PInvoke marshaling a char* inside a struct

From: deleria010 (deleria010_at_hotmail.com)
Date: 03/17/05 

Date: 17 Mar 2005 08:01:19 -0800

I have a C struct that contains a char* and I am tring to use PInvoke
(and C#) to marshal it as a string but it is not working. I was hoping
someone has done something like this and will give me a hint on how to
get it to work. I also have to add a uchar* that points to the start
of a large uchar array. I don't know what C# type this should be
marshaled as.

Something odd is happening when the char* was added. After calling
takeStruct, the C#'s variable members were not set but after calling
returnStruct, the variable members were set correctly!! Why would the
char* be breaking the marshaling of the reset of the members.

Here is the code so far:

.h

struct myStruct {
      ushort c1;
      uchar c2;
      uchar c3;
      uchar c4;
      uchar c5;
      uchar c6;
      uchar c7;
      bool c8;
      char* c9;
    };
struct myStruct* st;

void takeStruct(struct myStruct* s);
struct myStruct* returnStruct();

.cc

extern "C" void takeStruct(struct myStruct* s)
{
  s->c1 = 1;
  s->c2 = 2;
  s->c3 = 3;
  s->c4 = 4;
  s->c5 = 5;
  s->c6 = 6;
  s->c7 = 7;
  s->c8 = true;
  s->c9 = "hello";
}

struct myStruct* returnStruct()
{
  if (st == 0) {
    st = new struct myStruct;
  }
  st->c1 = 1;
  st->c2 = 2;
  st->c3 = 3;
  st->c4 = 4;
  st->c5 = 5;
  st->c6 = 6;
  st->c7 = 7;
  st->c8 = true;
  st->c9 = "world";

  return st;
}

C#

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi, Pack=4)]
public class MyStruct
{
        public ushort cs1;
        public byte cs2;
        public byte cs3;
        public byte cs4;
        public byte cs5;
        public byte cs6;
        public byte cs7;
        public bool cs8;
        [MarshalAs(UnmanagedType.LPStr)]
        public string cs9;
}

[DllImport("ctisr_csharp.dll")]
public static extern void
takeStruct([MarshalAs(UnmanagedType.LPStruct)] MyStruct c);

[DllImport("ctisr_csharp.dll")]
[return : MarshalAs(UnmanagedType.LPStruct)]
public static extern MyStruct returnStruct();

Thanks for reading

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