How to marshall pointer to array of char[256] from C++ to C#

Question

I have C++ method which has following signature:

typedef char TNameFile[256];

void Foo(TNameFile** output);

I've run out of ideas how to marshall it.

Answer 1

Assuming they return an empty string as last element:

static extern void Foo(ref IntPtr output);

IntPtr ptr = IntPtr.Zero;
Foo(ref ptr);
while (Marshal.ReadByte(ptr) != 0)
{
   Debug.Print(Marshal.PtrToStringAnsi(ptr, 256).TrimEnd('\0'));
   ptr = new IntPtr(ptr.ToInt64() + 256);
}

EDIT: Since I've written the above code on my smartphone, I tested the code this morning and it seems like it should work (I just had to add the TrimEnd('\0')). Here is my test case:

class Program
{
    const int blockLength = 256;

    /// <summary>
    /// Method that simulates your C++ Foo() function
    /// </summary>
    /// <param name="output"></param>
    static void Foo(ref IntPtr output)
    {
        const int numberOfStrings = 4;
        byte[] block = new byte[blockLength];
        IntPtr dest = output = Marshal.AllocHGlobal((numberOfStrings * blockLength) + 1);
        for (int i = 0; i < numberOfStrings; i++)
        {
            byte[] source = Encoding.UTF8.GetBytes("Test " + i);
            Array.Clear(block, 0, blockLength);
            source.CopyTo(block, 0);
            Marshal.Copy(block, 0, dest, blockLength);
            dest = new IntPtr(dest.ToInt64() + blockLength);
        }
        Marshal.WriteByte(dest, 0); // terminate
    }

    /// <summary>
    /// Method that calls the simulated C++ Foo() and yields each string
    /// </summary>
    /// <returns></returns>
    static IEnumerable<string> FooCaller()
    {
        IntPtr ptr = IntPtr.Zero;
        Foo(ref ptr);
        while (Marshal.ReadByte(ptr) != 0)
        {
            yield return Marshal.PtrToStringAnsi(ptr, blockLength).TrimEnd('\0');
            ptr = new IntPtr(ptr.ToInt64() + blockLength);
        }
    }

    static void Main(string[] args)
    {
        foreach (string fn in FooCaller())
        {
            Console.WriteLine(fn);
        }
        Console.ReadKey();
    }
}

One question remains: who is going to free the buffer?