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Tip of the Day
Language: VB7
Expertise: Intermediate
Oct 28, 2002

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Retrieving the hi/low byte/word of a value, and other operations

As explained in the tip about the ARGBColor structure (look at the end of this tip for the link), we can define a structure and have its fields that point to the same memory address, but that read a different number of bytes. This makes easier to define a structure that allows us to set a value to a field, and then retrieve the single bytes/words of that value with the other fields. Here's how we can define this structure:

<StructLayout(LayoutKind.Explicit)> Structure IntegerTypes
    ' A 64-bit integer
    <FieldOffset(0)> Dim Long0 As Long
    ' Two 32-bit integers
    <FieldOffset(0)> Dim Integer0 As Integer
    <FieldOffset(4)> Dim Integer1 As Integer
    ' Four 16-bit integers
    <FieldOffset(0)> Dim Short0 As Short
    <FieldOffset(2)> Dim Short1 As Short
    <FieldOffset(4)> Dim Short2 As Short
    <FieldOffset(6)> Dim Short3 As Short
    ' Eight 8-bit integers
    <FieldOffset(0)> Dim Byte0 As Byte
    <FieldOffset(1)> Dim Byte1 As Byte
    <FieldOffset(2)> Dim Byte2 As Byte
    <FieldOffset(3)> Dim Byte3 As Byte
    <FieldOffset(4)> Dim Byte4 As Byte
    <FieldOffset(5)> Dim Byte5 As Byte
    <FieldOffset(6)> Dim Byte6 As Byte
    <FieldOffset(7)> Dim Byte7 As Byte

    ' Low byte of a word
    Function LowByte(ByVal Value As Long) As Byte
        Long0 = Value
        Return Byte0
    End Function

    ' High byte of a word
    Function HighByte(ByVal Value As Long) As Byte
        Long0 = Value
        Return Byte1
    End Function

    ' Low word of a doubleword
    Function LowWord(ByVal Value As Long) As Short
        Long0 = Value
        Return Short0
    End Function

    ' High word of a doubleword
    Function HighWord(ByVal Value As Long) As Short
        Long0 = Value
        Return Short1
    End Function

End Structure
Here's how you can test the IntegerTypes structure to retrieve the hi/low byte/word of a value, and to do other operations:

Dim it As IntegerTypes

it.Short0 = 517                  ' hex 0205
Console.WriteLine(it.Byte0)        ' => 5
Console.WriteLine(it.Byte1)        ' => 2

Console.WriteLine(it.LowByte(517))           ' => 5
Console.WriteLine(it.HighByte(517))          ' => 2
Console.WriteLine(it.LowWord(&HFFFF1000))    ' => 4096
Console.WriteLine(it.HighWord(&HFFFF1000))   ' => -1
--------------------------
Note: this code is taken from Francesco Balena's Programming Microsoft Visual Basic .NET book (MS Press 2002)
Francesco Balena
 
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