I’m trying to understand different types of GC handles and their purposes. Here’s what I know so far:
Weak handles - These don’t stop objects from being collected by garbage collector. WeakReference uses this type.
Normal handles - These prevent objects from being garbage collected. Regular strong references work this way.
RefCounted handles - The runtime uses these internally, especially with COM objects.
Pinned handles - This is where I need help. What’s the main reason we need pinned GC handles? Is it only to stop objects from moving around in memory, or are there other important reasons?
I have a specific case where I’m working with a DataTable array that’s connected to a WPF grid control. When I close the form, I can see in memory analysis tools that there’s a pinned GC handle pointing to this array even after I set the grid’s data source to null. I’m not using any unsafe code blocks. Why would this array get pinned in memory and what component is responsible for pinning it?
wpf controls pin handles automatically when binding to data sources, even if u don’t use unsafe code. your grid might be pinning the datatable for optimizing performance. try using GC.Collect() and GC.WaitForPendingFinalizers() after setting the datasource to null. sometimes pinned handles linger longer than expected.
Yeah, pinned GC handles stop objects from moving during garbage collection. This matters when you’re working with unmanaged code that expects memory addresses to stay put. If you pass a managed object’s address to unmanaged code, that code assumes the memory won’t move until it’s done. For your DataTable issue - WPF’s data binding creates pinned handles internally for performance. The binding system needs direct data access for fast UI updates and change notifications. Even after you set DataSource to null, the binding engine might still hold references through its internal caching. That pinned handle you’re seeing is probably from WPF’s ItemsControl or DataGrid, not your code directly. This is normal behavior with complex WPF controls that do their own data management for virtualization and performance.
Pinned handles do more than just stop objects from moving - they’re critical when managed memory needs to talk to unmanaged code. The CLR uses them to guarantee memory won’t shift around when passing data to native libraries or COM components.
For your DataTable issue, WPF’s data virtualization is the culprit. The DataGrid creates pinned handles to keep direct access to row data for smooth scrolling and rendering. Even after you null the DataSource, WPF’s virtualization container hangs onto these handles temporarily for performance reasons. It’s probably the VirtualizingStackPanel or similar virtualization logic doing this.
This is intentional behavior - WPF assumes you might rebind similar data soon. The handles will get released eventually during garbage collection, but that happens on WPF’s schedule, not when you explicitly set things to null.