How to: Create a C/C++ Union by Using Attributes (C# and Visual Basic)
Visual Studio 2012
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.
[System.Runtime.InteropServices.StructLayout(LayoutKind.Explicit)] struct TestUnion { [System.Runtime.InteropServices.FieldOffset(0)] public int i; [System.Runtime.InteropServices.FieldOffset(0)] public double d; [System.Runtime.InteropServices.FieldOffset(0)] public char c; [System.Runtime.InteropServices.FieldOffset(0)] public byte b; }
The following is another example where fields start at different explicitly set locations.
[System.Runtime.InteropServices.StructLayout(LayoutKind.Explicit)] struct TestExplicit { [System.Runtime.InteropServices.FieldOffset(0)] public long lg; [System.Runtime.InteropServices.FieldOffset(0)] public int i1; [System.Runtime.InteropServices.FieldOffset(4)] public int i2; [System.Runtime.InteropServices.FieldOffset(8)] public double d; [System.Runtime.InteropServices.FieldOffset(12)] public char c; [System.Runtime.InteropServices.FieldOffset(14)] public byte b; }
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.
