Import C library in C#

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C

C#

DLL

I have an USB device with a "C" dll interface which can make communication between PC & USB device. Also I have API document for this C library.
I am not familiar with C language, so I decide to use C# to import this C dll, but I got an error "No SPLib DLL was found in the Waterfall procedure".

You can find the C dll and document from this git repository.
GitHub - ctzuchiang/TriaxialVibrationAnalyser: TriaxialVibrationAnalyser[^]

What I have tried:

public static class FTPA_Service
    {
        internal const CallingConvention CC = 0;
        internal const string LIBUSB_DLL = @"ExternalLib\fasttracerpa.dll";

        [DllImport(LIBUSB_DLL, CallingConvention = CC, SetLastError = false, EntryPoint = "FTPA_enumerateAllDeviceInfo")]
        internal static extern IntPtr FTPA_enumerateAllDeviceInfo(IntPtr devCount);

        [DllImport(LIBUSB_DLL, CallingConvention = CC, SetLastError = false, EntryPoint = "FTPA_getAllDeviceInfo")]
        internal static extern FASTTRACERPA_RETURN_CODE FTPA_getAllDeviceInfo(IntPtr rDI, IntPtr prSize);

        public static IntPtr EnumAllDeviceInfo()
        {
            return FTPA_enumerateAllDeviceInfo(new IntPtr(0));
        }
    }