calls to Dispose() thrown an exception? Do you swallow it (which is "bad") so that you can continue on to dispose the next element?

Or, do you suggest that I just call myDataSet.Dispose(), and forget about disposing the DataTables in myDataSet.Tables?

Aircool

Dispose is not supposed to throw any exceptions. If it does—it’s not well written, so… try { some.Dispose(); } catch {} should be enough. - blogs.msdn.com/b/clyon/archive/2004/09/23/233464.aspx – Malnourished

It introduces a lot of confusion that a Dispose() implementation is not required after all. I only wished this had been better documented at MSDN, exactly because of the exception character of these classes. – Topi

it's not strictly true that its not required. Just at this point in time there are no unmanaged resources. Other implementations may introduce them. It is up to you to determine what is best, but know that Dispose will most likely be at worst a no-op on MS implementations. – Ullage

I noticed an apparent memory leak in one of my apps that uses a lot of DataSet objects. I had not been calling .Dispose() or using "using" blocks for those objects. So, I went through the code and added a "using" block to every place I was creating a DataSet or a DataTable, and voila the memory is now released. Seems to me a solid indication that .Dispose() is, in fact, necessary for DataSet and DataTable. – Hurt

Answer 1

Here are a couple of discussions explaining why Dispose is not necessary for a DataSet.

To Dispose or Not to Dispose ?:

The Dispose method in DataSet exists ONLY because of side effect of inheritance-- in other words, it doesn't actually do anything useful in the finalization.

Should Dispose be called on DataTable and DataSet objects? includes some explanation from an MVP:

The system.data namespace (ADONET) does not contain unmanaged resources. Therefore there is no need to dispose any of those as long as you have not added yourself something special to it.

Understanding the Dispose method and datasets? has a with comment from authority Scott Allen:

In pratice we rarely Dispose a DataSet because it offers little benefit"

So, the consensus there is that there is currently no good reason to call Dispose on a DataSet.

Slipshod

The links provided totally missed the point that DataTable is a type of Finalizable object. Please see Nariman's answer below. – Embryectomy

Interesting answer but what about SqlConnection, SqlCommand and SqlDataAdapter, should the Dispose be called explicitly? – Rattray

@Rattray yes those should absolutely be disposed because they use unmanaged resources. Whether it's called explicitly or implicitly using a using block is up to you. – Gray

Answer 2

Update (December 1, 2009):

I'd like to amend this answer and concede that the original answer was flawed.

The original analysis does apply to objects that require finalization – and the point that practices shouldn’t be accepted on the surface without an accurate, in-depth understanding still stands.

However, it turns out that DataSets, DataViews, DataTables suppress finalization in their constructors – this is why calling Dispose() on them explicitly does nothing.

Presumably, this happens because they don’t have unmanaged resources; so despite the fact that MarshalByValueComponent makes allowances for unmanaged resources, these particular implementations don’t have the need and can therefore forgo finalization.

(That .NET authors would take care to suppress finalization on the very types that normally occupy the most memory speaks to the importance of this practice in general for finalizable types.)

Notwithstanding, that these details are still under-documented since the inception of the .NET Framework (almost 8 years ago) is pretty surprising (that you’re essentially left to your own devices to sift though conflicting, ambiguous material to put the pieces together is frustrating at times but does provide a more complete understanding of the framework we rely on everyday).

After lots of reading, here’s my understanding:

If an object requires finalization, it could occupy memory longer than it needs to – here’s why: a) Any type that defines a destructor (or inherits from a type that defines a destructor) is considered finalizable; b) On allocation (before the constructor runs), a pointer is placed on the Finalization queue; c) A finalizable object normally requires 2 collections to be reclaimed (instead of the standard 1); d) Suppressing finalization doesn’t remove an object from the finalization queue (as reported by !FinalizeQueue in SOS) This command is misleading; Knowing what objects are on the finalization queue (in and of itself) isn’t helpful; Knowing what objects are on the finalization queue and still require finalization would be helpful (is there a command for this?)

Suppressing finalization turns a bit off in the object's header indicating to the runtime that it doesn’t need to have its Finalizer invoked (doesn’t need to move the FReachable queue); It remains on the Finalization queue (and continues to be reported by !FinalizeQueue in SOS)

The DataTable, DataSet, DataView classes are all rooted at MarshalByValueComponent, a finalizable object that can (potentially) handle unmanaged resources

• Because DataTable, DataSet, DataView don’t introduce unmanaged resources, they suppress finalization in their constructors
• While this is an unusual pattern, it frees the caller from having to worry about calling Dispose after use
• This, and the fact that DataTables can potentially be shared across different DataSets, is likely why DataSets don’t care to dispose child DataTables
• This also means that these objects will appear under the !FinalizeQueue in SOS
• However, these objects should still be reclaimable after a single collection, like their non-finalizable counterparts

4 (new references):

• http://www.devnewsgroups.net/dotnetframework/t19821-finalize-queue-windbg-sos.aspx
• http://blogs.msdn.com/tom/archive/2008/04/28/asp-net-tips-looking-at-the-finalization-queue.aspx
• http://issuu.com/arifaat/docs/asp_net_3.5unleashed
• http://msdn.microsoft.com/en-us/magazine/bb985013.aspx
• http://blogs.msdn.com/tess/archive/2006/03/27/561715.aspx

Original Answer:

There are a lot of misleading and generally very poor answers on this - anyone who's landed here should ignore the noise and read the references below carefully.

Without a doubt, Dispose should be called on any Finalizable objects.

DataTables are Finalizable.

Calling Dispose significantly speeds up the reclaiming of memory.

MarshalByValueComponent calls GC.SuppressFinalize(this) in its Dispose() - skipping this means having to wait for dozens if not hundreds of Gen0 collections before memory is reclaimed:

With this basic understanding of finalization we can already deduce some very important things:

First, objects that need finalization live longer than objects that do not. In fact, they can live a lot longer. For instance, suppose an object that is in gen2 needs to be finalized. Finalization will be scheduled but the object is still in gen2, so it will not be re-collected until the next gen2 collection happens. That could be a very long time indeed, and, in fact, if things are going well it will be a long time, because gen2 collections are costly and thus we want them to happen very infrequently. Older objects needing finalization might have to wait for dozens if not hundreds of gen0 collections before their space is reclaimed.

Second, objects that need finalization cause collateral damage. Since the internal object pointers must remain valid, not only will the objects directly needing finalization linger in memory but everything the object refers to, directly and indirectly, will also remain in memory. If a huge tree of objects was anchored by a single object that required finalization, then the entire tree would linger, potentially for a long time as we just discussed. It is therefore important to use finalizers sparingly and place them on objects that have as few internal object pointers as possible. In the tree example I just gave, you can easily avoid the problem by moving the resources in need of finalization to a separate object and keeping a reference to that object in the root of the tree. With that modest change only the one object (hopefully a nice small object) would linger and the finalization cost is minimized.

Finally, objects needing finalization create work for the finalizer thread. If your finalization process is a complex one, the one and only finalizer thread will be spending a lot of time performing those steps, which can cause a backlog of work and therefore cause more objects to linger waiting for finalization. Therefore, it is vitally important that finalizers do as little work as possible. Remember also that although all object pointers remain valid during finalization, it might be the case that those pointers lead to objects that have already been finalized and might therefore be less than useful. It is generally safest to avoid following object pointers in finalization code even though the pointers are valid. A safe, short finalization code path is the best.

Take it from someone who's seen 100s of MBs of non-referenced DataTables in Gen2: this is hugely important and completely missed by the answers on this thread.

References:

1 - http://msdn.microsoft.com/en-us/library/ms973837.aspx

2 - http://vineetgupta.spaces.live.com/blog/cns!8DE4BDC896BEE1AD!1104.entry http://www.dotnetfunda.com/articles/article524-net-best-practice-no-2-improve-garbage-collector-performance-using-finalizedispose-pattern.aspx

3 - http://codeidol.com/csharp/net-framework/Inside-the-CLR/Automatic-Memory-Management/

Strident

Good point. How do you usually structure your code when you have a DataSet with many DataTables? Tons of nested using statements? A single try..finally to clean it all up at once? – Aircool

The statement "However, it turns out that DataSets, DataViews, DataTables suppress finalization in their constructors – this is why calling Dipose() on them explicitly does nothing." is a non-sequitur: the two concepts are largely unrelated; something that suppresses finalisation could still do something in Dispose(). Indeed , it actually makes more sense if we reverse it: Dispose() does nothing, which is why it suppresses finalisation in the constructor, i.e. because there would be nothing to do it doesn't want to bother the GC with calling the finaliser (which typically calls dispose). – Disagree

Thanks. Does this discussion apply to TableAdapters as well? – Icono

Fix dead link http://blogs.msdn.com/tess/archive/2006/03/27/561715.aspx – Advertise

Answer 3

You should assume it does something useful and call Dispose even if it does nothing in current .NET Framework incarnations. There's no guarantee it will stay that way in future versions leading to inefficient resource usage.

Immoralist

There's no guarantee that it will implement IDisposable in the future, either. I would agree with you if it were as simple as using(...), but in the case of DataSet, it seems like a lot of hassle for nothing. – Aircool

It fairly safe to assume that it will always implement IDisposable. Adding or removing the interface is a breaking change, whereas changing the implementation of Dispose is not. – Marcellus

Also, a different provider may have an implementation that actually does something with IDisposable. – Clarettaclarette

Not to mention that DataTable isn't sealed - not a big deal when you're doing new DataTable, but quite important when taking a DataTable as an argument or as a result of a method call. – Herbivore

Answer 4

Even if an object has no unmanaged resources, disposing might help GC by breaking object graphs. In general, if an object implements IDisposable, Dispose() should be called.

Whether Dispose() actually does something or not depends on the given class. In case of DataSet, Dispose() implementation is inherited from MarshalByValueComponent. It removes itself from container and calls Disposed event. The source code is below (disassembled with .NET Reflector):

protected virtual void Dispose(bool disposing)
{
    if (disposing)
    {
        lock (this)
        {
            if ((this.site != null) && (this.site.Container != null))
            {
                this.site.Container.Remove(this);
            }
            if (this.events != null)
            {
                EventHandler handler = (EventHandler) this.events[EventDisposed];
                if (handler != null)
                {
                    handler(this, EventArgs.Empty);
                }
            }
        }
    }
}

Carley

Indeed. I saw some code quite recently where lots of DataTables were created in a very large loop without being Disposed. This lead to all of the memory being consumed on the computer and the process crashing as it ran out of memory. After I told the developer to call dispose on the DataTable the problem went away. – Network

Answer 5

Do you create the DataTables yourself? Because iterating through the children of any Object (as in DataSet.Tables) is usually not needed, as it's the job of the Parent to dispose all its child members.

Generally, the rule is: If you created it and it implements IDisposable, Dispose it. If you did NOT create it, then do NOT dispose it, that's the job of the parent object. But each object may have special rules, check the Documentation.

For .NET 3.5, it explicitly says "Dispose it when not using anymore", so that's what I would do.

Griffin

From what I understand, the general consensus is that an object should dispose its own unmanaged resources. However, a collection of IDisposable objects will not in general iterate through its elements to dispose each one, because there might be other references to its elements outside of the collection: #497222 – Aircool

True, Collections are always something I consider special because they are usually not "doing" anything, they are merely... Containers, so I never bothered about that. – Griffin

Answer 6

I call dispose anytime an object implements IDisposeable. It's there for a reason.

DataSets can be huge memory hogs. The sooner they can be marked for clean up, the better.

update

It's been 5 years since I answered this question. I still agree with my answer. If there is a dispose method, it should be called when you are done with the object. The IDispose interface was implemented for a reason.

Dinodinoflagellate

Calling dispose doesn't speed up the reclaiming of memory, to do that you would have to manually start the garbage collector which is generally a bad plan. – Mcfarland

If Dispose sets a bunch of references to null, it can cause objects to be candidates for collection that might otherwise be skipped. – Marcellus

The point of the Dispose is not to clear up the memory of managed objects - that is the garbage collector's job. The point is to clear up unmanaged objects. There seems to be evidence that DataSets do not have any unmanaged references so theoretically don't need to have disposed called them. That being said, I've never been in a situation where I've had to go out of my way to call Dispose - I would just call it anyway. – Mcdaniels

The primary use of IDisposable is to release unmanaged resources. Often times it also modifies state in a manner which makes sense for a disposed instance. (i.e. properties set to false, references set to null, etc.) – Marcellus

If there is a dispose method on an object it was put there for a reason regardless if it's for cleaning up unmanaged objects or not. – Dinodinoflagellate

Hi all. Now, we can check the source code at "referencesource.microsoft.com/#System.Data/fx/src/data/System/…" "DataSet" doesn't override the "Dispose" method, it comes form "MarshalByValueComponent". – Premonition

Answer 7

If your intention or the context of this question is really garbage collection, then you can set the datasets and datatables to null explicitly or use the keyword using and let them go out of scope. Dispose does not do much as Tetraneutron said it earlier. GC will collect dataset objects that are no longer referenced and also those that are out of scope.

I really wish SO forced people down voting to actually write a comment before downvoting the answer.

Ionogen

+ 1 I guess some folks don't want to allow others to consider different points of view. – Slipshod

down voting, in no way disallows people from considering different points of view. – Marcellus

Answer 8

Datasets implement IDisposable thorough MarshalByValueComponent, which implements IDisposable. Since datasets are managed there is no real benefit to calling dispose.

Mcfarland

It may now, who knows what it will do later. – Marcellus

This attitude in which you speculate that any code will not in the future do what it is supposed to do is a pain in the assumption for all involved. – Umber

Answer 9

Try to use Clear() function. It works great for me for disposing.

DataTable dt = GetDataSchema();
//populate dt, do whatever...
dt.Clear();

Octillion

Answer 10

No need to Dispose() because DataSet inherit MarshalByValueComponent class and MarshalByValueComponent implement IDisposable Interface

Slue

Answer 11

This is the right way to properly Dispose the DataTable.

private DataTable CreateSchema_Table()
{
    DataTable td = null;
    try
    {
        td = new DataTable();
        //use table DataTable here
        
        return td.Copy();
    }
    catch {  }
    finally
    {
        if (td != null)
        {
            td.Constraints.Clear();
            td.Clear();
            td.Dispose();
            td = null;
        }
    }
}

Caravaggio

Answer 12

And this can be the best/proper way to Dispose and release the memory consumed by DataSet.

try
    {
        DataSet ds = new DataSet("DS");
        //use table DataTable here
        
    }
    catch {  }
    finally
    {
        if (ds != null)
                {
                    ds.EnforceConstraints = false;
                    ds.Relations.Clear();
                    int totalCount = ds.Tables.Count;

                    for (int i = totalCount - 1; i >= 0; i--)
                    {
                        DataTable td1 = ds.Tables[i];
                        if (td1 != null)
                        {
                            td1.Constraints.Clear();
                            td1.Clear();
                            td1.Dispose();
                            td1 = null;
                        }
                    }

                    ds.Tables.Clear();
                    ds.Dispose();
                    ds = null;
                }
    }

Caravaggio