Question

I’m using an API (written in c++) to connect to a DVR machine, actually I only have the .dll and the .lib files, and I want to do the job in .NET (C#).
So, the API doc contains definitions to all functions inside the dll, and I’m having a hard time trying to figure out how to translate those functions to equivalente C# definitions. Some of them are straightforward, but some I cannot make it work.

For example, there is this function:

bool searchEvent(int channel, const LONG* condition, bool next)  

In my c# class I put the following:

[DllImport("search.dll")]  
protected static extern bool searchEvent(int channel,  
[MarshalAs(UnmanagedType.I8)]long condition, bool next);

but when I call the searchEvent function, it raises an error (memory error), I guess because of that long var. So my question is how can I translate “const LONG* condition” to c#?
Also, that searchEvent function triggers a callback that returns a struct. I am unable to translate some vars of that struct, like:

char _version[2]  
time_t _time  
BYTE* _minute  
unsigned short int _dwell[MAX]  

Can someone help me with this, please? I´m no expert in c++, so any tips are greatly appreciated. Thanks!

Answer 1

    [DllImport("search.dll")]
    [return: MarshalAs(UnmanagedType.U1)]
    private static extern bool searchEvent(
        int channel,
        ref int condition,
        [MarshalAs(UnmanagedType.U1)] bool next
    );

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
    private struct Mumble {
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 2)]
        public string version;
        public long time;
        public IntPtr minute;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = MAX)]
        public ushort _dwell[];
    }

You will have to marshal the Mumble.Minute value yourself with Marshal.ReadXxxx(). These declarations are not great for interop, writing a wrapper in the C++/CLI language is advisable.