How to use the correct unmanaged DLL file according CPU architecture? (32 / 64 bits)

Question

I have to use a C++ DLL file from an ASP.NET site. The site will be hosted on both 32 and 64 bits environments.

I have a 32 and 64 bits version of the unmanaged DLL

Answer 1

Simplest approach would be to give the two native libraries the same filename in two different directories, then adjust your application DLL search path depending on the bitness.

http://www.pinvoke.net/default.aspx/kernel32.setdlldirectory

Answer 2

As we know that we cannot add unmanaged DLL files using add reference. So we need an alternative way to handle unmanaged DLLS files according to CPU architecture that is add DLL files in your project directories.

1. Create new folders into your project named x32 and x64.
2. Copy your DLL files into these folders according to architecture.
3. Select the files and open properties, Change "Copy to output directory: Copy always"
4. Repeat this on all files in both folders

Now build your project in any platform when you will build the project both x86 and x64 folder will be copyed in bin folder. When this will run on server it will use unmanaged DLL files according to CPU architecture.

Answer 3

To get the compiler/framework to do most of the work you need to

1. have multiple build 'platforms' (typically x86, x64 - remove AnyCPU)
2. set each "platform target" configuration for each build config
3. we added conditional compilation symbols __WIN32 & __X64

List the different implementations for your functions according to platform, including different dll names if you need to have both installed at once.

#if __WIN32
        public delegate int Move(int target);
        [DllImport("my.dll", SetLastError = true, CharSet = CharSet.Auto)]
#elif __X64
        public delegate int Move(int target);
        [DllImport("my64.dll", SetLastError = true, CharSet = CharSet.Auto)]
#endif

Otherwise you can use loadlib and manage the marshalling yourself.