March 2002
Intermediate to advanced
864 pages
31h 8m
English
Retrieving attributes at runtime is done using reflection via one of System.Attribute’s GetCustomAttribute or GetCustomAttributes overloads. Here is an example
that uses and inspects the CrossRefAttribute using GetCustomAttribute:
// XRefTest.cs - apply and inspect a CrossRefAttribute
// Compile with: csc /r:XRef.dll XRefTest.cs
using System;
class Bar { }
[CrossRef(typeof(Bar), Description="Foos often hang around Bars")]
class Foo {
static void Main() {
// Retrieve the custom attribute from type Foo
Attribute attr =
Attribute.GetCustomAttribute(typeof(Foo),
typeof(CrossRefAttribute));
// Display the attribute.
if (attr != null) {
CrossRefAttribute cr = (CrossRefAttribute)attr;
Console.WriteLine(cr);
}
}
}This is one of the few circumstances where the difference between custom
attributes and pseudocustom attributes becomes apparent, since pseudocustom
attributes can’t be retrieved with GetCustomAttribute.
Here is another example that uses reflection to determine which attributes
are on a specific type:
using System;
[Serializable, Obsolete]
class Test {
static void Main() {
Type t = typeof(Test);
object[] caarr = Attribute.GetCustomAttributes(t);
Console.WriteLine("{0} has {1} custom attribute(s)",
t, caarr.Length);
foreach (object ca in caarr)
Console.WriteLine(ca);
}
}Although the Test class of the preceding example
has two attributes specified, the sample produces the following output:
Test has 1 custom attribute(s) System.ObsoleteAttribute ...