Attributes in C#

This example shows the use of class and method attributes.
The IsTested class is a user-defined custom attribute class. It can be applied to any declaration including
- types (struct, class, enum, delegate)
- members (methods, fields, events, properties, indexers).
It is used with no arguments.

// AttributesTutorial.cs
using System;
using System.Reflection;
using System.Collections;

public class IsTestedAttribute : Attribute
{
    public override string ToString()
    {
        return "Is Tested";
    }
}

The AuthorAttribute class is a user-defined attribute class.It can be applied to classes and struct declarations only. It takes one unnamed string argument (the author's name). It has one optional named argument Version, which is of type int.

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)]
public class AuthorAttribute : Attribute
{
    // This constructor specifies the unnamed arguments to the attribute class.
    public AuthorAttribute(string name)
    {
        this.name = name;
        this.version = 0;
    }


    // This property is readonly (it has no set accessor)
    // so it cannot be used as a named argument to this attribute.
    public string Name
    {
        get
        {
            return name;
        }
    }


    // This property is read-write (it has a set accessor)
    // so it can be used as a named argument when using this
    // class as an attribute class.
    public int Version
    {
        get
        {
            return version;
        }
        set
        {
            version = value;
        }
    }


    public override string ToString()
    {
        string value = "Author : " + Name;
        if (version != 0)
        {
            value += " Version : " + Version.ToString();
        }
        return value;
    }


    private string name;
    private int version;
}

Here you attach the AuthorAttribute user-defined custom attribute to the Account class. The unnamed string argument is passed to the AuthorAttribute class's constructor when creating the attributes.

[Author("Joe Programmer")]
class Account
{
    // Attach the IsTestedAttribute custom attribute to this method.
    [IsTested]
    public void AddOrder(Order orderToAdd)
    {
        orders.Add(orderToAdd);
    }


    private ArrayList orders = new ArrayList();
}


// Attach the AuthorAttribute and IsTestedAttribute custom attributes 
// to this class.
// Note the use of the 'Version' named argument to the AuthorAttribute.
[Author("Jane Programmer", Version = 2), IsTested()]
class Order
{
    // add stuff here ...
}


class MainClass
{
    private static bool IsMemberTested(MemberInfo member)
    {
        foreach (object attribute in member.GetCustomAttributes(true))
        {
            if (attribute is IsTestedAttribute)
            {
                return true;
            }
        }
        return false;
    }


    private static void DumpAttributes(MemberInfo member)
    {
        Console.WriteLine("Attributes for : " + member.Name);
        foreach (object attribute in member.GetCustomAttributes(true))
        {
            Console.WriteLine(attribute);
        }
    }


    public static void Main()
    {
        // display attributes for Account class
        DumpAttributes(typeof(Account));


        // display list of tested members
        foreach (MethodInfo method in (typeof(Account)).GetMethods())
        {
            if (IsMemberTested(method))
            {
                Console.WriteLine("Member {0} is tested!", method.Name);
            }
            else
            {
                Console.WriteLine("Member {0} is NOT tested!", method.Name);
            }
        }
        Console.WriteLine();


        // display attributes for Order class
        DumpAttributes(typeof(Order));


        // display attributes for methods on the Order class
        foreach (MethodInfo method in (typeof(Order)).GetMethods())
        {
            if (IsMemberTested(method))
            {
                Console.WriteLine("Member {0} is tested!", method.Name);
            }
            else
            {
                Console.WriteLine("Member {0} is NOT tested!", method.Name);
            }
        }
        Console.WriteLine();
    }
}