BadImageFormatException occurring when loading a native DLL in .NET 4.0

Question

I'm attempting to use a CAN device over USB that comes with a native DLL that needs to be wrapped by a .NET C# class (source code provided by the vendor) that gets included in one's project. Their sample applications target .NET 2.0 where my application targets .NET 4.0. I'm able to use the code in their sample apps and debug everything just fine, however, when I try to debug my application, I get a BadImageFormatException:

System.TypeInitializationException: The type initializer for 'TotalPhase.KomodoApi' threw an exception. ---> System.BadImageFormatException: An attempt was made to load a program with an incorrect format. (Exception from HRESULT: 0x8007000B)

The only differences between their code and mine appears to be that their code is an application built for .NET 2.0 and (currently) my code is running as an MSTest unit test in .NET 4.0. Both solutions target AnyCPU. I'm running on a Windows 7 Ultimate 64-bit install. Even changing from AnyCPU to x86 didn't make any difference. How can I get this native DLL to load in an AnyCPU project?

Answer 1

If you get a BadImageFormatException when interfacing with a native DLL, it almost always means that you are trying to interface with a 32-bit DLL while running in the 64-bit CLR, or vice versa.

When you run the sample applications, do the processes have *32 in the "Image Name" column of Task Manager's "Processes" tab? That indicates the applications are running in the 32-bit CLR. Check your own application as well. It is possible that the machine you are testing on only has a 32-bit .NET 2.0 runtime, but both 32-bit and 64-bit .NET 4.0 runtimes, or the other way around.

If you are distributing a native DLL with your .NET application, then you should set your startup project to target x86 or x64 (as opposed to AnyCPU), depending on whether the native libraries are 32-bit or 64-bit. You can always ship both 32-bit and 64-bit versions, and let the installer choose which binaries to install based on the client architecture.

Alternatively, you can ship both 32-bit and 64-bit DLLs with different file names, define separate P/Invoke stubs for each version, and decide which one to call at runtime. The easiest way to do this would probably be to wrap your native calls in an interface (e.g., INativeMethods) and choose which implementation to instantiate at runtime based on IntPtr.Size. With this method, you could still target AnyCPU.