Information
The topic you requested is included in another documentation set. For convenience, it's displayed below. Choose Switch to see the topic in its original location.

How to: Create a C/C++ Union Using Attributes (C# Programming Guide)

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

Was this page helpful?
(1500 characters remaining)
Thank you for your feedback

Community Additions

Show:
© 2015 Microsoft