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C# in a Nutshell by Peter Drayton, Ted Neward, Ben Albahari

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Mapping a Struct to a Binary Format

You can use the LayoutKind.Sequential argument with the StructLayout attribute to tell .NET to ensure that struct fields are laid out in the order in which they are defined. You can use this to create a struct that maps to a binary format, such as a GIF image. If you use this in conjunction with Win32 APIs for mapping a file into memory, you can define such a struct, load a file into memory, and superimpose the struct over that region of memory.

The next example features a struct, Gif89a, whose fields map directly to the layout of a GIF image. Because you can’t define a fixed-length array in a C# struct, use a sequence of three bytes to hold the GIF signature (“GIF”) and the version (“89a”). The Signature and Version properties each have an accessor method that turns these bytes into a string. The Dump() method displays the version and the dimensions of the GIF.

The Main() method uses three functions defined in kernel32.dll to map a GIF file into memory. Then, it casts that memory region (starting at baseAddress) to a Gif89a. This maps the struct onto the binary file format, eliminating the need to write code that parses each individual field of the GIF header.

// GIFInspects.cs - compile with /unsafe using System; using System.IO; using System.Runtime.InteropServices; [StructLayout(LayoutKind.Sequential)] public struct Gif89a { byte sig0, sig1, sig2; // Signature byte ver0, ver1, ver2; // Version public ushort ScreenWidth; public ushort ScreenHeight; ...

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