I have a C struct and a DLL with a single function.  I've used the
MarshalAsAttribute, ref, DLllImport, StructLayout, UnmanagedType, etc...
declarations and I'm not having any luck.

I'm not really clear on when you can use the special declarations for
automatic marshalling or when (or how) you use IntPtr, Marshal.AllocHGlobal,
etc...

Any suggestions on how to translate my C to C# and/or any high-level
feedback on when to use the different approached (especially when the
structures have pointers to other structs or arrays)

Thank you --- Ted

// C
int Populate(int flags,PhotoBuffer *photo);

typedef struct {
 byte *buffer;
 int bufferSize;
 long mode;
}  PhotoBuffer

// C#

[DllImport("photo.dll",CharSet=CharSet.Ansi,CallingConvention=CallingConvention.StdCall,SetLastError=true)]
public static extern int populate(int flags, ref aPhotoBuffer);

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Auto)]
public class PhotoBuffer
{
    [MarshalAs(UnmanagedType.LPArray)] public byte[] travBuff;
    [MarshalAs(UnmanagedType.I4)] public int bifferSize
    [MarshalAs(UnmanagedType.U8)] public long ,mode;
}

PhotoBuffer photo=new PhotoBuffer();
photo.mode=1;
       
byte[] imageData=new byte[2048];
photo.buffer=imageData;           
photo.bufferSize=2048;

int returnCode=Populate(2,ref photo);