Builder Pattern

This pattern creates object based on the Interface, but also lets the subclass decide which class to instantiate. It also has finer control over the construction process.

There is a concept of Director in Builder Pattern implementation. The director actually creates the object and also runs a few tasks after that.

http://3.bp.blogspot.com/_DSP5FXX4Isw/S_RYTZhHAXI/AAAAAAAAC9o/L6J0-mjdR_I/s320/builder.JPG

public interface  IBuilder
    {
        string RunBulderTask1();
        string RunBuilderTask2();
    }
 
    public class  Builder1 : IBuilder
    {
 
        #region IBuilder Members
 
        public string  RunBulderTask1()
        {
            throw new  ApplicationException("Task1");
        }
 
        public string  RunBuilderTask2()
        {
            throw new  ApplicationException("Task2");
        }
 
        #endregion
    }
 
    public class  Builder2 : IBuilder
    {
        #region IBuilder Members
 
        public string  RunBulderTask1()
        {
            return "Task3";
        }
 
        public string  RunBuilderTask2()
        {
            return "Task4";
        }
 
        #endregion
    }
 
    public class  Director
    {
        public IBuilder CreateBuilder(int type)
        {
            IBuilder builder = null;
            if (type == 1)
                builder = new  Builder1();
            else
                builder = new  Builder2();
            builder.RunBulderTask1();
            builder.RunBuilderTask2();
            return builder;
        }
    }

In case of Builder pattern you can see the Director is actually using CreateBuilder to create the instance of the builder. So when the Bulder is actually created, we can also invoke a few common task in it.