Referencing unmanaged libraries in a PaaS environment

This is a proof of concept demonstrating that an Azure Web App can include a C++ unmanaged library and call functions from it.

The unmanaged library

The project Ugly is an unmanaged C++ project.

The file Ugly.cpp contains a function called fnUgly:

UGLY_API int fnUgly(void)
{
    return 42;
}

The file Ugly.h exports the fnUgly function:

extern "C" {
	UGLY_API int fnUgly(void);
}

You'll need to compile the DLL against the expected architecture (32 vs 64 bits).

Include the unamanged library in your project

All you need to do to use an unamanged library is to copy it to your bin directory.

I achieved this by adding the DLLs to the project, have a look at the Api\Ugly\x64 folder.

• Build Action: None
• Copy to Output Directory: Copy if newer

Call into an unmanaged library

The code is located in HomeController

/*
* The path is relative to the bin folder.
* The function name has to match the function name in the C++ DLL,
* if required you can override this via the EntryPoint property
* in the DllImportAttribute
*/
[DllImport("Ugly\\x64\\Ugly.dll")]
public static extern int fnUgly();

/* [...] */

var value1 = fnUgly().ToString();

Logging

By default Serilog will write to D:\home\LogFiles\, it's a hack I know but it was the fastest way to get logging up and running. You can download the log files via the Kudu console.

If you need to log somewhere else you can create a LogPath key in your Web.config.

Troubleshooting

If you can't manage to call into your unmanaged DLL you can uncomment LoadNativeAssemblies in WebApiApplication. This will attempt to load the unmanaged DLLs manually and generate better errors in the logs.

Process monitor

Your unmanaged DLL might depend on other unmanaged libraries that are not deployed on the target machine.

I don't know if you can enable Fusion logs in a PaaS environment or use the dependency walker. The fastest way to figure out which native modules are required is to use Process Monitor to see which files are being accessed by the process.