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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, where 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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To determine whether a file is hidden, you would execute code like this:
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String file = @"C:\Boot.ini"; FileAttributes attributes = File.GetAttributes(file); Console.WriteLine("Is {0} hidden {1}", file, (attributes & FileAttributes.Hidden) != 0);
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And here s code demonstrating how to change a file s attributes to read only and hidden:
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File.SetAttributes(@"C:\Boot.ini", FileAttributes.ReadOnly | FileAttributes.Hidden);
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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 flags, 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 to each of the symbols numeric values that map to individual bits. It s also highly recommended that you apply the System.FlagsAttribute custom attribute type to the enumerated type, as shown here: 244
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[Flags] // The C# compiler allows either "Flags" or "FlagsAttribute". enum Actions { Read = 0x0001, Write = 0x0002, Delete = 0x0004, Query = 0x0008, Sync = 0x0010 }
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Because Actions is an enumerated type, you can use all the methods described in the previous section when working with bit flag enumerated types. However, it would be nice if some of those functions behaved a little differently. For example, let s say you had the following code:
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Actions actions = Actions.Read | Actions.Write; // 0x0003 Console.WriteLine(actions.ToString()); // "Read, Write"
When ToString is called, it attempts to translate the numeric value into its symbolic equivalent. The numeric value is 0x0003, which has no symbolic equivalent. However, the ToString method detects the existence of the [Flags] attribute on the Actions type, and ToString now treats the numeric value not as a single value but as a set of bit flags. Because the 0x0001 and 0x0002 bits are set, ToString generates the following string: "Read, Write". If you remove the [Flags] attribute from the Actions type, ToString would produce the following string: "3". I discussed the ToString method in the previous section, and I showed that it offered three ways to format the output: "G" (general), "D" (decimal), and "X" (hex). When you re formatting an instance of an enumerated type by using the general format, the type is first checked to see whether the [Flags] attribute is applied to it. If this attribute is not applied, a symbol matching the numeric value is looked up and returned. If the [Flags] attribute is applied, a symbol matching each 1 bit is looked up and concatenated to a string; each symbol is separated by a comma. If you prefer, you could define the Actions type without the [Flags] attribute and still get the correct string by using the "F" format:
// [Flags] // Commented out now enum Actions { Read = 0x0001, Write = 0x0002, Delete = 0x0004, Query = 0x0008, Sync = 0x0010 } Actions actions = Actions.Read | Actions.Write; // 0x0003 Console.WriteLine(actions.ToString("F")); // "Read, Write"
As I mentioned in the previous section, the ToString method actually calls System.Enum s static Format method internally. This means that you can use the "F" format when calling the static Format method. If the numeric value contains 1 bit with no matching symbols, the returned string will contain just a decimal number and none of the symbols will appear in the string. Note that the symbols you define in your enumerated type don t have to be pure powers of 2. For example, the Actions type could define a symbol called All with a value of 0x001F. If an instance of the Actions type has a value of 0x001F, formatting the instance will produce a string that contains "All". The other symbol strings won t appear.
So far, I ve discussed how to convert numeric values into a string of flags. It s also possible to convert a string of comma delimited symbols into a numeric value by calling Enum s static Parse method. Here s some code demonstrating how to use this method:
// Because Query is defined as 8, a is initialized to 8. Actions a = (Actions) Enum.Parse(typeof(Actions), "Query", true); // Because Query and Read are defined, a is initialized to 9. Actions a = (Actions) Enum.Parse(typeof(Actions), "Query,Read", false); // Creates an instance of the Actions enum with a value of 28 Actions a = (Actions) Enum.Parse(typeof(Actions), "28", false); Console.WriteLine(a.ToString()); // "Delete, Query, Sync" // Creates an instance of the Actions enum with a value of 333 Actions a = (Actions) Enum.Parse(typeof(Actions), "333", false); Console.WriteLine(a.ToString()); // "333"
Again, when Parse is called, it checks whether the [Flags] custom attribute has been applied to the enumerated type. If the attribute exists, Parse splits the string into individual symbols, looks up each symbol, and bitwise ORs the corresponding numeric value into the resulting instance of the enumerated type. See 16 for more information about custom attributes. The [Flags] attribute affects how ToString, Format, and Parse behave. Compilers are also encouraged to look for this attribute and ensure that the enumerated type is being manipulated as a set of bit flags. For example, a compiler could allow only bit operations on the bit flag enumerated type and disallow other arithmetic operations, such as multiplication and division. The C# compiler ignores the [Flags] attribute completely; anything you can do with an enumerated type you can do with a bit flag enumerated type. When using a Visual Studio .NET form designer, you can use a property window to make various settings at design time. If some of these settings are enumerated types, the form designer checks whether the [Flags] attribute is applied to the type and displays the possible values accordingly.
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