Understanding Piece of C# code

Question

I have been coding in c# for a bit but I came across some piece of code which does not make any sense to me at all. It looks something like below:

[DllImport(DllName, SetLastError = true, 
    CallingConvention=CallingConvention.Cdecl)]
static extern byte QLIB_UploadCEFS_File(UInt32 handle, string sFileName, 
    string sPartitionFileName);

I have no idea how to interpret this code. Can anyone explain me what are they trying to achieve here?

Answer 1

The is a P/Invoke declaration. It declares a function, external to this module, implemented in an unmanaged DLL.

The DllImport attribute specifies the name of the unmanaged DLL, DllName in this instance. The other properties of the DllImport attribute specify, in this instance, the calling convention, and that the function sets the Win32 last error variable.

The function declaration itself specifies the signature of the unmanaged function. In this case the parameters mean that the function has the following unmanaged declaration:

unsigned char QLIB_UploadCEFS_File(
    unsigned int handle, 
    const char *sFileName, 
    const char *sPartitionFileName
);

From the perspective of the managed code that calls the function, it's just like any other function. You call it passing parameters as specified in the code in your question.

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For what it is worth, I do suspect that the declaration is incorrect. The first parameter is a handle and these are almost always pointer sized. So, whilst the code is probably just fine under 32 bit, it is likely to break under 64 bit. I would expect to see that first parameter declared as IntPtr. Of course, this is speculation because I cannot see the actual unmanaged function declaration.