Marshalling Unmanaged DLL

[Jim shedden]

Hi,

I've really flailed at this today, and read many posts and MSDN help
areas. I would really appreciate a sanity check.

The DLL is for OziExplorer API, the function declaration from the
documentation is:

function oziGetExePath(var Pathansichar;
var DataLenth:integer):integer;stdcall;

I believe that PAnsiChar is a Delphi type for "pointer to Ansi Char"
and can be used similar to LPSTR to point at null terminated strings.

In general the API states return values as -1 for timeout, 0 for
completion, or other numbers as indicated by the specific funtion.
This function should return the path to the EXE file, so I expect it
should return only 0 or -1.

In C# I write the following:
________

StringBuilder sb = new StringBuilder(512);

[DllImport("OziAPI.dll", CharSet = CharSet.Ansi, CallingConvention =
CallingConvention.StdCall)]

static extern int oziGetExePath(ref StringBuilder s, ref int length);

public string getOziExePath()
{
int size = 0;
int ret;

ret=oziGetExePath(ref sb, ref size);
//if (ret != 0)
// throw new Exception("Could not get Ozi exe Path");
return (sb.ToString());
}
___________

1. While the return value is correct, the "ret" value is 1, so my API
error trap will always throw an exception. I'm a bit leery about
ignoring return values.

2. In examples I do not see the StringBuilder passed by reference. I
do not get the correct result if I do not pass by reference. I also
tried defining the path parameter as String. It also required a "ref"
and the behaviour is identical to using StringBuilder.

3. I think that the unmanaged code allocates a buffer, and I don't
want the CLR to try and release it. The API provides a "close"
function to free up storage, as it reuses it internally. I tried some
tricks with "IntPtr" and "Marshal.PtrToStringAuto()" that did not seem
to work either.

I would be happy to learn more from anyone in the group! Thanks in
advance for any help.

Regards,
Jim

 

[Guest]

Id try again with IntPtr
Did you try Marshal.PtrToStringAnsi (using the size returned by the function)?

maybe you could try Marshal.Copy to get the data into a byte array and see
whats in it.

Does size return a value?

Scott

 

[Jim shedden]

Id try again with IntPtr
Did you try Marshal.PtrToStringAnsi (using the size returned by the function)?

maybe you could try Marshal.Copy to get the data into a byte array and see
whats in it.

Does size return a value?

Hi Scott,

Thanks for the reply. I do not have a problem getting the data back in
both the string and integer parameters. I was looking for a sanity
check on the most accestable way to do it, as I am new to C#.

My problem is the return value of the function. It doesn't make sense
to get back expected data, with a return value inconsistent with
proper execution. I've never used this DLL before, but I imagine the
author is reliable.

Regards,
Jim Shedden