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garbage collection runs, the memory for this object cannot be compacted (moved) . This is typically useful when you want to hand the address of the memory out to unmanaged code . The unmanaged code can write to this memory in the managed heap knowing that the location of the managed object will not be moved due to a garbage collection . When you call GCHandle s static Alloc method, it scans the AppDomain s GC handle table, looking for an available entry where the address of the object you passed to Alloc is stored, and a flag is set to whatever you passed for the GCHandleType argument . Then, Alloc returns a GCHandle instance back to you . A GCHandle is a lightweight value type that contains a single instance field, an IntPtr, which refers to the index of the entry in the table . When you want to free this entry in the GC handle table, you take the GCHandle instance and call the Free method (which also invalidates the instance by setting the IntPtr field to zero) . Here s how the garbage collector uses the GC handle table . When a garbage collection occurs: 1. The garbage collector marks all of the reachable objects (as described at the beginning of this chapter) . Then, the garbage collector scans the GC handle table; all Normal or Pinned objects are considered roots, and these objects are marked as well (including any objects that these objects refer to via their fields) . 2. The garbage collector scans the GC handle table looking for all of the Weak entries . If a Weak entry refers to an object that isn t marked, the pointer identifies an unreachable object (garbage), and the entry has its pointer value changed to null . 3. The garbage collector scans the finalization list . If a pointer in the list refers to an unmarked object, the pointer identifies an unreachable object, and the pointer is moved from the finalization list to the freachable queue . At this point, the object is marked because the object is now considered reachable . 4. The garbage collector scans the GC handle table looking for all of the WeakTrackResurrection entries . If a WeakTrackResurrection entry refers to an object that isn t marked (which now is an object pointed to by an entry in the freachable queue), the pointer identifies an unreachable object (garbage), and the entry has its pointer value changed to null . 5. The garbage collector compacts the memory, squeezing out the holes left by the unreachable objects . Note that the garbage collector sometimes decides not to compact memory if it determines that the amount of fragmentation isn t worth the time to compact . Pinned objects are not compacted (moved); the garbage collector will move other objects around them . Now that you have an understanding of the mechanism, let s take a look at when you d use them . The easiest flags to understand are the Normal and Pinned flags, so let s start with these two . Both of these flags are typically used when interoperating with unmanaged code .
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The Normal flag is used when you need to hand a reference to a managed object to unmanaged code because, at some point in the future, the unmanaged code is going to call back into managed code, passing it the reference . You can t actually pass a pointer to a managed object out to unmanaged code because if a garbage collection occurs, the object could move in memory, invalidating the pointer . So to work around this, you would call GCHandle s Alloc method, passing in a reference to the object and the Normal flag . Then you d cast the returned GCHandle instance to an IntPtr and pass the IntPtr into the unmanaged code . When the unmanaged code calls back into managed code, the managed code would cast the passed IntPtr back to a GCHandle and then query the Target property to get the reference (or current address) of the managed object . When the unmanaged code no longer needs the reference, you d call GCHandle s Free method, which will allow a future garbage collection to free the object (assuming no other root exists to this object) . Notice that in this scenario, the unmanaged code is not actually using the managed object itself; the unmanaged code wants a way just to reference the object . In some scenarios, the unmanaged code needs to actually use the managed object . In these scenarios, the managed object must be pinned . Pinning prevents the garbage collector from moving/compacting the object . A common example is when you want to pass a managed String object to a Win32 function . In this case, the String object must be pinned because you can t pass the reference of a managed object to unmanaged code and then have the garbage collector move the object in memory . If the String object were moved, the unmanaged code would either be reading or writing to memory that no longer contained the String object s characters this will surely cause the application to run unpredictably . When you use the CLR s P/Invoke mechanism to call a method, the CLR pins the arguments for you automatically and unpins them when the unmanaged method returns . So, in most cases, you never have to use the GCHandle type to explicitly pin any managed objects yourself . You do have to use the GCHandle type explicitly when you need to pass the address of a managed object to unmanaged code and then, the unmanaged function returns, but unmanaged code might still need to use the object later . The most common example of this is when performing asynchronous I/O operations . Let s say that you allocate a byte array that should be filled as data comes in from a socket . Then, you would call GCHandle s Alloc method, passing in a reference to the array object and the Pinned flag . Then, using the returned GCHandle instance, you call the AddrOfPinnedObject method . This returns an IntPtr that is the actual address of the pinned object in the managed heap; you d then pass this address into the unmanaged function, which will return back to managed code immediately . While the data is coming from the socket, this byte array buffer should not move in memory; preventing this buffer from moving is accomplished by using the Pinned flag . When the asynchronous I/O operation has completed, you d call GCHandle s Free method, which will allow a future garbage collection to move the buffer . Your managed code should still have a reference to the buffer so that you can access the data, and this reference will prevent a garbage collection from freeing the buffer from memory completely .
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