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How do I unload a DLL which has been loaded using DllImport in C#?

g t
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subbu
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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. – RBT Feb 09 '17 at 03:32

5 Answers5

29

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.

Hans Passant
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Mitch Wheat
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    You 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(). – Ants Mar 15 '10 at 07:59
  • Take a look here http://www.codeproject.com/KB/cs/dyninvok.aspx?fid=2892&df=90&mpp=25&noise=3&sort=Position&view=Quick&select=3436363&fr=26#xx0xx – Goran Apr 13 '10 at 08:37
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    @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. – Technophile Mar 18 '14 at 19:22
  • At the time I commented, the original answer didn't say that FreeLibrary() should be called twice. – Ants Mar 20 '14 at 13:07
  • Why does CLR fails to recognize if we have unloaded a native module using native `FreeLibrary` calls. Even the `FreeLibrary` calls go through P/invoke only. Isn't it? – RBT Feb 09 '17 at 03:29
  • Because the CLR doesn't know that you are *manually* calling `FreeLibrary()` directly, so it doesn't know to clear out any references that it itself had loaded before. And what `FreeLibrary()` were being called via `GetProcAddress()` instead of `DllImport`? So the CLR would have to install a hook into `FreeLibrary()` itself to intercept all calls. – Remy Lebeau Aug 24 '21 at 17:39
13

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);
        }
    }
}
IS4
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    That will *usually* work. But module names can be ambiguous, it might release the wrong DLL. – Hans Passant Aug 11 '12 at 13:56
  • Hans is right. So perhaps better using Path.GetFileName(mod.FileName) instead of mod.ModuleName ? – Peter Jan 05 '14 at 18:02
  • Or hang onto the pointer you get from LoadLibrary and compare BaseAddress to that. – yoyo Apr 28 '15 at 00:23
  • (Or, if you save the IntPtr handle, just unload the handle directly instead of iterating the modules.) – yoyo Apr 28 '15 at 23:24
  • An improvisation for LINQ lovers - `var loadedAssemblyModule = Process.GetCurrentProcess().Modules.OfType() .FirstOrDefault(x => x.ModuleName == moduleName); if (loadedAssemblyModule != null) FreeLibrary(loadedAssemblyModule.BaseAddress);` – RBT Feb 07 '23 at 06:02
6

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);
            }
        }
    }
stefan.seeland
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0

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.

mcpiroman
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-2

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:

https://gist.github.com/amirebrahimi/d7b02c01bcd3ca7da144

Amir Ebrahimi
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