How to: Create a C/C++ Union by Using Attributes (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.

Example

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  

Example

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.

See also