Writing custom attributes in C#

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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:

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);
    }
}

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)