How to: Create a C/C++ Union by Using Attributes (C# and Visual Basic)

By using attributes you can customize how structs are laid out in memory. For example, you can create what is known as a union in C/C++ by using the StructLayout(LayoutKind.Explicit) and FieldOffset attributes.

In this code segment, all of the fields of TestUnion start at the same location in memory.

' Add an Imports statement for System.Runtime.InteropServices.

<System.Runtime.InteropServices.StructLayout( 
      System.Runtime.InteropServices.LayoutKind.Explicit)> 
Structure TestUnion
    <System.Runtime.InteropServices.FieldOffset(0)> 
    Public i As Integer

    <System.Runtime.InteropServices.FieldOffset(0)> 
    Public d As Double

    <System.Runtime.InteropServices.FieldOffset(0)> 
    Public c As Char

    <System.Runtime.InteropServices.FieldOffset(0)> 
    Public b As Byte 
End Structure

The following is another example where fields start at different explicitly set locations.

 ' Add an Imports statement for System.Runtime.InteropServices.

 <System.Runtime.InteropServices.StructLayout( 
      System.Runtime.InteropServices.LayoutKind.Explicit)> 
Structure TestExplicit
     <System.Runtime.InteropServices.FieldOffset(0)> 
     Public lg As Long

     <System.Runtime.InteropServices.FieldOffset(0)> 
     Public i1 As Integer

     <System.Runtime.InteropServices.FieldOffset(4)> 
     Public i2 As Integer

     <System.Runtime.InteropServices.FieldOffset(8)> 
     Public d As Double

     <System.Runtime.InteropServices.FieldOffset(12)> 
     Public c As Char

     <System.Runtime.InteropServices.FieldOffset(14)> 
     Public b As Byte 
 End Structure

The two integer fields, i1 and i2, share the same memory locations as lg. This sort of control over struct layout is useful when using platform invocation.

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