Clean Code_Note8_Eliminate Null Checks

- 8月 09, 2021

在一段程式中當觀察到物件中充斥很多 method重覆查檢非null的判斷時

就可能是沒有從源頭下手的情境

以下為一個範例

using System;
using System.Collections.Generic;

namespace CleanUpConditionals
{
    public class EliminateNullChecks
    {
        public class BattleRobot
        {
            public int Team { get; set; }
            public int Health { get; set; }
            public int XCoordinate { get; set; }
            public int YCoordinate { get; set; }

            #region Methods

            public void Attack(int x, int y)
            {
                //shoot missiles implementation omitted...
            }

            public List<Tuple<int, int>> GetPossibleMoves()
            {
                //some complex implementation omitted...
                return null;
            }

            public List<Tuple<int, int>> GetEnemyLocations()
            {
                //some complex implementation omitted...
                return null;
            }

            #endregion
        }

        private BattleRobot _robot;

        public BattleRobot Robot { get => _robot; set => _robot = value; }

        public bool CanStillFight()
        {
            if (_robot != null)
            {
                return true;
            }
            return false;
        }

        public void AttackPosition(int x, int y)
        {
            if (_robot != null)
            {
                _robot.Attack(x, y);
            }
        }

        public void UpdateStrategy()
        {
            if (_robot != null)
            {
                List<Tuple<int, int>> posssibleMoves = _robot.GetPossibleMoves();
                List<Tuple<int, int>> enemyLocations = _robot.GetEnemyLocations();

                foreach (var move in posssibleMoves)
                {
                    DevelopStrategy(move, enemyLocations);
                }
            }
        }

        #region Other methods
        private void DevelopStrategy(Tuple<int, int> move, List<Tuple<int, int>> locations)
        {

        }
        #endregion
    }
}

可以觀察到在EliminateNullChecks一整個class中

CanStillFight() 、AttackPosition()、UpdateStrategy()這三個方法不斷重複在做null查檢。

此時可引入Null物件並去繼承自BattleRobot，覆寫這些方法

EliminateNullChecks原本在這三個方法中呼叫到的private欄位全部改為用屬性封裝的Robot呼叫方法，在Robot屬性中已經改為當程式被呼叫執行結果為空時，直接返回Null Object，而非Null。

改完後的程式

using System;
using System.Collections.Generic;
using static CleanUpConditionals.EliminateNullChecks;

namespace CleanUpConditionals
{
    public class NullRobot : BattleRobot
    {
        public override bool CanStillFight()
        {
            return base.CanStillFight();
        }

        public override void Attack(int x, int y)
        {
            base.Attack(x, y);
        }

        public override List<Tuple<int, int>> GetEnemyLocations()
        {
            return base.GetEnemyLocations();
        }

        public override List<Tuple<int, int>> GetPossibleMoves()
        {
            return base.GetPossibleMoves();
        }
    }
    public class EliminateNullChecks
    {
        public class BattleRobot
        {
            public int Team { get; set; }
            public int Health { get; set; }
            public int XCoordinate { get; set; }
            public int YCoordinate { get; set; }

            public virtual bool CanStillFight()
            {
                return true;
            }


            #region Methods

            public virtual void Attack(int x, int y)
            {
                //shoot missiles implementation omitted...
            }

            public virtual List<Tuple<int, int>> GetPossibleMoves()
            {
                //some complex implementation omitted...
                return null;
            }

            public  virtual List<Tuple<int, int>> GetEnemyLocations()
            {
                //some complex implementation omitted...
                return null;
            }

            #endregion
        }

        private BattleRobot _robot;

        public BattleRobot Robot { get => _robot ?? new NullRobot(); set => _robot = value; }

        public bool CanStillFight()
        {
            return Robot.CanStillFight();
        }

        public void AttackPosition(int x, int y)
        {
            Robot.Attack(x, y);
        }

        public void UpdateStrategy()
        {
            List<Tuple<int, int>> posssibleMoves = Robot.GetPossibleMoves();
            List<Tuple<int, int>> enemyLocations = Robot.GetEnemyLocations();

            foreach (var move in posssibleMoves)
            {
                DevelopStrategy(move, enemyLocations);
            }
        }

        #region Other methods
        private void DevelopStrategy(Tuple<int, int> move, List<Tuple<int, int>> locations)
        {

        }
        #endregion
    }
}

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