CODEWITHSUNDEEP
c#
dorin.petre
dorin.petre

Reputation: 103

DllImport C Sharp const char*

Hy all,

I want to make from a unmanaged dll to a managed dll in C#.

in the documentation it is:

typedef void*    AP_HANDLE
typedef uint32_t ap_u32

AP_HANDLE ap_CreateVirtual(const char *szFilename)

void ap_SetState(AP_HANDLE apbase, const char *szState, int nValue)

unsigned char *ap_ColorPipe(AP_HANDLE      apbase, 
                            unsigned char *pInBuffer,
                            ap_u32         nInBufferSize, 
                            ap_u32        *rgbWidth, 
                            ap_u32        *rgbHeight, 
                            ap_u32        *rgbBitDepth)

In C++ works fine, but in C# the syntax is the problem It seems I can't make not evan the first function work

public unsafe class appbase
{
    [DllImport("D:\\apbase.dll", EntryPoint = "ap_CreateVirtual")]
    //, CharSet = UnicodeEncoding
    //, CallingConvention = CallingConvention.StdCall

    public static extern void* ap_CreateVirtual(char* szFilename);
}

and 

    public static void Main()
    {
        unsafe
        {
            void* ap_handle = null;

            appbase APPbase = new appbase();

            string s = "D:\\ASX.xsdat";

            fixed (char* p = s)
            {
                ap_handle = APPbase.ap_CreateVirtual(p);
            }

            return;
        }
    }

tried with argument for ap_CreateVirtual string, String*, char*, char[] and put a break point on return; the ap_handle was always with value 0x0000

what is the proper way to import these functions ?

Upvotes: 0

Views: 2511

Answers (2)

Luaan
Luaan

Reputation: 63732

Just copying over the C++ headers isn't enough. You need to understand what the arguments actually mean and how they are represented. The first function is simple enough:

[DllImport("...", EntryPoint="ap_CreateVirtual")]
public static extern IntPtr ap_CreateVirtual(
  [In, MarshalAs(UnmanagedType.LPStr)] string szFilename);

I assume that in the second, szState is again a simple LPStr, while the pInBuffer is most likely a fixed-length byte[] with size determined by nInBufferSize. rgbXXX seem to be ref arguments. The return type may again be a LPStr.

Upvotes: 0

Ben Voigt
Ben Voigt

Reputation: 283674

char* is the default marshaling for a .NET string.

[DllImport("D:\\apbase.dll", EntryPoint = "ap_CreateVirtual", CharSet = CharSet.Ansi)]
//, CallingConvention = CallingConvention.StdCall
public static extern System.IntPtr ap_CreateVirtual(string szFilename);

Your original attempt was wrong because char in C# is C++'s wchar_t, not C++ char.

If you need a C++ char in C#, it's either byte or sbyte. But p/invoke will simply do the right thing with System.String.

Upvotes: 2

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