StructLayout
Learn to control memory layout with StructLayout.
What is StructLayout?
The StructLayout attribute controls the memory layout of a struct or class. [StructLayout(LayoutKind.Sequential)] places fields in declaration order; this matches C-style structs and is the most common option.
With LayoutKind.Explicit and [FieldOffset(...)] you can place each field by hand. This is critical when calling native APIs that expect a C-style struct memory layout.
Explicit and FieldOffset
With LayoutKind.Explicit each field gets a position such as [FieldOffset(0)]. This lets a field share memory with fields of different types, producing a union.
For example a 4-byte int and a 4-byte float can share the same offset. This technique brings C union behavior to C# and is useful in low-level data structures.
Exercise Define a Point struct
Define [StructLayout(LayoutKind.Sequential)] struct Point { public int X; public int Y; }. In Main write Point p; p.X = 3; p.Y = 4; and print the sum with Console.WriteLine(p.X + p.Y);.
Show example solution
Point p; p.X = 3; p.Y = 4; Console.WriteLine(p.X + p.Y);
Example solution:
using System;
using System.Runtime.InteropServices;
[StructLayout(LayoutKind.Sequential)]
struct Point
{
public int X;
public int Y;
}
class Program
{
static void Main()
{
Point p;
p.X = 3;
p.Y = 4;
Console.WriteLine(p.X + p.Y);
}
}Test yourself
-
Which LayoutKind orders fields by declaration?
- Auto
- Explicit
- Random
- Sequential
Show answer
Sequential — Correct. Sequential places fields in declaration order.
-
Complete the StructLayout attribute
[____(LayoutKind.Sequential)] struct Point { public int X; public int Y; }Show answer
[StructLayout(LayoutKind.Sequential)] struct Point { public int X; public int Y; } -
Match the LayoutKind types
- LayoutKind.Sequential
- Declaration order
- LayoutKind.Explicit
- Manual offset
- FieldOffset(0)
- Offset 0
- StructLayout
- Layout attribute