Passing string between C# and C++ dll

From C# to C++ dll

C++

void foo(const char *str)
{
    // do something with str
}

C#

[DllImport("...", CallingConvention = CallingConvention.Cdecl)
static extern void foo(string str);

....

foo("bar");

In the other direction you would typically expect the caller to allocate the buffer, into which the callee can write:

C++

void foo(char *str, int len)
{
    // write no more than len characters into str
}

C#

[DllImport("...", CallingConvention = CallingConvention.Cdecl)
static extern void foo(StringBuilder str, int len);

....

StringBuilder sb = new StringBuilder(10);
foo(sb, sb.Capacity);

From C++ dll to C#

C++ 裡面：

TCHAR g_awcMessage[] = L"Hello中文";
char g_aszMessage[] = "Hello中文";

extern "C" __declspec(dllexport) TCHAR* __stdcall GetHelloL()
{ 
 return g_awcMessage; 
}

extern "C" __declspec(dllexport) CHAR* __stdcall GetHello()
{ 
 return g_aszMessage;
}

extern "C" __declspec(dllexport) int __stdcall GetInt()
{ 
 return 100;
}



C# 裡面：

[DllImport("gpDll.dll")]
public static extern IntPtr GetHello();

[DllImport("gpDll.dll")]
public static extern IntPtr GetHelloL();

[DllImport("gpDll.dll")]
public static extern int GetInt();



private void button1_Click(object sender, EventArgs e)
{
            // Not support in CF
            //Marshal.PtrToStringAnsi
            //Marshal.PtrToStringAuto



            // Multibytes 會變成亂碼
            string str =  Marshal.PtrToStringUni(GetHello());



            // Wide Character 顯示正常
            string str = Marshal.PtrToStringAnsi(GetHello());




            // Wide Character 顯示正常
            string strL = Marshal.PtrToStringUni(GetHelloL());



            int n = GetInt();
}