exporting function with parm pointer to struct

  • Thread starter Thread starter Angel
  • Start date Start date
A

Angel

I'm exporting (with DllImport) a C-style function with this syntax:

int z9indqry (4_PARM *parm);

4_PARM is a structure declared in a proprietary header file that cannot be
included in my project (due to C# limits).

What would be the other best way to do this? I would like to be able to use
the original struct because these structs have over 40 members each and
there are several more Structs similar to 4_PARM. Also, they are modified by
the company every two months so I'd need to compare and modify my C# structs
everytime they change the h structs.
I'm not going to use wrappers because the dll function calls are working
well when the parms are simple data types (int, char, string, etc...). I
just need a way to be able to call these specialized structs.

Thanks again.

P.S. - I'm pretty sure this is the last question I have regarding unmanaged
code :-))
 
Angel,

If your structures are really that flexible, then I would recommend that
you not use a pointer to the struct, but rather, a pointer to void in the
declaration. This will prevent you from having to recompile, as well as
redefine the C# code (at least for the declaration).

Also, it would imply that you need a better design that is more
flexible, IMO. Changing the structure every two months is very time
consuming. It might make sense to have a keyed collection of some kind to
use.

That being said, with what you have now, you will have to declare the
structure in C#. You will want it to have the same layout in memory as the
structure in C# (remember longs in C are ints in C#, etc, etc). Also, you
will want to adorn the structure with the StructLayout attribute, passing
LayoutKind.Sequential to the constructor.

Hope this helps.
 
Thanks for the post.

I did what you suggested but I still get "This type can not be marshaled as
a structure field" when I call the function. This is my initial code:

[DllImport("C:\\zm7\\Developm\\DLL\\ZIP4_W32.DLL")]
public static extern int z9indqry(4_PARM parm);
public static void getParms ()
{
int i;
Zip4_parm parm = new Zip4_parm();
i = z9indqry(parm); // Error: "This type can not be marshaled as a
structure field"
// IntPtr buffer = Marshal.AllocCoTaskMem( Marshal.SizeOf( parm ));
}
What can this error mean?
Also, I'm trying to delare some const vars from the initial H file (eg.
#define Z4_INVADDR 10) but adding it to the cs file that contains the
structs generates error "Expected class, delegate, enum, interface, or
struct".

Any help would be appreciated.

Nicholas Paldino said:
Angel,

If your structures are really that flexible, then I would recommend that
you not use a pointer to the struct, but rather, a pointer to void in the
declaration. This will prevent you from having to recompile, as well as
redefine the C# code (at least for the declaration).

Also, it would imply that you need a better design that is more
flexible, IMO. Changing the structure every two months is very time
consuming. It might make sense to have a keyed collection of some kind to
use.

That being said, with what you have now, you will have to declare the
structure in C#. You will want it to have the same layout in memory as the
structure in C# (remember longs in C are ints in C#, etc, etc). Also, you
will want to adorn the structure with the StructLayout attribute, passing
LayoutKind.Sequential to the constructor.

Hope this helps.


--
- Nicholas Paldino [.NET/C# MVP]
- (e-mail address removed)

Angel said:
I'm exporting (with DllImport) a C-style function with this syntax:

int z9indqry (4_PARM *parm);

4_PARM is a structure declared in a proprietary header file that cannot be
included in my project (due to C# limits).

What would be the other best way to do this? I would like to be able to use
the original struct because these structs have over 40 members each and
there are several more Structs similar to 4_PARM. Also, they are
Click to expand...
modified
by
the company every two months so I'd need to compare and modify my C# structs
everytime they change the h structs.
I'm not going to use wrappers because the dll function calls are working
well when the parms are simple data types (int, char, string, etc...). I
just need a way to be able to call these specialized structs.

Thanks again.

P.S. - I'm pretty sure this is the last question I have regarding unmanaged
code :-))
Click to expand...
Click to expand...
 
Hi Angel,

Thank you for posting in the community!

I am confused by your code. In your function declaration:
public static extern int z9indqry(4_PARM parm)
It needs 4_PARM structure, but you construct a Zip4_parm, then pass it to
z9indqry.

What is "Zip4_parm"?

I think the most recommanded way is do the marshaling and declaration in C#
for your 4_PARM structure. Then use it as:
public static extern int z9indqry(ref 4_PARM parm)

If you still do not want to declare the structure. Just as Nicholas said,
you may use IntPtr, which you need to read and write the structure memory
all by yourself(Which is not recommanded)
Below is the steps:

1). Initialize a byte[] which is the same length as the structure.
2). Initilize the structure byte[] array(Do some write to this byte[] array)
3). Use Marshal.AllocCoTaskMem() to alloc a structure size in unmanaged
memory
4). Use Marshal.Copy(Byte[], Int32, IntPtr, Int32) to copy the managed
byte[] array to the unmanaged memory.
5). Use the function like this: public static extern int z9indqry(ref
IntPtr parm)

===========================================
Please apply my suggestion above and let me know if it helps resolve your
problem.

Thank you for your patience and cooperation. If you have any questions or
concerns, please feel free to post it in the group. I am standing by to be
of assistance.

Best regards,
Jeffrey Tan
Microsoft Online Partner Support
Get Secure! - www.microsoft.com/security
This posting is provided "as is" with no warranties and confers no rights.
 
Thanks for your help. Instead of recreating the C-style struct as a C#
struct, I decided to use it as a class. I think that it's at least
communicating with the dll. This is how my class looks like:

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi) ]
public class city
{
public city()
{
}
//[MarshalAs(UnmanagedType.LPStr)]
[MarshalAs( UnmanagedType.ByValArray, SizeConst=5 )]
public char detail_code;
public char[] zip_code;
public char[] city_key;
public char zip_class_code;
public char[] city_name;
}

The original:
typedef struct
{
char detail_code;
char zip_code[5+1];
char city_key[6+1];
char zip_class_code;
char city_name[28+1];
} CITY_REC;


When I invoke the function, I receive the error: "Can not marshal field
detail_code of type ZM7.city: Invalid managed/unmanaged type combination
(chars must be paired with U1 or U2)". At least the error specifies a field
(detail_code) inside the class, which I think may be a good sign.

What could it be?

Thanks.
 
Hi, Angel ;)

Try the following

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi) ]
public class city
{
public city()
{
}
[MarshalAs(UnmanagedType.U1)]
public char detail_code;
[MarshalAs( UnmanagedType.ByValArray, SizeConst=5 )]
public char[] zip_code;
[MarshalAs( UnmanagedType.ByValArray, SizeConst=6 )]
public char[] city_key;
[MarshalAs(UnmanagedType.U1)]
public char zip_class_code;
[MarshalAs( UnmanagedType.ByValArray, SizeConst=29 )]
public char[] city_name;
}

You should marshal 'char' as 'UnmanagedType.U1' which means C 'char' and
'char[]' as 'UnmanagedType.ByValArray, SizeConst=size' which means C 'char[size]'.

Also you may need to compile C structure with '#pragma pack push(1)' to remove data alignment.
Thanks for your help. Instead of recreating the C-style struct as a C#
struct, I decided to use it as a class. I think that it's at least
communicating with the dll. This is how my class looks like:

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi) ]
public class city
{
public city()
{
}
//[MarshalAs(UnmanagedType.LPStr)]
[MarshalAs( UnmanagedType.ByValArray, SizeConst=5 )]
public char detail_code;
public char[] zip_code;
public char[] city_key;
public char zip_class_code;
public char[] city_name;
}

The original:
typedef struct
{
char detail_code;
char zip_code[5+1];
char city_key[6+1];
char zip_class_code;
char city_name[28+1];
} CITY_REC;


When I invoke the function, I receive the error: "Can not marshal field
detail_code of type ZM7.city: Invalid managed/unmanaged type combination
(chars must be paired with U1 or U2)". At least the error specifies a field
(detail_code) inside the class, which I think may be a good sign.

What could it be?

Thanks.



Hi Angel,

Thank you for posting in the community!

I am confused by your code. In your function declaration:
public static extern int z9indqry(4_PARM parm)
It needs 4_PARM structure, but you construct a Zip4_parm, then pass it to
z9indqry.

What is "Zip4_parm"?

I think the most recommanded way is do the marshaling and declaration in
C#

for your 4_PARM structure. Then use it as:
public static extern int z9indqry(ref 4_PARM parm)

If you still do not want to declare the structure. Just as Nicholas said,
you may use IntPtr, which you need to read and write the structure memory
all by yourself(Which is not recommanded)
Below is the steps:

1). Initialize a byte[] which is the same length as the structure.
2). Initilize the structure byte[] array(Do some write to this byte[]
array)

3). Use Marshal.AllocCoTaskMem() to alloc a structure size in unmanaged
memory
4). Use Marshal.Copy(Byte[], Int32, IntPtr, Int32) to copy the managed
byte[] array to the unmanaged memory.
5). Use the function like this: public static extern int z9indqry(ref
IntPtr parm)
Click to expand...
Click to expand...
 
Hi Angel,

Please apply Dmitry's suggestion to see if resolve your problem. Thanks

Best regards,
Jeffrey Tan
Microsoft Online Partner Support
Get Secure! - www.microsoft.com/security
This posting is provided "as is" with no warranties and confers no rights.
 
Hi Angel,

Is your problem resolved?

Please feel free to feedback, I will help you.

Best regards,
Jeffrey Tan
Microsoft Online Partner Support
Get Secure! - www.microsoft.com/security
This posting is provided "as is" with no warranties and confers no rights.
 
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