How to Pass & Return Unsigned Integers to DLLs from VB

MORE INFORMATION

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).

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:

1. Start a new project in Visual Basic. Form1 is created by default.
2. 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
   						

3. Add a command button (Comamnd1) to Form1.
4. 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
   						

5. Run the program.