大学大学, 大概学学 之 Null Object Pattern

最近在网上看到这个设计模式, 感觉很赞 :)

写代码的时候, 经常会遇到抛出NullReferenceException的情况, 举个最简单的例子：

public class Student
{
    public void Method()
    {
        Console.WriteLine("Chen.");
    }
}

public class Test
{
    public static void Main(string[] args)
    {
        Student student = new Student();
        student.Method();                //Chen.
        student=null;
        student.method();                //抛出NullReferenceException
    }
}

Null Object Pattern可以很好地解决这一问题。
Null Object Pattern的作用就是让null值变为null object, 代码如下:

namespace DesignPatternTest.NullObject
{
    public class Student
    {
        public Student() { }

        public virtual bool IsNull
        {
            get { return false; }
        }

        public virtual void Method()
        {
            Console.WriteLine("Chen.");
        }
    }
}

namespace DesignPatternTest.NullObject
{
    public class NullStudent : Student
    {
        private NullStudent() { }

        public static NullStudent GetInstance()
        {
            return new NullStudent();
        }

        public override bool IsNull
        {
            get { return true; }
        }

        public override void Method()
        {
            //nothing
        }
    }
}

通过StudentManager管理:

namespace DesignPatternTest.NullObject
{
    class StudentManager
    {
        private Student student=new Student();

        public Student Student
        {
            set { student = value ?? NullStudent.GetInstance(); }
            get { return student; }
        }
    }
}

对于Student类型的引用, 一旦为null值, 则指向NullStudent对象, 测试类如下:

namespace DesignPatternTest.NullObject
{
    public class Test
    {
        public static void TestApp()
        {
            StudentManager m = new StudentManager();
            Console.WriteLine(m.Student.IsNull);    //false
            m.Student.Method();                     //Chen.
            m.Student = null;
            Console.WriteLine(m.Student.IsNull);    //true
            m.Student.Method();                     //不会抛出NullReferenceException , 什么也不做
        }
    }
}

成功解决方法调用时的NullReferenceException问题。