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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