Memory leaks in .NET [closed]

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Closed 7 years ago.

What are all the possible ways in which we can get memory leaks in .NET?

I know of two:

1. Not properly un-registering Event Handlers/Delegates.
2. Not disposing dynamic child controls in Windows Forms:

Example:

// Causes Leaks  
Label label = new Label();  
this.Controls.Add(label);  
this.Controls.Remove(label);  

// Correct Code  
Label label = new Label();  
this.Controls.Add(label);  
this.Controls.Remove(label);  
label.Dispose();

Update: The idea is to list common pitfalls which are not too obvious (such as the above). Usually the notion is that memory leaks are not a big problem because of the garbage collector. Not like it used to be in C++.

---

Great discussion guys, but let me clarify... by definition, if there is no reference left to an object in .NET, it will be Garbage Collected at some time. So that is not a way to induce memory leaks.

In the managed environment, I would consider it a memory leak if you had an unintended reference to any object that you aren't aware of (hence the two examples in my question).

So, what are the various possible ways in which such a memory leak can happen?

Answer 1

There's no way to provide a comprehensive list... this is very much like asking "How can you get wet?"

That said, make sure you're calling Dispose() on everything that implements IDisposable, and make sure you implement IDisposable on any types that consume unmanaged resources of any kind.

Every now and then, run something like FxCop on your codebase to help you enforce that rule - you'd be surprised how deep some disposable objects get buried within an application framework.

Answer 2

I have 4 additional items to add to this discussion:

1. Terminating threads (Thread.Abort()) that have created UI Controls without properly preparing for such an event may lead to memory being used expectantly.

2. Accessing unmanaged resources through Pinvoke and not cleaning them up may lead to memory leaks.

3. Modifying large string objects. Not necessarily a memory leak, once out of scope, GC will take care of it, however, performance wise, your system may take a hit if large strings are modified often because you can not really depend on GC to ensure your program's foot print is minimal.

4. Creating GDI objects often to perform custom drawing. If performing GDI work often, reuse a single gdi object.

Answer 3

One thing that was really unexpected for me is this:

Region oldClip = graphics.Clip;
using (Region newClip = new Region(...))
{
    graphics.Clip = newClip;
    // draw something
    graphics.Clip = oldClip;
}

Where's the memory leak? Right, you should have disposed oldClip, too! Because Graphics.Clip is one of the rare properties that returns a new disposable object every time the getter is invoked.

Answer 4

3. Keeping around references to objects that you no longer need.

Re other comments - one way to ensure Dispose gets called is to use using... when code structure allows it.

Answer 5

That doesn't really cause leaks, it just makes more work for the GC:

// slows GC
Label label = new Label();  
this.Controls.Add(label);  
this.Controls.Remove(label);  

// better  
Label label = new Label();  
this.Controls.Add(label);  
this.Controls.Remove(label);  
label.Dispose();

// best
using( Label label = new Label() )
{ 
    this.Controls.Add(label);  
    this.Controls.Remove(label);  
}

Leaving disposable components lying around like this is never much of a problem in a managed environment like .Net - that's a big part of what managed means.

You'll slow you app down, certainly. But you won't leave a mess for anything else.

Answer 6

Setting the GridControl.DataSource property directly without using an instance of the BindingSource class (http://msdn.microsoft.com/en-us/library/system.windows.forms.bindingsource.aspx).

This caused leaks in my application that took me quite a while to track down with a profiler, eventually I found this bug report that Microsoft responded to: http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=92260

It's funny that in the documentation for the BindingSource class Microsoft try to pass it off as a legitmate well thought out class, but I think they just created it to solve a fundamental leak regarding currency managers and binding data to grid controls.

Watch out for this one, I bet there are absolutely loads of leaky applications out there because of this!