C# in Depth

Fixed-size buffers in unsafe code199checksum pragma to tell the C# compiler to use that checksum instead of calculatingone from the generated page.The syntax of the checksum pragma is#pragma checksum "filename" "{guid}" "checksum bytes"The GUID indicates which hashing algorithm has been used to calculate the checksum.The documentation for the CodeChecksumPragma class gives GUIDs for SHA-1 andMD5, should you ever wish to implement your own dynamic compilation frameworkwith debugger support.It’s possible that future versions of the C# compiler will include more pragmadirectives, and other compilers (such as the Mono compiler, mcs) could have theirown support for different features. Consult your compiler documentation for themost up-to-date information.The next feature is another one that you may well never use—but at the same time,if you ever do, it’s likely to make your life somewhat simpler.7.6 Fixed-size buffers in unsafe codeWhen calling into native code with P/Invoke, it’s not particularly unusual to find yourselfdealing with a structure that is defined to have a buffer of a particular lengthwithin it. Prior to C# 2, such structures were difficult to handle directly, even withunsafe code. Now, you can declare a buffer of the right size to be embedded directlywith the rest of the data for the structure.This capability isn’t just available for calling native code, although that is its primaryuse. You could use it to easily populate a data structure directly corresponding toa file format, for instance. The syntax is simple, and once again we’ll demonstrate itwith an example. To create a field that embeds an array of 20 bytes within its enclosingstructure, you would usefixed byte data[20];This would allow data to be used as if it were a byte* (a pointer to byte data),although the implementation used by the C# compiler is to create a new nested typewithin the declaring type and apply the new FixedBuffer attribute to the variableitself. The CLR then takes care of allocating the memory appropriately.One downside of this feature is that it’s only available within unsafe code: theenclosing structure has to be declared in an unsafe context, and you can only use thefixed-size buffer member within an unsafe context too. This limits the situations inwhich it’s useful, but it can still be a nice trick to have up your sleeve at times. Also,fixed-size buffers are only applicable to primitive types, and can’t be members ofclasses (only structures).There are remarkably few Windows APIs where this feature is directly useful. Thereare numerous situations where a fixed array of characters is called for—the TIME_ZONE_INFORMATION structure, for example—but unfortunately fixed-size buffers of charactersappear to be handled poorly by P/Invoke, with the marshaler getting in the way.Licensed to Rhona Hadida