C# 4 and .NET 4

Custom attributes ❘ 335
error. For example, if the attribute class indicates that the attribute can be applied only to classes, but you
have applied it to a struct definition, a compilation error will occur.
To continue with the example, assume that you have defined the FieldName attribute like this:
[AttributeUsage(AttributeTargets.Property,
AllowMultiple=false,
Inherited=false)]
public class FieldNameAttribute: Attribute
{
private string name;
public FieldNameAttribute(string name)
{
this.name = name;
}
}
The following sections discuss each element of this definition.
attributeUsage attribute
The first thing to note is that the attribute class itself is marked with an attribute — the System
.AttributeUsage attribute. This is an attribute defined by Microsoft for which the C# compiler provides
special support. (You could argue that AttributeUsage isn’t an attribute at all; it is more like a
meta-attribute, because it applies only to other attributes, not simply to any class.) The primary purpose
of AttributeUsage is to identify the types of program elements to which your custom attribute can be
applied. This information is given by the first parameter of the AttributeUsage attribute — this parameter
is mandatory, and is of an enumerated type, AttributeTargets. In the previous example, you have
indicated that the FieldName attribute can be applied only to properties, which is fine, because that is
exactly what you have applied it to in the earlier code fragment. The members of the AttributeTargets
enumeration are:
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All
Assembly
Class
Constructor
Delegate
Enum
Event
Field
GenericParameter (.NET 2.0 and higher only)
Interface
Method
Module
Parameter
Property
ReturnValue
Struct
This list identifies all the program elements to which you can apply attributes. Note that when applying the
attribute to a program element, you place the attribute in square brackets immediately before the element.
However, two values in the preceding list do not correspond to any program element: Assembly and
Module. An attribute can be applied to an assembly or module as a whole instead of to an element in your
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