Simple Factory Pattern 简单工厂模式简介与 C# 示例【创建型】【设计模式来了】

// 形状接口，画动作的方法
public interface IShape
{
    void Draw();
}
public class Circle : IShape
{
    public void Draw()
    {
        Console.WriteLine("画圆：〇");
    }
}
public class Rectangle : IShape
{
    public void Draw()
    {
        Console.WriteLine("画方：口");
    }
}
public class ShapeFactory // 简单工厂实现
{
    public static IShape CreateShape(string shapeType)
    {
        switch (shapeType) // 当需要扩展时，就需要修改这里的 case 也是本模式的缺点所在
        {
            case "圆":
                return new Circle();
            case "方":
                return new Rectangle();
            default:
                throw new ArgumentException("输入形状不支持！");
        }
    }
}

static void Main(string[] args)
{
    // 简单工厂模式写法
    IShape circle = ShapeFactory.CreateShape("圆"); // 把对象的创建交给工厂类
    circle.Draw();
    IShape rectangle = ShapeFactory.CreateShape("方");
    rectangle.Draw();
    // 不用简单工厂模式的写法
    Circle circle = new Circle(); // 单个类实例化，把创建对象的工作放在了客户端
    circle.Draw();
    Rectangle rectangle = new Rectangle();
    rectangle.Draw();
}

// System.Private.CoreLib, Version=5.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e
// System.Text.Encoding
using System.Globalization;

public static Encoding GetEncoding(int codepage)
{
    Encoding encoding = FilterDisallowedEncodings(EncodingProvider.GetEncodingFromProvider(codepage));
    if (encoding != null)
    {
        return encoding;
    }
    switch (codepage)
    {
        case 0:
            return Default;
        case 1200:
            return Unicode;
        case 1201:
            return BigEndianUnicode;
        case 12000:
            return UTF32;
        case 12001:
            return BigEndianUTF32;
        case 65001:
            return UTF8;
        case 20127:
            return ASCII;
        case 28591:
            return Latin1;
        case 1:
        case 2:
        case 3:
        case 42:
            throw new ArgumentException(SR.Format(SR.Argument_CodepageNotSupported, codepage), "codepage");
        case 65000:
            {
                if (LocalAppContextSwitches.EnableUnsafeUTF7Encoding)
                {
                    return UTF7;
                }
                string p = string.Format(CultureInfo.InvariantCulture, "http://aka.ms/dotnet-warnings/{0}", "SYSLIB0001");
                string message = SR.Format(SR.Encoding_UTF7_Disabled, p);
                throw new NotSupportedException(message);
            }
        default:
            if (codepage < 0 || codepage > 65535)
            {
                throw new ArgumentOutOfRangeException("codepage", SR.Format(SR.ArgumentOutOfRange_Range, 0, 65535));
            }
            throw new NotSupportedException(SR.Format(SR.NotSupported_NoCodepageData, codepage));
    }
}

public abstract class Encoding : ICloneable
{
    // 。。。
}