C#簡單工廠模式（文件案例）

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace 讀文件案例
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("請輸入文件名");
            string fileName = Console.ReadLine();//獲得用戶輸入
            File file = Factory.GetFileName(fileName);
            file.OpenFile();
            Console.ReadKey();
        }
    }
    /// <summary>
    /// 父類，等待子類實現
    /// </summary>
    public abstract class File
    {
        private string _fileName;//文件名
        public string FileName
        {
            get { return _fileName; }
            set { _fileName = value; }
        }
        private string _extension;//后綴名
        public string Extension
        {
            get { return _extension; }
            set { _extension = value; }
        }
        /// <summary>
        /// 構造函數
        /// </summary>
        /// <param name="fileName"></param>
        public File(string fileName)
        {
            //在本類中，用私有字段即可，不必要使用屬性來保護對應的字段
            this._fileName = Path.GetFileNameWithoutExtension(fileName);
            this._extension = Path.GetExtension(fileName);
        }
        public abstract void OpenFile();
    }
    public class TxtFile : File
    {
        /// <summary>
        /// 子類構造函數，調用父類構造函數為字段賦值
        /// </summary>
        /// <param name="fileName"></param>
        public TxtFile(string fileName)
            : base(fileName)
        { }
        public override void OpenFile()
        {
            //此時就需要用屬性來保護字段了
            Console.WriteLine("文件名為{0}，后綴名為{1}", this.FileName, this.Extension);
        }
    }
    public class AviFile : File
    {
        public AviFile(string fileName)
            : base(fileName)
        { }
        public override void OpenFile()
        {
            Console.WriteLine("文件名為{0}，后綴名為{1}", this.FileName, this.Extension);
        }
    }
    public class Factory
    {
        public static File GetFileName(string fileName)
        {
            File file = null;
            switch (Path.GetExtension(fileName).ToLower())
            {
                case "txt":
                case ".txt": file = new TxtFile(fileName); break;
                case "avi":
                case ".avi": file = new AviFile(fileName); break;
                //父類是虛方法的寫法
                //case "txt":
                //case ".txt": return new TxtFile(fileName);
                //default: return new File(fileName);
            }
            return file;
        }
    }
}