string array when c# dllimport from c/c++

Question

[C/C++ Code]

extern "C" __declspec(dllexport) int Analyze_input_text(char* input_text, char *ppArray){
   int size;
   // code...
   return size;
}

[C# Code]

[DllImport("PP_TextAnalyzer.dll",CallingConvention = CallingConvention.Cdecl, CharSet=CharSet.Unicode)]
extern public static int Analyze_input_text(IntPtr data, [MarshalAs(UnmanagedType.LPArray, SizeConst = 100)]  string[] ppArray);

public int GetResultData(string input_text)
{
    IntPtr pStr = Marshal.StringToCoTaskMemUni(input_text);
    List<string> ppArray = new List<string>();
    int size = **Analyze_input_text(pStr,ppArray.ToArray());** //Array(List) is still null.
    return size;
}

I don't know how to fix this problem anymore...

Anyone help?

What's the contract around the ppArray parameter? who's responsible for allocating it? freeing it? If it's a char*, it's not an array of char*. — Simon Mourier
– Simon Mourier, Commented Apr 14, 2013 at 10:54

Hans Passant · Accepted Answer · 2013-04-14 12:40:16Z

3

CharSet=CharSet.Unicode

Well, it isn't. This function takes a char*, an 8-bit character type. It is also rather unclear how ppArray could be an array of strings, that would be char**. As declared, the proper pinvoke declaration is:

[DllImport("PP_TextAnalyzer.dll", CallingConvention = CallingConvention.Cdecl, 
              CharSet=CharSet.Ansi)]
extern public static int Analyze_input_text(string data, string ppArray);

If you truly meant to pass an array of strings then you will have to modify the C function to at least:

extern "C" __declspec(dllexport) 
int Analyze_input_text(char* input_text, char** ppArray, int arraySize)

And declare the ppArray argument as string[] in your C# code. Do note that you really ought to use Unicode in your C code, a wchar_t*. Trying to "analyze" text that got whacked from Unicode to the local 8-bit system code page is a lossy proposition from the get-go. And of course make sure that this code wouldn't already be fast enough and less lossy when you write it in C#. .NET has rather good support for Unicode.

answered Apr 14, 2013 at 12:40

Hans Passant

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1 Comment

user1645680 Over a year ago

Thanks for the answer. Actually I treat Korean character (I think it's unicode..) with .NET.

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