Defining Enum Values for Frequent Combinations in C#

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Listing 816 Defining Enum Values for Frequent Combinations
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enum DistributedChannel { Transacted = 1, Queued = 2, Encrypted = 4, Persisted = 16, FaultTolerant = Transacted | Queued | Persisted }
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However, you should avoid enum values corresponding to things like Maximum as the last enum, because Maximum could be interpreted as a valid enum value To check whether a value is included within an enum use the SystemEnumIsDefined() method
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Advanced Topic: FlagsAttribute
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If you decide to use flag-type values, the enum should include FlagsAttribute The attribute appears in square brackets (see 14), just prior to the enum declaration, as shown in Listing 817
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Listing 817 Using FlagsAttribute
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// FileAttributes defined in SystemIO [Flags] // Decorating an enum with FlagsAttribute public enum FileAttributes { ReadOnly = 2^0, // 000000000000001 Hidden = 2^1, // 000000000000010 // } using System; using SystemDiagnostics; using SystemIO; class Program { public static void Main() { // string fileName = @"enumtesttxt"; FileInfo file = new FileInfo(fileName); fileAttributes = FileAttributesHidden | FileAttributesReadOnly; ConsoleWriteLine("\"{0}\" outputs as \"{1}\"", fileAttributesToString()Replace(",", " |"), fileAttributes); FileAttributes attributes = (FileAttributes) EnumParse(typeof(FileAttributes), fileAttributesToString()); ConsoleWriteLine(attributes); // } }
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The results of Listing 817 appear in Output 86
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Output 86
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"ReadOnly | Hidden" outputs as "ReadOnly, Hidden"
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The flag documents that the enum values can be combined Furthermore, it changes the behavior of the ToString() and Parse() methods For example, calling ToString() on an enum that is decorated with FlagsAttribute writes out the strings for each enum flag that is set In Listing 817, fileAttributesToString() returns ReadOnly, Hidden rather than the 3 it would have returned without the FileAttributes flag If two enum values are the same, the ToString() call would return the first value As mentioned earlier, however,
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you should use this with caution because it is not localizable Parsing a value from a string to the enum also works Each enum value identifier is separated by a comma It is important to note that FlagsAttribute does not automatically assign unique flag values or check that they have unique values Doing this wouldn't make sense, since duplicates and combinations are often desirable Instead, you must assign the values of each enum item explicitly
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Summary
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This chapter began with a discussion of how to define custom value types One of the key guidelines that emerge is to create immutable value types Boxing also was part of the value type discussion The idiosyncrasies introduced by boxing are subtle, and the vast majority of them lead to issues at execution time rather than at compile time Although it is important to know about these in order to try to avoid them, in many ways, focused attention on the potential pitfalls overshadows the usefulness and performance advantages of value types Programmers should not be overly concerned about using value types Value types permeate virtually every chapter of this book, and yet the idiosyncrasies do not I have staged the code surrounding each issue to demonstrate the concern, but in reality, these types of patterns rarely occur The key to avoiding most of them is to follow the guideline of not creating mutable value types; this is why you don't encounter many of them within the primitive types Perhaps the only issue to occur with some frequency is repetitive boxing operations within loops However, C# 20 greatly reduces the chance of this with the addition of generics, and even without that, performance is rarely affected enough to warrant avoidance until a particular algorithm with boxing is identified as a delay Furthermore, custom structs (value types) are relatively rare They obviously play an important role within C# development, but when compared to the number of classes, custom structs are rare and most frequently are defined in frameworks targeted at interoperating with managed code or a particular problem space In addition to demonstrating structs, this chapter introduced enums This is a standard construct available in most programming languages, and it deserves prominent consideration if you want to improve API usability and code readability The next chapter highlights more guidelines to creating well-formed types, both structs and otherwise It begins by looking at overriding the virtual members of objects and defining operatoroverloading methods These two topics apply to both structs and classes, but they are somewhat more critical in completing a struct definition and making it well formed
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