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

Using a union to retrieve the RGB components of a color

In VB.NET you can create what in C is called a union, i.e. a particular structure where you can access the same memory with different names. Here's how you declare such a structure by using the StructLayout attribute, and specifying that you want to define the structure layout explicitly, namely how the fields will be saved in memory within the type.

<StructLayout(LayoutKind.Explicit)> Structure ARGBColor
  <FieldOffset(0)> Dim Red As Byte
  <FieldOffset(1)> Dim Green As Byte
  <FieldOffset(2)> Dim Blue As Byte
  <FieldOffset(3)> Dim Alpha As Byte
  <FieldOffset(0)> Dim Value As Integer
End Structure
If you set the Value field, you're actually setting all the 4 bytes of the structure in a single step. Otherwise you can get and set the single bytes of the structure with the other fields. Here's how you can set the Value field of a variable of type ARGBColor to an hexadecimal value that represents a color, and then read the other fields to retrieve its RGB components:

' Split a color into its components.
Dim rgb As ARGBColor
rgb.Value = &H112233         ' This is equal to 1122867.
Console.WriteLine(String.Format("Red={0}, Green={1}, Blue={2}", rgb.Red, _
    rgb.Green, rgb.Blue))
    ' => Red=51, Green=34, Blue=17
--------------------------
Note: this code is taken from Francesco Balena's Programming Microsoft Visual Basic .NET book (MS Press 2002)
Francesco Balena
 
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