Unable to load DLL (Module could not be found HRESULT: 0x8007007E)

I have a 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 several 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 the 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?

From what I remember on Windows the search order for a dll is:

1. Current Directory
2. System folder, C:\windows\system32 or c:\windows\SysWOW64 (for 32-bit process on 64-bit box).
3. Reading from the Path environment variable

In addition I'd check the dependencies of the DLL, the dependency walker provided with Visual Studio can help you out here, it can also be downloaded for free: http://www.dependencywalker.com

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.

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".