Programming .NET Components, 2nd Edition

Implementing Custom Attributes

Implementing a custom attribute (and the accompanying reflection code to make use of it) is easy and straightforward. For example, suppose you want to provide a custom attribute that adds a color option to your classes and interfaces. The color is an enum defined as:

    public enum ColorOption {Red,Green,Blue};

The color attribute should also have:

• A default constructor that assigns ColorOption.Red to the target class or interface

• A parameterized constructor that accepts the color to assign to the target class or interface

• A property that accepts the color to assign to the target class or interface

Example C-3 shows the implementation of the ColorAttribute class.

Example C-3. Implementing a custom attribute

public enum ColorOption {Red,Green,Blue};

[AttributeUsage(AttributeTargets.Class|AttributeTargets.Interface)]
public class ColorAttribute : Attribute
{
   ColorOption m_Color;
   public ColorOption Color
   {
      get
      {
         return m_Color;
      }
      set
      {
         m_Color = value;
      }
   }
   public ColorAttribute()
   {
      Color = ColorOption.Red;
   }
   public ColorAttribute(ColorOption color)
   {
      this.Color = color;
   }
}

Before walking though Example C-3, here are a few examples that use the ColorAttribute custom attribute:

• Using the default constructor:

  [Color]
  public class MyClass1
  {}

• Using the parameterized constructor:

  [Color(ColorOption.Green)]
  public class MyClass2
  {}

• Using the property:

  [Color(Color = ColorOption.Blue)]
  public class MyClass3
  {}

• Using it on an interface:

  [Color] public interface IMyInterface {} ...