Develop Reference

use of forcefully calling Garbage collection method [duplicate]

The general advice is that you should not call GC.Collect from your code, but what are the exceptions to this rule?
I can only think of a few very specific cases where it may make sense to force a garbage collection.
One example that springs to mind is a service, that wakes up at intervals, performs some task, and then sleeps for a long time. In this case, it may be a good idea to force a collect to prevent the soon-to-be-idle process from holding on to more memory than needed.
Are there any other cases where it is acceptable to call GC.Collect?

If you have good reason to believe that a significant set of objects - particularly those you suspect to be in generations 1 and 2 - are now eligible for garbage collection, and that now would be an appropriate time to collect in terms of the small performance hit.
A good example of this is if you've just closed a large form. You know that all the UI controls can now be garbage collected, and a very short pause as the form is closed probably won't be noticeable to the user.
UPDATE 2.7.2018
As of .NET 4.5 - there is GCLatencyMode.LowLatency and GCLatencyMode.SustainedLowLatency. When entering and leaving either of these modes, it is recommended that you force a full GC with GC.Collect(2, GCCollectionMode.Forced).
As of .NET 4.6 - there is the GC.TryStartNoGCRegion method (used to set the read-only value GCLatencyMode.NoGCRegion). This can itself, perform a full blocking garbage collection in an attempt to free enough memory, but given we are disallowing GC for a period, I would argue it is also a good idea to perform full GC before and after.
Source: Microsoft engineer Ben Watson's: Writing High-Performance .NET Code, 2nd Ed. 2018.
See:
https://msdn.microsoft.com/en-us/library/system.runtime.gclatencymode(v=vs.110).aspx
https://msdn.microsoft.com/en-us/library/dn906204(v=vs.110).aspx

I use GC.Collect only when writing crude performance/profiler test rigs; i.e. I have two (or more) blocks of code to test - something like:
GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
TestA(); // may allocate lots of transient objects
GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
TestB(); // may allocate lots of transient objects
GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
...
So that TestA() and TestB() run with as similar state as possible - i.e. TestB() doesn't get hammered just because TestA left it very close to the tipping point.
A classic example would be a simple console exe (a Main method sort-enough to be posted here for example), that shows the difference between looped string concatenation and StringBuilder.
If I need something precise, then this would be two completely independent tests - but often this is enough if we just want to minimize (or normalize) the GC during the tests to get a rough feel for the behaviour.
During production code? I have yet to use it ;-p

The best practise is to not force a garbage collection in most cases. (Every system I have worked on that had forced garbage collections, had underlining problems that if solved would have removed the need to forced the garbage collection, and speeded the system up greatly.)
There are a few cases when you know more about memory usage then the garbage collector does. This is unlikely to be true in a multi user application, or a service that is responding to more then one request at a time.
However in some batch type processing you do know more then the GC. E.g. consider an application that.
Is given a list of file names on the command line
Processes a single file then write the result out to a results file.
While processing the file, creates a lot of interlinked objects that can not be collected until the processing of the file have complete (e.g. a parse tree)
Does not keep match state between the files it has processed.
You may be able to make a case (after careful) testing that you should force a full garbage collection after you have process each file.
Another cases is a service that wakes up every few minutes to process some items, and does not keep any state while it’s asleep. Then forcing a full collection just before going to sleep may be worthwhile.
The only time I would consider forcing
a collection is when I know that a lot
of object had been created recently
and very few objects are currently
referenced.
I would rather have a garbage collection API when I could give it hints about this type of thing without having to force a GC my self.
See also "Rico Mariani's Performance Tidbits"
These days I consider same of the above cases would be better to use a short lived worker process to do each batch of work and let the OS do the resource recovery.

One case is when you are trying to unit test code that uses WeakReference.

In large 24/7 or 24/6 systems -- systems that react to messages, RPC requests or that poll a database or process continuously -- it is useful to have a way to identify memory leaks. For this, I tend to add a mechanism to the application to temporarily suspend any processing and then perform full garbage collection. This puts the system into a quiescent state where the memory remaining is either legitimately long lived memory (caches, configuration, &c.) or else is 'leaked' (objects that are not expected or desired to be rooted but actually are).
Having this mechanism makes it a lot easier to profile memory usage as the reports will not be clouded with noise from active processing.
To be sure you get all of the garbage, you need to perform two collections:
GC.Collect();
GC.WaitForPendingFinalizers();
GC.Collect();
As the first collection will cause any objects with finalizers to be finalized (but not actually garbage collect these objects). The second GC will garbage collect these finalized objects.

You can call GC.Collect() when you know something about the nature of the app the garbage collector doesn't.
As the author, it's often tempting to think this is likely or normal. However, the truth is the GC amounts to a pretty well-written and tested expert system, and it's rare you'll know something about the low level code paths it doesn't.
The best example I can think of where you might have some extra information is an app that cycles between idle periods and very busy periods. You want the best performance possible for the busy periods and therefore want to use the idle time to do some clean up.
However, most of the time the GC is smart enough to do this anyway.

One instance where it is almost necessary to call GC.Collect() is when automating Microsoft Office through Interop. COM objects for Office don't like to automatically release and can result in the instances of the Office product taking up very large amounts of memory. I'm not sure if this is an issue or by design. There's lots of posts about this topic around the internet so I won't go into too much detail.
When programming using Interop, every single COM object should be manually released, usually though the use of Marshal.ReleseComObject(). In addition, calling Garbage Collection manually can help "clean up" a bit. Calling the following code when you're done with Interop objects seems to help quite a bit:
GC.Collect()
GC.WaitForPendingFinalizers()
GC.Collect()
In my personal experience, using a combination of ReleaseComObject and manually calling garbage collection greatly reduces the memory usage of Office products, specifically Excel.

As a memory fragmentation solution.
I was getting out of memory exceptions while writing a lot of data into a memory stream (reading from a network stream). The data was written in 8K chunks. After reaching 128M there was exception even though there was a lot of memory available (but it was fragmented). Calling GC.Collect() solved the issue. I was able to handle over 1G after the fix.

Have a look at this article by Rico Mariani. He gives two rules when to call GC.Collect (rule 1 is: "Don't"):
When to call GC.Collect()

I was doing some performance testing on array and list:
private static int count = 100000000;
private static List<int> GetSomeNumbers_List_int()
{
var lstNumbers = new List<int>();
for(var i = 1; i <= count; i++)
{
lstNumbers.Add(i);
}
return lstNumbers;
}
private static int[] GetSomeNumbers_Array()
{
var lstNumbers = new int[count];
for (var i = 1; i <= count; i++)
{
lstNumbers[i-1] = i + 1;
}
return lstNumbers;
}
private static int[] GetSomeNumbers_Enumerable_Range()
{
return Enumerable.Range(1, count).ToArray();
}
static void performance_100_Million()
{
var sw = new Stopwatch();
sw.Start();
var numbers1 = GetSomeNumbers_List_int();
sw.Stop();
//numbers1 = null;
//GC.Collect();
Console.WriteLine(String.Format("\"List<int>\" took {0} milliseconds", sw.ElapsedMilliseconds));
sw.Reset();
sw.Start();
var numbers2 = GetSomeNumbers_Array();
sw.Stop();
//numbers2 = null;
//GC.Collect();
Console.WriteLine(String.Format("\"int[]\" took {0} milliseconds", sw.ElapsedMilliseconds));
sw.Reset();
sw.Start();
//getting System.OutOfMemoryException in GetSomeNumbers_Enumerable_Range method
var numbers3 = GetSomeNumbers_Enumerable_Range();
sw.Stop();
//numbers3 = null;
//GC.Collect();
Console.WriteLine(String.Format("\"int[]\" Enumerable.Range took {0} milliseconds", sw.ElapsedMilliseconds));
}
and I got OutOfMemoryException in GetSomeNumbers_Enumerable_Range method the only workaround is to deallocate the memory by:
numbers = null;
GC.Collect();

You should try to avoid using GC.Collect() since its very expensive. Here is an example:
public void ClearFrame(ulong timeStamp)
{
if (RecordSet.Count <= 0) return;
if (Limit == false)
{
var seconds = (timeStamp - RecordSet[0].TimeStamp)/1000;
if (seconds <= _preFramesTime) return;
Limit = true;
do
{
RecordSet.Remove(RecordSet[0]);
} while (((timeStamp - RecordSet[0].TimeStamp) / 1000) > _preFramesTime);
}
else
{
RecordSet.Remove(RecordSet[0]);
}
GC.Collect(); // AVOID
}
TEST RESULT: CPU USAGE 12%
When you change to this:
public void ClearFrame(ulong timeStamp)
{
if (RecordSet.Count <= 0) return;
if (Limit == false)
{
var seconds = (timeStamp - RecordSet[0].TimeStamp)/1000;
if (seconds <= _preFramesTime) return;
Limit = true;
do
{
RecordSet[0].Dispose(); // Bitmap destroyed!
RecordSet.Remove(RecordSet[0]);
} while (((timeStamp - RecordSet[0].TimeStamp) / 1000) > _preFramesTime);
}
else
{
RecordSet[0].Dispose(); // Bitmap destroyed!
RecordSet.Remove(RecordSet[0]);
}
//GC.Collect();
}
TEST RESULT: CPU USAGE 2-3%

In your example, I think that calling GC.Collect isn't the issue, but rather there is a design issue.
If you are going to wake up at intervals, (set times) then your program should be crafted for a single execution (perform the task once) and then terminate. Then, you set the program up as a scheduled task to run at the scheduled intervals.
This way, you don't have to concern yourself with calling GC.Collect, (which you should rarely if ever, have to do).
That being said, Rico Mariani has a great blog post on this subject, which can be found here:
http://blogs.msdn.com/ricom/archive/2004/11/29/271829.aspx

One useful place to call GC.Collect() is in a unit test when you want to verify that you are not creating a memory leak (e. g. if you are doing something with WeakReferences or ConditionalWeakTable, dynamically generated code, etc).
For example, I have a few tests like:
WeakReference w = CodeThatShouldNotMemoryLeak();
Assert.IsTrue(w.IsAlive);
GC.Collect();
GC.WaitForPendingFinalizers();
Assert.IsFalse(w.IsAlive);
It could be argued that using WeakReferences is a problem in and of itself, but it seems that if you are creating a system that relies on such behavior then calling GC.Collect() is a good way to verify such code.

There are some situations where it is better safe than sorry.
Here is one situation.
It is possible to author an unmanaged DLL in C# using IL rewrites (because there are situations where this is necessary).
Now suppose, for example, the DLL creates an array of bytes at the class level - because many of the exported functions need access to such. What happens when the DLL is unloaded? Is the garbage collector automatically called at that point? I don't know, but being an unmanaged DLL it is entirely possible the GC isn't called. And it would be a big problem if it wasn't called. When the DLL is unloaded so too would be the garbage collector - so who is going to be responsible for collecting any possible garbage and how would they do it? Better to employ C#'s garbage collector. Have a cleanup function (available to the DLL client) where the class level variables are set to null and the garbage collector called.
Better safe than sorry.

The short answer is: never!

using(var stream = new MemoryStream())
{
bitmap.Save(stream, ImageFormat.Png);
techObject.Last().Image = Image.FromStream(stream);
bitmap.Dispose();
// Without this code, I had an OutOfMemory exception.
GC.Collect();
GC.WaitForPendingFinalizers();
//
}

Another reason is when you have a SerialPort opened on a USB COM port, and then the USB device is unplugged. Because the SerialPort was opened, the resource holds a reference to the previously connected port in the system's registry. The system's registry will then contain stale data, so the list of available ports will be wrong. Therefore the port must be closed.
Calling SerialPort.Close() on the port calls Dispose() on the object, but it remains in memory until garbage collection actually runs, causing the registry to remain stale until the garbage collector decides to release the resource.
From https://stackoverflow.com/a/58810699/8685342:
try
{
if (port != null)
port.Close(); //this will throw an exception if the port was unplugged
}
catch (Exception ex) //of type 'System.IO.IOException'
{
System.GC.Collect();
System.GC.WaitForPendingFinalizers();
}
port = null;

If you are creating a lot of new System.Drawing.Bitmap objects, the Garbage Collector doesn't clear them. Eventually GDI+ will think you are running out of memory and will throw a "The parameter is not valid" exception. Calling GC.Collect() every so often (not too often!) seems to resolve this issue.

i am still pretty unsure about this.
I am working since 7 years on an Application Server. Our bigger installations take use of 24 GB Ram. Its hightly Multithreaded, and ALL calls for GC.Collect() ran into really terrible performance issues.
Many third party Components used GC.Collect() when they thought it was clever to do this right now.
So a simple bunch of Excel-Reports blocked the App Server for all threads several times a minute.
We had to refactor all the 3rd Party Components in order to remove the GC.Collect() calls, and all worked fine after doing this.
But i am running Servers on Win32 as well, and here i started to take heavy use of GC.Collect() after getting a OutOfMemoryException.
But i am also pretty unsure about this, because i often noticed, when i get a OOM on 32 Bit, and i retry to run the same Operation again, without calling GC.Collect(), it just worked fine.
One thing i wonder is the OOM Exception itself...
If i would have written the .Net Framework, and i can't alloc a memory block, i would use GC.Collect(), defrag memory (??), try again, and if i still cant find a free memory block, then i would throw the OOM-Exception.
Or at least make this behavior as configurable option, due the drawbacks of the performance issue with GC.Collect.
Now i have lots of code like this in my app to "solve" the problem:
public static TResult ExecuteOOMAware<T1, T2, TResult>(Func<T1,T2 ,TResult> func, T1 a1, T2 a2)
{
int oomCounter = 0;
int maxOOMRetries = 10;
do
{
try
{
return func(a1, a2);
}
catch (OutOfMemoryException)
{
oomCounter++;
if (maxOOMRetries > 10)
{
throw;
}
else
{
Log.Info("OutOfMemory-Exception caught, Trying to fix. Counter: " + oomCounter.ToString());
System.Threading.Thread.Sleep(TimeSpan.FromSeconds(oomCounter * 10));
GC.Collect();
}
}
} while (oomCounter < maxOOMRetries);
// never gets hitted.
return default(TResult);
}
(Note that the Thread.Sleep() behavior is a really App apecific behavior, because we are running a ORM Caching Service, and the service takes some time to release all the cached objects, if RAM exceeds some predefined values. so it waits a few seconds the first time, and has increased waiting time each occurence of OOM.)

one good reason for calling GC is on small ARM computers with little memory, like the Raspberry PI (running with mono).
If unallocated memory fragments use too much of the system RAM, then the Linux OS can get unstable.
I have an application where I have to call GC every second (!) to get rid of memory overflow problems.
Another good solution is to dispose objects when they are no longer needed. Unfortunately this is not so easy in many cases.

This isn't that relevant to the question, but for XSLT transforms in .NET (XSLCompiledTranform) then you might have no choice. Another candidate is the MSHTML control.

If you are using a version of .net less than 4.5, manual collection may be inevitable (especially if you are dealing with many 'large objects').
this link describes why:
https://blogs.msdn.microsoft.com/dotnet/2011/10/03/large-object-heap-improvements-in-net-4-5/

Since there are Small object heap(SOH) and Large object heap(LOH)
We can call GC.Collect() to clear de-reference object in SOP, and move lived object to next generation.
In .net4.5, we can also compact LOH by using largeobjectheapcompactionmode

Memory Leak in C# WPF

I need to reduce the memory leak in c# WPF after disposing all the used objects. But I couldn't reduce the memory consumption completely by using the following code snippet.
Here is my code:
string str;
Uri uri;
private void Button_Click(object sender, RoutedEventArgs e) // "Load" Button
{
if(img.Source!=null)
Unload();
str = "F://Photos//Parthi//IMG_20141128_172826244.jpg"; // File Size: 0.643 MB
uri = new Uri(str);
img.Source = new BitmapImage(uri);
}
private void Button_Click_1(object sender, RoutedEventArgs e) //"Unload Button"
{
Unload();
}
private void Unload()
{
Bitmap bmp = GetBitmap(img.Source as BitmapSource);
bmp.Dispose();
bmp = null;
img.Source = null;
str = string.Empty;
uri = null;
}
private Bitmap GetBitmap(BitmapSource source)
{
Bitmap bmp = new Bitmap(source.PixelWidth, source.PixelHeight, System.Drawing.Imaging.PixelFormat.Format32bppPArgb);
BitmapData data = bmp.LockBits(new System.Drawing.Rectangle(System.Drawing.Point.Empty, bmp.Size), ImageLockMode.WriteOnly, System.Drawing.Imaging.PixelFormat.Format32bppPArgb);
source.CopyPixels(Int32Rect.Empty, data.Scan0, data.Height * data.Stride, data.Stride);
bmp.UnlockBits(data);
data = null;
source = null;
return bmp;
}
After Running the sample, While checking it in Task manager, the following memory consumption reading has produced,
Before "Load" Button is clicked : 10.0 MB
After "Load" Button is clicked : 47.8 MB
After "Unload" Button is clicked : 26.0 MB
After Unloading, I need to reduce the memory closely to 10.0 MB. So please help me regarding this.
Thanks in Advance.

Using the .net platform does not have control over the memory as if it works in c / c ++
The garbage collector operates very complex policies and the fact that you see down the memory to 26 but 10 is normal.
The .net reserves a space to recharge faster data and guarantee a free space in main memory without having to require continuous OS.
What I've noticed is perfectly normal.
It 'a question that in the past I personally subjected to a Microsoft MVP during a course
View this:
http://msdn.microsoft.com/en-us/library/ee787088%28v=vs.110%29.aspx
and this: Best Practice for Forcing Garbage Collection in C#

First of all, do not use Task Manager to view your memory usage, because it only shows the memory that the windows process allocates. There are plenty of much better tools out there and even the Performance Monitor, which comes on Windows, will give you a better idea of your application performance and any memory leaks. You may start it by running perfmon.exe.
In this sample application, GC does not become aggressive with collections until heap reaches approximately 85MB. And why would I want it to? It's not a lot of memory and it works very well as a caching solution, if I decide to use the same objects again.
So, I suggest taking a look at that tool. It gives a nice overview of what's going on and it's free.
Second of all, just because you called a .Dispose() to release those resources, does not mean that memory is released right away. You're basically making it eligible for the garbage collection and when GC gets to it, it'll take care of it.
The default garbage collection behavior for a WPF application is Concurrent(<4.0)/Background(4.0=<) Workstation GC. It runs on a special thread and most of the time tries to run concurrently to the rest of the application (I say most of the time, because once in a while it'll extremely briefly suspend other threads, so that it may complete its cleanup). It doesn't wait until you're out of memory, but at the same time, it does collection only when it doesn't affect the performance greatly -- sort of a balanced trade-off.
Another factor to consider is that you have a 0.643 MB jpeg file and once you count in BitmapImage, it's a bit more... well, anything above 85,000 bytes is considered to be a large object and thus, it is placed into generation 2, which contains the large object heap. Generation 2 garbage collection is expensive and is done infrequently. However, if you're running .NET 4.5.1, and I'm not sure if you are, you may force a compaction:
GCSettings.LargeObjectHeapCompactionMode = GCLargeObjectHeapCompactionMode.CompactOnce;
GC.Collection();
As with .Dipose(), this is not going happen right away, as it's an expensive process and it'll take a little longer than the generation 1 less-than-a-millisecond sweep. And honestly, you probably wouldn't even benefit from it, since I doubt that you would have a high fragmentation in the LOH with that application. I just mentioned it so that you're aware of that option if you ever need it.
So, why am I giving you a brief lesson on the default GC behavior of a WPF application (and most of .NET applications for that matter)? Well, it's important to understand the behavior of GC and acknowledge its existence. Unlike a C++ application, where you're granted a lot of control and freedom over your memory allocation, a .NET application utilizes a GC. The trade-off is that the memory is freed when it's freed by the GC. There may even be times when it frees it prematurely, from your point of view, and that's when you would explicitly keep an object alive by calling GC.KeepAlive(..). A lot of embedded systems do not make use of a GC and if you want very precise control over your memory, I suggest that you do not either.
If you want to be aware of how memory is being handled in a .NET application, I strongly recommend educating yourself on the inner workings of the garbage collector. What I've told you about is an incredibly brief snapshot of the default behavior and there is a lot more to it. There are a few modes, which provide different behaviors.

try to manage the usage of your variables. you can do something like this :
private void Button_Click(object sender, RoutedEventArgs e)
{
img.Source = new BitmapImage(new Uri(#"F://Photos//Parthi//IMG_20141128_172826244.jpg"));
}
if 'str' and 'uri' is not necessary needed by the whole class you can just add it directly it to your object. Or if you really need to store it and reuse your data that really has a huge amount of size you can just store it on Physical file on a temp file to prevent memory leakage; instead of storing it to memory you can store it on physical storage.
hope it helps

Your Unload method calls GetBitmap, but GetBitmap returns a new object every time, so as far as I can tell, you're never actually going to dispose of img.Source properly

How to change a dynamic snippet to a static?

I am getting out of memory alert in this function, called 500K times:
public void DoStuff(string msg)
{
rtfTerminal.Invoke(new EventHandler(delegate
{
rtfTerminal.SelectedText = string.Empty;
rtfTerminal.AppendText(msg);
rtfTerminal.ScrollToCaret();
}));
}
In the global scope I declared:
private System.Windows.Forms.RichTextBox rtfTerminal;
I guess I get the out of memory since I do new EventHandler each
call, How can I avoid that?
Is it reasonable that I get the out of memory due to
rtfTerminal.AppendText(msg);?

The new EventHandler is not a massive problem, nor is the anonymous object that you can't really see (but which exists): both of these will be short-lived, i.e. GEN-0 and will virtually free to collect. Additionally, GC will kick in if things start to run low, so that shouldn't be a problem. Continually adding text to a RichTextBox without limit could be a problem. I would try to avoid that. Files are usually a better option for dumping output from long-running processes.
Ultimately, if that doesn't help: you need to use something like a memory profiler to see where the memory is going.

The out of memory exception may show against the line with the "invoke" call but is almost certainly coming from the add to the richTextBox.
Tick "thrown" against debug->exceptions->CLR Exceptions to see this while debugging.
100 unicode chars every ms should eat up about 686MB an hour, check Available MB in task manager to see if this is the culprit.

C# not releasing memory after task complete

The following code is a simplified example of an issue I am seeing. This application consumes approx 4GB of memory before throwing an exception as the dictionary is too big.
class Program
{
static void Main(string[] args)
{
Program program = new Program();
while(true)
{
program.Method();
Console.ReadLine();
}
}
public void Method()
{
WasteOfMemory memory = new WasteOfMemory();
Task tast = new Task(memory.WasteMemory);
tast.Start();
}
}
public class WasteOfMemory
{
public void WasteMemory()
{
Dictionary<string, string> aMassiveList = new Dictionary<string, string>();
try
{
long i = 0;
while (true)
{
aMassiveList.Add(i.ToString(), "I am a line of text designed to waste space.... I am exceptionally useful........");
i++;
}
}
catch(Exception e)
{
Console.WriteLine("I have broken myself");
}
}
}
This is all as expected, although what we cannot currently work out is when this memory should be released from the CLR.
We have let the task complete and then simulated a memory overload situation, but the memory consumed by the dictionary is not released. As the OS is running out of memory, is it not putting pressure on the CLR to release the memory?
However and even more confusing, if we wait until the task has completed, then hit enter to run the task again the memory is released, so obviously the previous dictionary has been garbage collected (hasn't it?).
So, why is the memory not being released? And how can we get the CLR to release the memory?
Any explanations or solutions would be greatly appreciated.
EDIT: Following replies, particularly Beska's, it is obvious my description of the issue is not the the clearest, so I will try to clarify.
The code may not be the best example, sorry! It was a quick crude piece of code to try to replicate the issue.
The dictionary is used here to replicate the fact we have a large custom data object, which fills a large chunk of our memory and it is not then released after the task has completed.
In the example, the dictionary fills up to the limit of the dictionary and then throws an exception, it does NOT keep filling forever! This is well before our memory is full, and it does not cause an OutOfMemoryException. Hence the result is a large object in memory, and then the task completes.
At this point we would expect the dictionary to be out of scope, as both the task and the method 'Method' have completed. Hence, we would expect the dictionary to be garbage collected and the memory reclaimed. In reality, the memory is not freed until 'Method' is called again, creating a new WasteOfMemory instance and starting a new task.
Hopefully that will clarify the issue a bit

The garbage collector only frees locations in memory that are no longer in use that are objects which have no pointer pointing to them.
(1) your program runs infinitely without termination and
(2) you never change the pointer to your dictionary, so the GC has certainly no reason to touch the dictionary.
So for me your program is doing exactly what it is supposed to do.

Okay, I've been following this...I think there are a couple issues, some of which people have touched on, but I think not answering the real question (which, admittedly, took me a while to recognize, and I'm not sure I'm answering what you want even now.)
This is all as expected, although what we cannot currently work out is when this memory should be released from the CLR.
As others have said, while the task is running, the dictionary will not be released. It's being used. It gets bigger until you run out of memory. I'm pretty sure you understand this.
We have let the task complete and then simulated a memory overload situation, but the memory consumed by the dictionary is not released. As the OS is running out of memory, is it not putting pressure on the CLR to release the memory?
Here, I think, is the real question.
If I understand you correctly, you're saying you set this up to fill up memory. And then, after it crashes (but before you hit return to start a new task) you're trying other things outside of this program, such as running other programs in Windows to try to get the GC to collect the memory, right? Hoping that the OS would talk to the GC, and start pressuring it to do it's thing.
However and even more confusing, if we wait until the task has completed, then hit enter to run the task again the memory is released, so obviously the previous dictionary has been garbage collected (hasn't it?).
I think you answered your own question...it has not been necessarily been released until you hit return to start a new task. The new task needs memory, so it goes to the GC, and the GC happily collects the memory from the previous task, which has now ended (after throwing from full memory).
So, why is the memory not being released? And how can we get the CLR to release the memory?
I don't know that you can force the GC to release memory. Generally speaking, it does it when it wants (though some hacker types might know some slick way to force its hand.) Of course, .NET decides when to run the GC, and since nothing is happening while the program is just sitting there, it may well be deciding that it doesn't need to. As to whether the OS can pressure the GC to run, it seems from your tests the answer is "no". A bit counter-intuitive perhaps.
Is that what you were trying to get at?

The memory is not being released because the scope aMassiveList is never finished. When a function returns, it releases all non-referenced resources created inside it.
In your case, aMassiveList never leaves context. If you want your function never to return you have to find a way to 'process' your info and release it instead of storing all of them forever.
If you create a function that increasingly allocates resources and never release it you will end up consuming all the memory.

GC will only release unreferenced objects, so as the dictionary is being referenced by your program it can't be released by the GC

The way you've written the WasteMemory method, it will never exit (unless the variable "i" overflows, which won't happen this year) and BECAUSE IT WILL NEVER EXIT it will keep IN USE the reference to the internal Dictionary.
Daniel White is right, you should read about how GC works.
If the references are in use, GC will not collect the referenced memory. Otherwise, how would any program work?
I don't see what you expect the CLR/GC to do here. There's nothing to garbage-collect inside one run of your WasteMemory method.
However and even more confusing, if we wait until the task has completed, then hit enter to run the task again the memory is released, so obviously the previous dictionary has been garbage collected (hasn't it?).
When you press Enter, a new task is created and started. It's not the same task, it's a new task - a new object holding a reference to a new WasteOfMemory instance.
The old task will keep running and the memory it uses will NOT be collected because the old task keeps running in background and it keeps USING that memory.
I'm not sure why - and most importantly HOW - you observe the memory of the old task being released.

Change your method to be a using statement
Example:
Using (WateOfMemory memory = new WateOfMemory())
{
Task tast = new Task(memory.WasteMemory);
tast.Start();
}
And add disposible WateOfMemoryClass (by the way your constructor is WasteOfMemory)
#region Dispose
private IntPtr handle;
private Component component = new Component();
private bool disposed = false;
public WateOfMemory()
{
}
public WateOfMemory(IntPtr handle)
{
this.handle = handle;
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
private void Dispose(bool disposing)
{
if(!this.disposed)
{
if(disposing)
{
component.Dispose();
}
CloseHandle(handle);
handle = IntPtr.Zero;
}
disposed = true;
}
[System.Runtime.InteropServices.DllImport("Kernel32")]
private extern static Boolean CloseHandle(IntPtr handle);
~WateOfMemory()
{
Dispose(false);
}
#endregion

working with unmanaged resources

I use a 3rd party library which is a wrapper around a native dll. the library contains a type XImage, XImage has some properties and a IntPtr Data() method. XImage also Implements IDisposable but I don't know if it is implemented correctly.
I get many XImages from TCP connection and show them as a movie in a PictureBox.
I used to convert 'XImage' to System.Drawing.Image and view them in a PictureBox but I got AccessViolationException.
So I made a wrapper around XImage called Frame.
public class Frame : IDisposable
{
public uint size { get; private set; }
private Image image;
public XImage XImage { get; set; }
public Image Image { get { return image ?? (image = GetBitmap(this.XImage)); } }
public DateTime Time { get; set; }
public Frame(XImage xImage)
{
this.XImage = xImage;
this.size = XImage.ImageBufferSize();
GC.AddMemoryPressure(size);
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
~Frame()
{
Dispose(false);
}
protected virtual void Dispose(bool disposing)
{
if (disposing)
{
try
{
image.Dispose();
}
catch { }
finally
{
image = null;
}
try
{
MImage.Dispose();
}
catch { }
finally { XImage = null; }
}
GC.RemoveMemoryPressure(size);
}
}
and by handling references to Frame I solved the AccessViolationException.
now I have another Issue, when I run the program from visual studio (F5 - Start Debugging) everything is okay, but when I run it from the .exe file or (ctrl + F5 - Start without debugging) the memory usage is growing larger and larger until I get OutOfMemoryException.(Biuld Configuration: Release - X86). what should I do ?
---- EDIT ----
I found that GC.AddMemoryPressure or GC.RemoveMemoryPressure just makes the garbage collection to run more often and, my problem is now that I have small objects that have a handle to large unmanaged memory, and GC is not collecting these small objects.
---- EDIT ----
calling GC.Collect will solve the problem during run-time, I set up a timer and call GC.Collect periodically, but it makes the application freeze for a short period, so I don't want to use this approach.

I have found that GC has limitations and may not work too well under very heavy pressure and for memory intense application. I have an application that does not do anything with unmanaged resources directly, all standard .NET components, and it can still choke on memory. It can use GBs of RAM but not because of enormous memory requirements but because big objects are created and destroyed relatively quickly and apparently not collected too often. Application has no memory leaks as it is all freed when collection is forced. It looks like GC is not always able to collect unused objects on time i.e. before OutOfMemoryException. It waits to find the best moment but before it makes up its mind it is too late. When I periodically force the collection, application runs with no problems.
It is worth mentioning that OutOfMemoryException does not always mean that you're actually out of free memory. It can also mean there is no big enough contiguous memory chunk available. This may be the case especially when working with video and images. GC might think there is still plenty of memory free but it is too fragmented for your application. I am sure GC takes fragmentation into account but it is possible that it's not always getting it right.
If you are sure the library is not the problem, my advise is to experiment more with memory pressure methods (AddMemoryPressure and RemoveMemoryPressure) to help GC do its thing on time. It may solve your problems since you work with an unmanaged library that may be handling significant amounts of memory behind GC's back. Alternatively do as you do with GC.Collect. Manual collection may not be ideal but I believe there are cases when it is justified. Of course do expect that manual collection of lots of objects may have an impact on your application performance.
EDIT
If manual collection introduces too much of a performance impact, try to use overloaded versions of GC.Collect that give you a bit more control.