How To Convert Between Signed and Unsigned Numbers

Visual Basic for Applications only supports signed 2- and 4-byte Integers, while other languages, such as C, support both signed and unsigned Integers. This article provides conversion functions between signed and unsigned 4-byte Integers and between signed and unsigned 2-byte Integers.

In VBA, the range of Integer values is from -32768 to +32767, and for Long values from -2147483648 to 2147483647. When making API calls or calling a DLL written in C, you may be requested to pass in or receive unsigned values in the range of 0 to 65535 or 0 to 4294967296. The conversion functions provided below convert an unsigned Integer to a Long and also from an unsigned Long to a Double for purposes of input and display or other calculations.

The four functions are:

UnsignedToLong

The function takes a Double containing a value in the range of an unsigned Long and returns a Long that you can pass to an API that requires an unsigned Long.

LongToUnsigned

The function takes an unsigned Long from an API and converts it to a Double for display or arithmetic purposes.

UnsignedToInteger

The function takes a Long containing a value in the range of an unsigned Integer and returns an Integer that you can pass to an API that requires an unsigned Integer.

IntegerToUnsigned

The function takes an unsigned Integer from and API and converts it to a Long for display or arithmetic purposes. In the above example, the MyAPI API accepts an unsigned Long as a parameter and returns an unsigned Long as a result code. Because VBA only understands signed Longs, the DECLARE statement uses signed Longs. The UnsignedToLong range of an unsigned Long) into a signed Long for purposes of calling the API. The LongToUnsigned function performs the opposite conversion.

Without these functions, the input argument would have to have been specified as a signed Long, in this case -994967296, and the return value would have to be displayed as a signed value, possibly also as a negative number and non-intuitive.

Step by Step Example

1. Create a new VBA project.
2. Add the following code to a Module:

         Option Explicit
   
         Private Const OFFSET_4 = 4294967296#
         Private Const MAXINT_4 = 2147483647
         Private Const OFFSET_2 = 65536
         Private Const MAXINT_2 = 32767
   
         Function UnsignedToLong(Value As Double) As Long
           If Value < 0 Or Value >= OFFSET_4 Then Error 6 ' Overflow
           If Value <= MAXINT_4 Then
             UnsignedToLong = Value
           Else
             UnsignedToLong = Value - OFFSET_4
           End If
         End Function
   
         Function LongToUnsigned(Value As Long) As Double
           If Value < 0 Then
             LongToUnsigned = Value + OFFSET_4
           Else
             LongToUnsigned = Value
           End If
         End Function
   
         Function UnsignedToInteger(Value As Long) As Integer
           If Value < 0 Or Value >= OFFSET_2 Then Error 6 ' Overflow
           If Value <= MAXINT_2 Then
             UnsignedToInteger = Value
           Else
             UnsignedToInteger = Value - OFFSET_2
           End If
         End Function
   
         Function IntegerToUnsigned(Value As Integer) As Long
           If Value < 0 Then
             IntegerToUnsigned = Value + OFFSET_2
           Else
             IntegerToUnsigned = Value
           End If
         End Function
   						

3. Visual Basic only: Run the project and pause it.
4. Type the following in the Immediate/Debug Window:

         ?UnsignedToLong(3300000000)
         ?LongToUnsigned(-55)
         ?UnsignedToInteger(45000)
         ?IntegerToUnsigned(-3000)

5. You will get the following results:
   -994967296
   4294967241
   -20536
   62536

For additional information, please see the following article in the Microsoft Knowledge Base: