To set the value of a two-dimensional array in c++ dll

Question

Hello friend The following code will not run properly. C++ codes: // mytest.cpp #include stdafx.h #include #include extern C{ _declspec(dllexport) void _setv(char ***s int row int column){ for (int i = 0; i for (int j = 0; j { strcat(s[i][j]The value in here.); } } } } C# codes: class Program { [DllImport(mytest.dll CallingConvention = CallingConvention.Cdecl)] public static extern void _setv(string[] s int row int column); static void Main(string[] args) { string[] _arr = new string[2 2]{{}{}}; _setv(_arr 2 2); } } Thank.

Vulpes · Answer

The P/Invoke marshaler seems to have problems with a 2D string array and the only way I could get this to work was to 'flatten' it into a single dimensional string array and then pass that by reference. There are a couple of other points to note: 1. As the unmanaged function is filling the array and then returning it to managed code a large enough buffer needs to be created for each string element including the terminating null character - I've used 80 characters. 2. Normally you'd use StringBuilder instead of string for a mutable string but an array of StringBuilders doesn't work and therefore you have to use a string array. However you need to specify the [In Out] attributes for this to work. 3. The marshaler needs to know how big the string array is and I've specified 4 as there are 2 x 2 elements. The 'row' parameter has been changed to 'size' to reflect this. So here's the revised code: // C++ code // mytest.cpp #include stdafx.h #include #include extern C { _declspec(dllexport) void _setv(char ***s int size int column) { int row = size/column; char **t = *s; for (int i = 0; i { for (int j = 0; j { strcpy(t[i * row + j] The value in here.); } } } } // C# code using System; using System.Runtime.InteropServices; class Program { [DllImport(mytest.dll CallingConvention = CallingConvention.Cdecl)] public static extern void _setv([In Out][MarshalAs(UnmanagedType.LPArray ArraySubType= UnmanagedType.LPStr SizeParamIndex=1)] ref string[] s int size int column); static void Main(string[] args) { string[] s = new string[4]; for(int i = 0; i for(int j = 0; j { s[i * 2 + j] = new string('\0' 80); } _setv(ref s 4 2); string[] ss = new string[2 2]; for(int i = 0; i for(int j = 0; j { ss[i j] = s[i * 2 + j]; Console.WriteLine(ss[i j]); } Console.ReadKey(); } } As expected the output is: The value in here. The value in here. The value in here. The value in here.

Vulpes · Answer

To deal with unicode as opposed to ANSI strings you'll need to make a few changes: // C++ code // mytest_uni.cpp #include stdafx.h #include #include extern C { _declspec(dllexport) void _setv( wchar_t ***s int size int column) { int row = size/column; wchar_t **t = *s; for (int i = 0; i { for (int j = 0; j { wcscpy (t[i * row + j] LThe value in here. ); } } } } // C# code using System; using System.Runtime.InteropServices; class Program { [DllImport( mytest_uni.dll  CharSet = CharSet.Unicode  CallingConvention = CallingConvention.Cdecl)] public static extern void _setv([In Out][MarshalAs(UnmanagedType.LPArray ArraySubType= UnmanagedType.LPWSt r SizeParamIndex=1)] ref string[] s int size int column); static void Main(string[] args) { string[] s = new string[4]; for(int i = 0; i for(int j = 0; j { s[i * 2 + j] = new string('\0' 80); } _setv(ref s 4 2); string[] ss = new string[2 2]; for(int i = 0; i for(int j = 0; j { ss[i j] = s[i * 2 + j]; Console.WriteLine(ss[i j]); } Console.ReadKey(); } }

Ken H · Answer

Thank for Vulpes. :)

Ken H · Answer

It can work. but there is a small problem and how to make it compatible with Unicode character set? Thank for you Vulpes. :)