根据类名创建实例

Source Code 最近有人问我一个问题，就是如何根据传入的字符串，由父类生成不同的子类。当然，肯定是不希望用switch语句了，也不希望每次创建instance时，都使用reflection。我觉得解决这个问题，应该使用工厂方法来处理。解释一下自己的code，如果有更好的方法，或者有什么可以改进的地方，还望请各位不吝赐教。

Shape class 的Create调用ShapeFactory返回一个Shape class
CreateShape根据传入到shape，使用反射创建一个ShapeFactory的instance，但是只创建一次，并且使用double check来保证多线程安全。所以这里的反射只会在需要的ShapeFactory不存在的时候才会创建，所以只是在第一次创建想要的shape时，才会使用到反射，以后都是直接使用instance

WinShape project: (class library)   

using System;
using System.Collections.Generic;
using System.Text;
using System.Reflection;

namespace WinShape
{
    public abstract class Shape
    {
        public abstract string Draw();

        public static Shape Create(string name)
        {
            return ShapeFactory.CreateShape(name);

        }
    }


    public class Line : Shape
    {

        public override string Draw()
        {
            return "Draw Line:  -----------------";
        }
    }

    public class Circle : Shape
    {
        public override string Draw()
        {
            return "Draw Circle: OOOOOOOOOOOOOOOO";
        }
    }

    public class Triangle : Shape
    {
        public override string Draw()
        {
            return "Draw Triangle: ^^^^^^^^^^^^^^";
        }
    }

    internal abstract class ShapeFactory
    {
        private static Dictionary<string, ShapeFactory> factories = new Dictionary<string, ShapeFactory>();

        protected abstract Shape GetShape();

        internal static Shape CreateShape(string shapeName)
        {
               //double check to prevent create multiple instances by multiple threads
               if(!factories.ContainsKey(shapeName)) 
               {
                   lock (factories)
                   {
                       if (!factories.ContainsKey(shapeName))
                       {
                           Assembly assembly = Assembly.GetExecutingAssembly();
                           Type t = assembly.GetType(string.Format("WinShape.ShapeFactory+{0}Factory", shapeName));
                           if (t == null)
                               return null;
                           ShapeFactory factory = System.Activator.CreateInstance(t) as ShapeFactory;
                           factories.Add(shapeName, factory);
                       }
                   }
               }

               ShapeFactory fac = factories[shapeName];
               return fac.GetShape();
        }

        private class LineFactory : ShapeFactory
        {
            protected override Shape  GetShape()
            {
                return new Line();
            }
        }

        private class CircleFactory : ShapeFactory
        {
            protected override Shape GetShape()
            {
                return new Circle();
            }
        }

        private class TriangleFactory : ShapeFactory
        {
            protected override Shape GetShape()
            {
                return new Triangle();
            }
        }

    }

    
    
}

window application:

Form1.Design.cs

  private System.Windows.Forms.Button btnCreate;
        private System.Windows.Forms.ComboBox cmbClassNames;
        private System.Windows.Forms.Button button1;
        private System.Windows.Forms.TextBox textBox1;
        private System.Windows.Forms.Label label1;
        private System.Windows.Forms.Label label2;

Form1.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Reflection;
using WinShape;

namespace CreateInstanceByClassName
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void btnCreate_Click(object sender, EventArgs e)
        {
            if (this.cmbClassNames.SelectedIndex < 0)
                return;

            string shapeName = this.cmbClassNames.Items[this.cmbClassNames.SelectedIndex].ToString();

            Shape shape = Shape.Create(shapeName);

            if (shape != null)
                this.textBox1.Text = shape.Draw();
            else
                this.textBox1.Text = "Null Reference";

        }

        private void Form1_Load(object sender, EventArgs e)
        {
            Assembly assembly = Assembly.Load("WinShape");

            Type shapeType = assembly.GetType("WinShape.Shape");

            Type[] types = assembly.GetTypes();

            foreach (Type t in types)
            {
                if (t.IsSubclassOf(shapeType) && !t.IsAbstract)
                {
                    this.cmbClassNames.Items.Add(t.Name);
                }
            }

        }

        private void button1_Click(object sender, EventArgs e)
        {
            this.Close();
            Application.Exit();
        }
    }
}