Passing a structure with arrays of fixed length strings to a DLL

Passing a structure with arrays of fixed length strings to a DLL

• alok ranjan

  The VBFixedString and VBFixedArray are meant to be used with VB File IO routines (FileOpen, FilePut, etc).

  For passing structures with fixed size strings through P/Invoke (Declare statement method) you probably need to use the ByValTStr type on the marshalas attribute instead of ByValArray:

  MarshalAs(Runtime.InteropServices.UnmanagedType.ByValTStr,sizeconst:=5)

  I don't really remember if the Ansi vs Unicode definition on declares affects the size of the characters here - you may also need to add a StructLayoutAttribute to the structure if you can't get the correct Char size.

  
  
  

• Malla

  Hi,

  ByValTStr does not work - this is as per the Microsoft Press VB6 to .NEt upgrade book. My testing supports this - changing the strings to char arrays and using the VBFixedString attribute worked.

  Regards

  Michael Green

  
  

• alan_k

  My test dll relevant code:

  struct repeatingRecord
  {
  char field1[2];
  char field2[5];
  };
  struct C1
  {
  char field1[7];
  repeatingRecord records[10];
  };
  extern "C"
  {
  TESTDLL_API int fnTestDLL(C1 arg);
  }

  TESTDLL_API int fnTestDLL(C1 arg)
  {
  MessageBoxA(0,arg.field1,"Title1", MB_OK);
  MessageBoxA(0,arg.records[9].field1 ,"Title2", MB_OK);
  MessageBoxA(0,arg.records[9].field2 ,"Title3", MB_OK);
  return 42;
  }

  On the vb side:

  Imports System.Runtime.InteropServices
  Module Module1
  
  Structure repeatingRecord
  <MarshalAs(UnmanagedType.ByValTStr, sizeconst:=2)> Dim field1 As String
  <MarshalAs(UnmanagedType.ByValTStr, sizeconst:=5)> Dim field2 As String
  End Structure

  Structure maintype
  <MarshalAs(UnmanagedType.ByValTStr, sizeconst:=7)> Dim field1 As String
  <MarshalAs(UnmanagedType.ByValArray, sizeconst:=10)> Dim records() As repeatingRecord
  End Structure

  Declare Ansi Function fnTestDLL Lib "C:\Documents and Settings\amoura\My Documents\Visual Studio 2005\Projects\TestDLL\debug\testdll.dll" (ByVal arg As maintype) As Integer

  Sub Main()
  Dim c As maintype
  c.field1 = "123456"
  c.records = New repeatingRecord(10) {}
  For i As Integer = 0 To 9
  c.records(i).field1 = "1"
  c.records(i).field2 = "1234"
  Next
  fnTestDLL(c)
  End Sub

  End Module

  This seems to work as expected - Let me know if this still isn't what you're looking for.

  
  
  

• Francy

  Hi,

  Given I've already had a look at this area (hence my comments on VBFixedString etc) a little more precise guidance would be appreciated e.g. a few URLs.......

  Regards

  Michael

  
  

• Karthick Sukumaran

  The following may help show the change from VB6 to VB.Net with regard to calling API's

  This is a simple example sending a file to the recycle bin but it defines a structure used in the API.

  http://www.developerfusion.co.uk/show/4334/

  fixed length strings and API functions

  http://groups.google.com/group/microsoft.public.dotnet.languages.vb/browse_thread/thread/282a4b3969498eb5/f7e6db7474437cb2%23f7e6db7474437cb2 sa=X&oi=groupsr&start=0&num=3

  
  

• Jvalente

  Hi,

  I still can't get this to work.

  The .NET structures are as follows. The only way I can get this to work is if I convert the array of sub-structures to dummy fields in the main structure. Then the data is correctly passed to the API. If I pass as is I get a 'Parameter Incorrect' error message.

  Structure repeatingRecord

  <VBFixedString(2), System.Runtime.InteropServices.MarshalAs(System.Runtime.InteropServices.UnmanagedType.ByValArray, SizeConst:=2)> _

  Public field1 As String

  <VBFixedString(5), System.Runtime.InteropServices.MarshalAs(System.Runtime.InteropServices.UnmanagedType.ByValArray, SizeConst:=5)> _

  Public field2 As String

  End Structure

  Structure maintype

  <VBFixedString(7), System.Runtime.InteropServices.MarshalAs(System.Runtime.InteropServices.UnmanagedType.ByValArray, SizeConst:=7)> _

  Public field1 As String

  <VBFixedArray(10)> Dim records() As repeatingRecord

  'UPGRADE_TODO: "Initialize" must be called to initialize instances of this structure. Click for more: 'ms-help://MS.VSCC.2003/commoner/redir/redirect.htm keyword="vbup1026"'

  Public Sub Initialize()

  ReDim records(10)

  End Sub

  End Structure

  
  

• Grozen

  Hi,

  sorry - but neither of these links address the core issue here which is how to pass to correctly arrays of subordinate structures.

  Regards

  Michael Green

  
  

• Varsha

  I am having a similar problem. Could you please reply and paste in a snippet of code that you used to convert the structure to a char array
  
  

• milTash

  Micheal...

  Take a look at interop services and Marshalling. There are way to describe those string that you won't find anywhere else.

  
  
  

• Margriet

  The Help files is what I was referring to......

  I refer to them often and the fora have settle into the phrase "have a look at" to mean either help or the object browser. In this case, help will be better.

  
  
  
• Passing a structure with arrays of fixed length strings to a DLL