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POGarbage Collection not happening even when needed
 

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    <p>I made a 64-bit WPF test app. With my app running and with Task Manager open, I watch my system memory usage. I see I'm using 2GB, and I have 6GB available.</p> <p>In my app, I click an Add button to add a new 1GB byte array to a list. I see my system memory usage increases by 1GB. I click Add a total of 6 times, filling the 6GB of memory I had available when I started.</p> <p>I click a Remove button 6 times to remove each array from the list. The removed byte arrays should not be referenced by any other object in my control.</p> <p>When I Remove, I don't see my memory go down. But that's OK with me, because I understand that GC is non-deterministic and all that. I figure the GC WILL collect as needed.</p> <p>So now with memory looking full, but expecting the GC to collect when needed, I Add again. My PC starts slipping in and out of a disk thrashing coma. Why didn't the GC collect? If that wasn't the time to do it, when is?</p> <p>As a sanity check, I have a button to force GC. When I push that, I quickly get 6GB back. Doesn't that prove my 6 arrays were not being referenced and COULD have been collected had the GC knew/wanted to?</p> <p>I've read a lot that says I shouldn't call GC.Collect() but if GC doesn't collect in this situation, what else can I do?</p> <pre><code> private ObservableCollection&lt;byte[]&gt; memoryChunks = new ObservableCollection&lt;byte[]&gt;(); public ObservableCollection&lt;byte[]&gt; MemoryChunks { get { return this.memoryChunks; } } private void AddButton_Click(object sender, RoutedEventArgs e) { // Create a 1 gig chunk of memory and add it to the collection. // It should not be garbage collected as long as it's in the collection. try { byte[] chunk = new byte[1024*1024*1024]; // Looks like I need to populate memory otherwise it doesn't show up in task manager for (int i = 0; i &lt; chunk.Length; i++) { chunk[i] = 100; } this.memoryChunks.Add(chunk); } catch (Exception ex) { MessageBox.Show(string.Format("Could not create another chunk: {0}{1}", Environment.NewLine, ex.ToString())); } } private void RemoveButton_Click(object sender, RoutedEventArgs e) { // By removing the chunk from the collection, // I except no object has a reference to it, // so it should be garbage collectable. if (memoryChunks.Count &gt; 0) { memoryChunks.RemoveAt(0); } } private void GCButton_Click(object sender, RoutedEventArgs e) { GC.Collect(); GC.WaitForPendingFinalizers(); } </code></pre>
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    1. COThe behavior of the Large Object Heap is well described in any backgrounder book on the CLR. Pay a visit to your library and check out anything by Richter. If allocating many one gigabyte arrays is something you do in your *real* code then you'll need more RAM. Any machine can be bogged down by artificial code.
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    2. COYou might also start with this link: http://msdn.microsoft.com/en-us/magazine/cc534993.aspx
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