8.14 Attributes
C# is an imperative language, but like all imperative languages it does have
some declarative elements. For
example, the accessibility of a method in a class is specified by declaring
it public, protected,
internal, protected internal, or private. C# generalizes this capability,
so that programmers can
invent new kinds of declarative information, attach this declarative
information to various program entities,
and retrieve this declarative information at run-time. Programs specify
this additional declarative
information by defining and using attributes (§24).
For instance, a framework might define a HelpAttribute attribute that can
be placed on program elements
such as classes and methods, enabling developers to provide a mapping from
program elements to
documentation for them. The example
using System;
[AttributeUsage(AttributeTargets.All)]
public class HelpAttribute: Attribute
{
public HelpAttribute(string url) {
this.url = url;
}
public string Topic = null;
private string url;
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public string Url {
get { return url; }
}
}
defines an attribute class named HelpAttribute, or Help for short, that has
one positional parameter
(string url) and one named parameter (string Topic). Positional parameters
are defined by the
formal parameters for public instance constructors of the attribute class,
and named parameters are defined
by public non-static read-write fields and properties of the attribute
class.
The example
[Help("http://www.mycompany.com/./Class1.htm )]
public class Class1
{
[Help("http://www.mycompany.com/./Class1.htm , Topic = "F")]
public void F() {}
}
shows several uses of the attribute Help.
Attribute information for a given program element can be retrieved at
run-time by using reflection support.
The example
using System;
class Test
{
static void Main() {
Type type = typeof(Class1);
object[] arr = type.GetCustomAttributes(typeof(HelpAttribute),
true);
if (arr.Length == 0)
Console.WriteLine("Class1 has no Help attribute.");
else {
HelpAttribute ha = (HelpAttribute) arr[0];
Console.WriteLine("Url = {0}, Topic = {1}", ha.Url, ha.Topic);
}
}
}
checks to see if Class1 has a Help attribute, and writes out the associated
Topic and Url values if the
attribute is present.
End of informative text.