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

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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.

[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.

See Also

Concepts

C# Programming Guide

Reference

Reflection (C# Programming Guide)

Attributes (C# Programming Guide)

Using Attributes (C# Programming Guide)

Disambiguating Attribute Targets (C# Programming Guide)

Creating Custom Attributes (C# Programming Guide)

Accessing Attributes With Reflection (C# Programming Guide)

System.Reflection

Attribute