14.6 Retrieving a Custom Attribute at Runtime
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
This demonstrates how the Serializable attribute
(a pseudocustom attribute) isn't accessible via
reflection, while the Obsolete attribute (a custom
attribute) is.
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