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.

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