How to Pass & Return Unsigned Integers to DLLs from VB

ID: Q112673

The information in this article applies to:

Microsoft Visual Basic Standard and Professional Editions for Windows, versions 1.0, 2.0, 3.0

SUMMARY

Visual Basic stores its integer variables in an 8-bit data field, as does C. Visual Basic uses signed integers only, so it reserves one of the bits as a sign bit. In C, you have the choice of an unsigned integer (the variable ranges from 0 to 65536) or a signed integer (the variable ranges from -32767 to +32767 as do Visual Basic integer variables).

MORE INFORMATION

Step-by-Step Example

Follow a process similar to the following to pass a value greater than 32767 as an integer from Visual Basic to a dynamic link library (DLL) that is expecting an unsigned integer or to return an integer value that is outside the range of valid Visual Basic integers:

Start a new project in Visual Basic. Form1 is created by default.

Add the following code to the general declarations section of Form1. Note that you must actually have a DLL that takes an unsigned integer as a parameter.


   ' MyLong is a function in a DLL that takes an unsigned integer as a
   ' parameter and returns the same value passed in. To run this sample you
   ' will have to create the MYLONG function. Enter the following Declare
   ' statement as one, single line:
   Declare Function MyLong Lib "MyLong.DLL" (ByVal iInt AS Integer)
      As Integer

Add a command button (Comamnd1) to Form1.

Add the following code to the Command1_Click event:


   Sub Command1_Click()
      Dim lValue As Long
      Dim i As Integer, w As Integer
      ' Initialize lvalue:
      lValue = 40000
      If lValue > 32767 Then
         w = lValue - 65536
      Else
         w = lValue     ' Just pass it on
      End If
      ' Call a DLL that is expecting an unsigned integer.
      ' For this example, the MyLong function will return
      ' the same value passed in.
      i = MyLong(w)

      ' Convert returned value:
      If i < 0 Then
         lValue = 65536 + i
      Else
         lValue = i
      End If
      ' Display the results:
      Print Str(lValue)
   End Sub

Run the program.

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