How do I unload a DLL which has been loaded using DllImport in C#?
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If you intend to do so just to save few KBs of memory foot print of a loaded module then it isn't worth. CLR does so on its own whenever appDomain gets unloaded. I would be curious to know if you have some real reason to try unloading the native dll.– RBTCommented Feb 9, 2017 at 3:32
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5 Answers
The most reliable way to unload an unmanaged DLL from a process that got loaded by a [DllImport] pinvoke declaration is to load it yourself, again, by pinvoking LoadLibrary(). That gives you a reliable handle to the DLL and works correctly even if the module name of the DLL is ambiguous. It doesn't have any affect at runtime, other than the Windows loader increasing the internal reference count on the DLL from 1 to 2.
You can then pinvoke FreeLibrary() twice to decrease the reference count to 0, passing it the IntPtr you got from LoadLibrary(). That unloads the DLL, as well as any dependent DLLs that got loaded.
Beware that you'll get very nasty failure when you try to pinvoke any exported function on the DLL again, any time after doing this. The pinvoke marshaller is unaware that the DLL isn't around anymore and will call the function at the address it thinks is still valid. Which bombs your program with an AccessViolation exception if you are lucky. Or runs a completely random bit of code if you are not so lucky and the address space formerly occupied by the DLL got re-used by another DLL. Anything can happen then, none of it good.
answered Mar 15, 2010 at 7:02
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4You still didn't answer subbu's question. He was asking how to unload a DLL that was loaded by DllImport, not by manually loading it via LoadLibrary().– AntsCommented Mar 15, 2010 at 7:59
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3@Ants, this WILL unload a DLL loaded by DllImport. Mitch is getting the DLL handle by trying to load the (already loaded) DLL. Then you call FreeLibrary() twice to get rid of both the (otherwise useless) reference you just added AND the reference that DllImport has. Commented Mar 18, 2014 at 19:22
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At the time I commented, the original answer didn't say that FreeLibrary() should be called twice.– AntsCommented Mar 20, 2014 at 13:07
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Why does CLR fails to recognize if we have unloaded a native module using native
FreeLibrarycalls. Even theFreeLibrarycalls go through P/invoke only. Isn't it?– RBTCommented Feb 9, 2017 at 3:29
This should free a module previously loaded when you called P/Invoke function.
[DllImport("kernel32", SetLastError=true)]
static extern bool FreeLibrary(IntPtr hModule);
public static void UnloadModule(string moduleName)
{
foreach(ProcessModule mod in Process.GetCurrentProcess().Modules)
{
if(mod.ModuleName == moduleName)
{
FreeLibrary(mod.BaseAddress);
}
}
}
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3That will usually work. But module names can be ambiguous, it might release the wrong DLL. Commented Aug 11, 2012 at 13:56
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Hans is right. So perhaps better using Path.GetFileName(mod.FileName) instead of mod.ModuleName ?– PeterCommented Jan 5, 2014 at 18:02
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Or hang onto the pointer you get from LoadLibrary and compare BaseAddress to that.– yoyoCommented Apr 28, 2015 at 0:23
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(Or, if you save the IntPtr handle, just unload the handle directly instead of iterating the modules.)– yoyoCommented Apr 28, 2015 at 23:24
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An improvisation for LINQ lovers -
var loadedAssemblyModule = Process.GetCurrentProcess().Modules.OfType<ProcessModule>() .FirstOrDefault(x => x.ModuleName == moduleName); if (loadedAssemblyModule != null) FreeLibrary(loadedAssemblyModule.BaseAddress);– RBTCommented Feb 7, 2023 at 6:02
Based on Peters recommendation this works for me:
[DllImport("kernel32", SetLastError = true)]
private static extern bool FreeLibrary(IntPtr hModule);
public static void UnloadImportedDll(string DllPath)
{
foreach (System.Diagnostics.ProcessModule mod in System.Diagnostics.Process.GetCurrentProcess().Modules)
{
if (mod.FileName == DllPath)
{
FreeLibrary(mod.BaseAddress);
}
}
}
answered Apr 2, 2015 at 9:14
Late to the party, but I've made a tool to do this. It's supposed to run in Unity, but I think it can be adopted to other C# solutions. It's available here: https://github.com/mcpiroman/UnityNativeTool.
Please note that it is essentionally a hack (but often used hack, see Harmony), so I don't quite recommend using it at production code.
Since I came across the information here while I was looking around for information I figure I will contribute back what I ended up doing to fix an issue with the Sixense SDK on OSX IN UNITY. You'll see in there an implementation of dynamically loading/unloading a dylib on OSX:
