C#
Custom attributes
Learn to write your own attribute class with AttributeUsage.
Writing a custom attribute
To write a custom attribute, you define a class that derives from Attribute. For convenience the name should end with ...Attribute. Example: public class NoteAttribute : Attribute { }.
You can add properties and a constructor to carry data. With [AttributeUsage] you specify where it can apply (class, method, property) and whether multiple uses are allowed.
AttributeUsage
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = true)] specifies where an attribute can apply and whether multiple uses are allowed. The AttributeTargets enumeration lists the target kinds.
Common targets: Class, Method, Property, Field, Assembly. AllowMultiple = true allows applying the attribute more than once to the same element. Inherited = true lets it propagate to derived classes.
Exercise Define NoteAttribute and apply it
Define the NoteAttribute class in the skeleton below: mark it with [AttributeUsage(AttributeTargets.Class)], derive from Attribute, add a Text property and a constructor taking string text. Apply [Note("Important")] to the Sample class. In Main, read the attributes with typeof(Sample).GetCustomAttributes(true) and print each one's type name with Console.WriteLine.
Show example solution
[AttributeUsage(AttributeTargets.Class)] public class NoteAttribute : Attribute { public string Text { get; set; } public NoteAttribute(string text) { Text = text; } }. Add [Note("Important")] to Sample. In Main read with typeof(Sample).GetCustomAttributes(true) and print with foreach (var a in attrs) { Console.WriteLine(a.GetType().Name); }.
Example solution:
using System;
[AttributeUsage(AttributeTargets.Class)]
public class NoteAttribute : Attribute
{
public string Text { get; set; }
public NoteAttribute(string text) { Text = text; }
}
[Note("Important")]
class Sample { }
class Program
{
static void Main()
{
var attrs = typeof(Sample).GetCustomAttributes(true);
foreach (var a in attrs)
{
Console.WriteLine(a.GetType().Name);
}
}
}
Test yourself
-
Complete the attribute class declaration
public class MyAttribute : ____ { }
Show answer
public class MyAttribute : Attribute { }
-
Which class does a custom attribute derive from?
- MemberInfo
- Attribute
- Type
- Object
Show answer
Attribute — Correct. A custom attribute derives from Attribute.
-
Match the element to its meaning
- AttributeUsage
- Specifies target
- AttributeTargets
- Target kinds
- AllowMultiple
- Multiple usage
- Inherited
- Applies to derived classes
Next lesson: Reading attributes
