Call VC++ dll in C# Application

Question

HI,
we are using one c++ dll to read data from Hardware. Following is the function defination from C++ dll.
LRESULT CALLBACK MT_Read_Holding_Reg(long llSocketHandle,UCHAR lucDevID,USHORT lunAddress,USHORT lunQuantity, USHORT* lunHLData);
To Call this from C# I have written following code:
[DllImport("MT_TCP_Read.dll", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
public static extern int MT_Read_Input_Reg(int llSocketHandle, byte lucDevID, short lunAddress, short lunQuantity, ref short lunHLData);
and while calling this function I have writtten following code:
short[] arr_hg = new short[3];
int ret = -1;
ret = MT_Read_Coils(handle, 1, 100, 3, ref arr_coils[0]);
This function return no data where as same dll used in VB6 return correct data.
Is there any problem in above code.
Any help appreciated.
Regards,
Vikram

Sam Hobbs · Answer

In C++ LRESULT can be either 32-bits or 64-bits so we need to use IntPtr for it. So change the type that is returned from int to IntPtr .

Vulpes · Answer

See if you have any better luck with this C# signature:
[DllImport("MT_TCP_Read.dll", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]    public static extern int MT_Read_Input_Reg(int llSocketHandle, byte lucDevID, ushort lunAddress, ushort lunQuantity, [MarshalAs(UnmanagedType.LPArray, SizeConst = 3)] ushort[] lunHLData);
and then call with:
ushort[] arr_hg = new ushort[3];   int ret = MT_Read_Coils(handle, 1, 100, 3, arr_hg);