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C# in a Nutshell by Peter Drayton, Ted Neward, Ben Albahari

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Predefined Attributes

The .NET Framework makes extensive use of attributes for purposes ranging from simple documentation to advanced support for threading, remoting, serialization, and COM interop. These attributes are all defined in the FCL, and can be used, extended, and retrieved by your own code.

However, certain attributes are treated specially by the compiler and the runtime. Three attributes considered general enough to be defined in the C# specification are AttributeUsage, Conditional, and Obsolete. Other attributes, such as CLSCompliant, Serializable, and NonSerialized, are also treated specially.

The AttributeUsage Attribute

[AttributeUsage(target-enum
   [, AllowMultiple=[true|false]]?
   [, Inherited=[true|false]]?
   ] (for classes)

The AttributeUsage attribute is applied to a new attribute class declaration. It controls how the new attribute should be treated by the compiler — specifically, which set of targets (classes, interfaces, properties, methods, parameters, etc.) the new attribute can be specified on. This is true whether multiple instances of this attribute may be applied to the same target, and whether this attribute propagates to subtypes of the target.

target-enum is a bitwise mask of values from the System.AttributeTargets enum, which looks like this:

namespace System { [Flags] public enum AttributeTargets { Assembly = 0x0001, Module = 0x0002, Class = 0x0004, Struct = 0x0008, Enum = 0x0010, Constructor = 0x0020, Method = 0x0040, Property = 0x0080, Field = 0x0100, Event ...

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