Writing custom attributes in C#

• Download demo project - 2.64 KB

Introduction

This small sample tries to show how to write custom attributes in C#. This sample makes no sense, but demonstrates the implementation of custom attributes in a simple way.

Description

Attributes are implemented in classes which have to inherit from Attribute. Further on, you have to put the AttributeUsageAttribute in front of your class. The AttributeUsageAttribute describes on which targets your attribute can be applied to. These are the valid targets:

Module	Attribute can be applied to a module
All	Attribute can be applied to any application element
Assembly	Attribute can be applied to an assembly
Class	Attribute can be applied to a class
Constructor	Attribute can be applied to a constructor
Delegate	Attribute can be applied to a delegate
Enum	Attribute can be applied to an enumeration
Event	Attribute can be applied to an event
Field	Attribute can be applied to a field
GenericParameter	Attribute can be applied to a generic parameter
Interface	Attribute can be applied to an interface
Method	Attribute can be applied to a method

In addition, you can also add some named properties:

• AllowMultiple - Gets or sets a boolean value indicating whether more than one instance of the indicated attribute can be specified for a single program element.
• Inherited - Gets or sets a boolean value indicating whether the indicated attribute can be inherited by derived classes and overriding members.

In this caseb our attribute can be applied on classes:

[AttributeUsage(AttributeTargets.Class)]
public class MyCustomAttribute : Attribute
{
    private string attributevalue;
    
    public MyCustomAttribute(string AttributeValue)
    {
        attributevalue = AttributeValue;
    }
    public string AttributeValue
    {
        get
        {
            return attributevalue;
        }
    }
}

For an easy access on the value of our new attribute, we need to implement a second class from which classes can be inherited:

public class MyCustom
{
    public string AttributeValue
    {
        get
        {
            string Value = null; 
            Type type = this.GetType();
            MyCustomAttribute[] attribs = (MyCustomAttribute[])
              type.GetCustomAttributes(typeof(MyCustomAttribute), true);
            if (attribs.Length > 0)
            {
                MyCustomAttribute attrib = attribs[0];
                Value = attrib.AttributeValue;
            }
            return Value;
        }
    }
}

The last step is to test the new attribute:

[MyCustom("Test")]
class TestClass : MyCustom
{
}

class Program
{
    static void Main(string[] args)
    {
        ///Create a instance of our class
        TestClass test = new TestClass();
        ///Write the value of the Property to the
        ///console
        Console.WriteLine("The value of the attribute is: " + 
                          test.AttributeValue);
        Console.WriteLine("Press any key...");
        Console.ReadKey(false);
    }
}