Attributes in C#

Attributes are a powerful method of associating declarative information with C# code (types, methods, properties, etc.). Once associated with a program entity, the attribute can be queried at run time and used in many ways. C# attributes are not to be confused with HTML attributes or object-oriented member variables which too are often called attributes.

An attribute is a tag of meta-data that can be applied to a particular construct. For example you can apply an attribute to a class, a method, or a property. At first, we must understand, that attributes are "just" classes that derives from System.Attribute.

For defining a new attribute, we must write a class. Let's look an example - Color attribute for figures

    public class ColorAttribute : Attribute
    {
    public string whichColor;
        public ColorAttribute(string color)
        {
	this.whichColor = color;
        }
    }

Le's see how we use it :

    [Color("No color")] // figures have no color by default
    public class Figure
    {
    }

    [Color("Red")]// All rectangles are to be colored red
    public class Rectangle : Figure
    {
    }

    [Color("Blue")]// Circles are to be colored blue
    public class Circle : Figure
    {
    }

Attributes can be queried during run-time:

public string WhichColorIsTheFigure(Figure fig)
 {
  Type type=fig.GetType();// Get object type
  object obj=type.GetCustomAttributes()[0];// Get first attribute
  if(obj is ColorAttribute)
  return ((ColorAttribute)obj).whichColor;
  }