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15 Enumerated Types and Bit Flags
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Important The IsDefined method is very convenient, but you must use it with caution . First,
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IsDefined always does a case-sensitive search, and there is no way to get it to perform a caseinsensitive search . Second, IsDefined is pretty slow because it uses reflection internally; if you
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wrote code to manually check each possible value, your application s performance would most certainly be better . Third, you should really use IsDefined only if the enum type itself is defined in the same assembly that is calling IsDefined . Here s why: Let s say the Color enum is defined in one assembly and the SetColor method is defined in another assembly . The SetColor method calls IsDefined, and if the color is White, Red, Green, Blue, or Orange, SetColor performs its work . However, if the Color enum changes in the future to include Purple, SetColor will now allow Purple, which it never expected before, and the method might execute with unpredictable results .
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Finally, the System.Enum type offers a set of static ToObject methods that convert an instance of a Byte, SByte, Int16, UInt16, Int32, UInt32, Int64, or UInt64 to an instance of an enumerated type . Enumerated types are always used in conjunction with some other type . Typically, they re used for the type s method parameters or return type, properties, and fields . A common question that arises is whether to define the enumerated type nested within the type that requires it or to define the enumerated type at the same level as the type that requires it . If you examine the FCL, you ll see that an enumerated type is usually defined at the same level as the class that requires it . The reason is simply to make the developer s life a little easier by reducing the amount of typing required . So you should define your enumerated type at the same level unless you re concerned about name conflicts .
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Programmers frequently work with sets of bit flags . When you call the System.IO.File type s GetAttributes method, it returns an instance of a FileAttributes type . A FileAttributes type is an instance of an Int32-based enumerated type, in which each bit reflects a single attribute of the file . The FileAttributes type is defined in the FCL as follows:
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[Flags, Serializable] public enum FileAttributes { ReadOnly = 0x0001, Hidden = 0x0002, System = 0x0004, Directory = 0x0010, Archive = 0x0020, Device = 0x0040, Normal = 0x0080, Temporary = 0x0100, SparseFile = 0x0200, ReparsePoint = 0x0400, Compressed = 0x0800, Offline = 0x1000, NotContentIndexed = 0x2000, Encrypted = 0x4000 }
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Part III Essential Types
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To determine whether a file is hidden, you would execute code like this:
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String file = Assembly.GetEntryAssembly().Location; FileAttributes attributes = File.GetAttributes(file); Console.WriteLine("Is {0} hidden {1}", file, (attributes & FileAttributes.Hidden) != 0);
Note The Enum class defines a HasFlag method defined as follows:
public Boolean HasFlag(Enum flag);
Using this method, you could rewrite the call to Console.WriteLine like this:
Console.WriteLine("Is {0} hidden {1}", file, attributes.HasFlag(FileAttributes.Hidden));
However, I recommend that you avoid the HasFlag method for this reason: Since it takes a parameter of type Enum, any value you pass to it must be boxed, requiring a memory allocation .
And here s code demonstrating how to change a file s attributes to read-only and hidden:
File.SetAttributes(file, FileAttributes.ReadOnly | FileAttributes.Hidden);
As the FileAttributes type shows, it s common to use enumerated types to express the set of bit flags that can be combined . However, although enumerated types and bit flags are similar, they don t have exactly the same semantics . For example, enumerated types represent single numeric values, and bit flags represent a set of bits, some of which are on, and some of which are off . When defining an enumerated type that is to be used to identify bit flags, you should, of course, explicitly assign a numeric value to each symbol . Usually, each symbol will have an individual bit turned on . It is also common to see a symbol called None defined with a value of 0, and you can also define symbols that represent commonly used combinations (see the ReadWrite symbol below) . It s also highly recommended that you apply the System.FlagsAttribute custom attribute type to the enumerated type, as shown here:
[Flags] // The C# compiler allows either "Flags" or "FlagsAttribute". internal enum Actions { None = 0 Read = 0x0001, Write = 0x0002, ReadWrite = Actions.Read | Actions.Write, Delete = 0x0004, Query = 0x0008, Sync = 0x0010 }
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