When compiling an application that calls functions or subroutines in a DLL and that passes large User Defined Types to the DLL, you may receive the following error message:

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The above error is the result of a bug in the Visual Basic compiler when allocating memory for passing User Defined Types (UDTs) to functions or subroutines in DLLs. The error may occur when this amount exceeds approximately 64KB.

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You can work around this problem by changing the way that such UDTs are passed to the DLL. When you pass a UDT to a DLL, you are actually passing a pointer to the first memory location of the UDT. Another way to pass this pointer is by copying the UDT to a Byte array and passing the first element of the Byte array by reference.

NOTE: You will need to calculate the size of this Byte array manually, taking into consideration the issues relating to byte-alignment.

For more information regarding byte-alignment, refer to Section 6 of the VB5DLL.DOC document located in the Tools\Docs folder of the Visual Basic CD-ROM.

Because the Visual Basic run-time library converts the strings from UNICODE to ANSI when using the CopyMemory function below, you should only allocate 1 byte for each character in your fixed-length strings.

Assuming you have the following UDT and Declare statement for your DLL:

   Type LargeUDT
      nAge as Integer
      sName As String * 4000
      lMiles as Long
   End Type

   Dim aLargeUDT As LargeUDT

   Declare Sub MySub Lib "MYDLL.DLL" (x As LargeUDT)
				

Follow these steps to implement a workaround to the problem:

1.	Manually calculate the size necessary for the UDT: Const UDT_SIZE = 4008
2.	Create the following Declare within your application (a Windows A function to copy memory from one location to another): Declare Sub CopyMemory Lib "KERNEL32" Alias "RtlMoveMemory" ( _ lpvDest as Any, lpvSource as Any, ByVal cbCopy as Long)
3.	Change the declaration of the subroutine within the DLL to accept 1 byte by reference in place of the UDT argument: Declare Sub MySub Lib "MYDLL.DLL" (x As Byte)
4.	Create a Byte Array to store the User Defined Type. Re-dimension the UDT using the manually-calculated constant UDT_SIZE in step 1: Dim ab() as byte Redim ab(UDT_SIZE)
5.	When calling the subroutine in the DLL, you must first call CopyMemory to copy the UDT into the Byte array. Next, call the subroutine, passing it the first element of the Byte array. Finally, call CopyMemory again to copy the Byte array back into the UDT: CopyMemory ab(0), aLargeUDT, UDT_SIZE MySub ab(0) CopyMemory aLargeUDT, ab(0), UDT_SIZE

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Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. We are researching this bug and will post new information here in the Microsoft Knowledge Base as it becomes available.

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      Option Explicit
      Private Type LargeUDT
         a As String * 4000
      End Type

      Dim aLargeUDT As LargeUDT

      Private Declare Sub MySub Lib "MYDLL.DLL" (x As LargeUDT)

      Private Sub Form_Load()
         MySub aLargeUDT
         MySub aLargeUDT
         MySub aLargeUDT
         MySub aLargeUDT
         MySub aLargeUDT

         MySub aLargeUDT
         MySub aLargeUDT
         MySub aLargeUDT
         'MySub aLargeUDT
      End Sub
						

For additional information on calling subroutines and functions in DLLs, please see the following document:

VB5DLL.DOC, Visual Basic 5 CD-ROM (Professional and Enterprise Editions Only), in the following folder ...\TOOLS\DOCS\.

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