How to pass const unsigned char *var to C++ unmanaged DLL

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Hi,

I'm using extern keyword to use function in C++ unmanaged DLL
one of the function parameters is const unsigned char *var,
I tried to pass that parameter as a string , Byte array but that was useless.

Would you please help me to find the correct data type to pass that parameter

C#

[DllImport("DMT.dll")]
        static extern int RequestData(int comm_type, int conn_num, int slave_addr, int func_code, const unsigned char *data, int datalen);

it's a library for ModBus communication and the data parameter is usually passed as string but what i get instead is the Hex code of each char.

Posted 8-Oct-11 1:43am

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Sergey Alexandrovich Kryukov · Answer 1 · 2011-10-10T10:19:00

Instead of const unsigned char *data use string with System.Runtime.InteropServicesMarshalAsAttribute(UnmanagedType); use System.Runtime.InteropServices.LPStr as a parameter.

See:
http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.marshalasattribute.aspx[^],
http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.unmanagedtype.aspx[^].

Also, look at calling conventions for the function. Figure out what calling conventions are used by "DMT.DLL" (maybe Cdecl?) and use appropriate value for System.Runtime.InteropServices.DllImportAttribute. See:
http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.dllimportattribute.aspx[^],
http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.callingconvention.aspx[^].

—SA