[C#] Unable to load DLL (Module could not be found HRESULT: 0x8007007E)



Answers

You can use the dumpbin tool to find out the required DLL dependencies:

dumpbin /DEPENDENTS my.dll

This will tell you which DLLs your DLL needs to load. Particularly look out for MSVCR*.dll. I have seen your error code occur when the correct Visual C++ Redistributable is not installed.

You can get the "Visual C++ Redistributable Packages for Visual Studio 2013" from the Microsoft website. It installs c:\windows\system32\MSVCR120.dll

In the file name, 120 = 12.0 = Visual Studio 2013.

Be careful that you have the right Visual Studio version (10.0 = VS 10, 11 = VS 2012, 12.0 = VS 2013...) right architecture (x64 or x86) for your DLL's target platform, and also you need to be careful around debug builds. The debug build of a DLL depends on MSVCR120d.dll which is a debug version of the library, which is installed with Visual Studio but not by the Redistributable Package.

Question

I have dll library with unmanaged C++ API code I need to use in my .NET 4.0 application. But every method i try to load my dll i get an error:

Unable to load DLL 'MyOwn.dll': The specified module could not be found. (Exception from HRESULT: 0x8007007E)

I have read and tried severa solutions i have found on the internet. Nothing works..

I have tried using following methods:

[DllImport("MyOwn.dll",  CallingConvention = CallingConvention.Cdecl)]
[return: MarshalAs((UnmanagedType.I4))]
public static extern Int32 MyProIni(string DBname, string DBuser_pass,
    string WorkDirectory, ref StringBuilder ErrorMessage);

When I tried following this article and when I run this example (from the downloaded code) it runs without a problem (the dll used is in the bin/debug folder)

I have copied my dll (along with all files the it depends on into my bin folder).

I also tried this approach but got the same error:

[DllImportAttribute(MyOwnLibDllPath, EntryPoint="TMproIni")]
[return: MarshalAs(UnmanagedType.I4)]
public static extern  int MyproIni(string DBname, string DBuser_pass, 
    string WorkDirectory, ref StringBuilder ErrorMessage);

Any suggestions?




I think your unmanaged library needs a manifest.
Here is how to add it to your binary. and here is why.

In summary, several Redistributable library versions can be installed in your box but only one of them should satisfy your App, and it might not be the default, so you need to tell the system the version your library needs, that's why the manifest.




Setup: 32-bit Windows 7

Context: Installed a PCI-GPIB driver that I was unable to communicate through due to the aforementioned issue.

Short Answer: Reinstall the driver.

Long Answer: I also used Dependency Walker, which identified several missing dependency modules. Immediately, I thought that it must have been a botched driver installation. I didn't want to check and restore each missing file.

The fact that I was unable to find the uninstaller under Programs and Features of the Control Panel is another indicator of bad installation. I had to manually delete a couple of *.dll in \system32 and registry keys to allow for driver re-installation.

Issue fixed.

The unexpected part was that not all dependency modules were resolved. Nevertheless, the *.dll of interest can now be referenced.




There is one very funny thing (and has a technical relevance) which might waste your hours so thought of sharing it here -

I created a console application project ConsoleApplication1 and a class library project ClassLibrary1.

All the code which was making the p/invoke was present in ClassLibrary1.dll. So before debugging the application from visual studio I simply copied the C++ unmanaged assembly (myUnmanagedFunctions.dll) into the \bin\debug\ directory of ClassLibrary1 project so that it can be loaded at run-time by the CLR.

I kept getting the

Unable to load DLL

error for hours. Later on I realized that all such unmanaged assemblies which are to be loaded need to be copied into the \bin\debug directory of the start-up project ConsoleApplication1 which is usually a win form, console or web application.

So please be cautious the Current Directory in the accepted answer actually means Current Directory of main executable from where you application process is starting. Looks like an obvious thing but might not be so at times.

Lesson Learnt - Always place the unamanaged dlls in the same directory as the start-up executable to ensure that it can be found.




The DLL has to be in the bin folder.

In Visual Studio, I add the dll to my project (NOT in References, but "Add existing file"). Then set the "Copy to Output Directory" Property for the dll to "Copy if newer".




Turn on the fusion logging, see this question for lots of advice on how to do that. Debugging mixed-mode apps loading problems can be a right royal pain. The fusion logging can be a big help.




If the DLL and the .NET projects are in the same solution and you want to compile and run both every time, you can right click the properties of the .NET project, Build events, then add something like the following to Post-build event command line:

copy $(SolutionDir)Debug\MyOwn.dll .

It's basically a DOS line, and you can tweak based on where your DLL is being built to.






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