How to match C++ LPSTR to C#

Question

I'm actually learning C#/WPF and I wrote a library in C++. And I have a function that have an output parameter in LPSTR type, the test work well in C++ and I'm actually getting the right value I wanted with the right size. But in C# after importing the dll and set the format to unicode/ansi, either I have a empty value or bad value.

C++ project build in myfunction.dll win32.

C++ export

/*
* getting the right value in outString 
* c++ test
* 
*/

API_DLLEXPORT long GetSomeString (LPSTR outString ,long * outNbChar);

for (long iChar = 0; iChar < *outNbofchar; ++iChar)
{
    outsLibClass[iChar] = (char)finder.GetFileName()[iChar];
}
outsLibClass[*outNbofchar] = '\0';
break; 

C# import

[DllImport("myfunction.dll",  CharSet = CharSet.Unicode)]
public static extern Int32 GetSomeString (StringBuilder outString, ref Int32 outNbChar);

StringBuilder out = new StringBuilder(256) ;
Int32 nbchar = new Int32() ;

int result = GetSomeString (out, ref nbchar ) ;

Console.WriteLine(out.ToString());

// I get the right nbchar

But I'm not getting the right outstring result:

㠴0휐ࠦ휠ࠦ鏺瓑휬ࠦ찞瑻쳁촀

And of course I try also to set the CharSet to Ansi but the result was empty.

So if anyone could help me that would be a great pleasure.

Answer 1

Thx to @john link in the comments, i resolve it : i was making a copy and not actualy referencing the address of outString.

// Replace LPSTR by LPSTR 
API_DLLEXPORT long GetSomeString (LPSTR * outString ,long * outNbChar);

and in c#

[DllImport("myfunction.dll",  CharSet = CharSet.Unicode)]
public static extern Int32 GetSomeString ([MarshalAs(UnmanagedType.LPStr)] ref string outString,
                                          ref Int64 outNbChar);