字典C中的GetHashCode和Equals实现＃

Question

我来到这个网站，在Dictionary中搜索对象比较，我开始知道重写GetHashCode和Equals是在C＃中进行对象比较的必要条件。 这是我尝试使用FOREACH迭代方法解决的一段代码。 但是由于性能问题，我的Boss说在不使用任何迭代（可能使用containskey或containsvalue方法）的情况下也这样做。 任何帮助都非常欢迎..

  public class employee
    {
        public string empname { get; set; }
        public string location { get; set; }
        public double kinid { get; set; }
        public double managerKin { get; set; }
        public override bool Equals(object obj)
        {
            return base.Equals(obj);
        }
        public override int GetHashCode()
        {
            return base.GetHashCode();
        }
    }

    public class manager
    {
        public string managername { get; set; }
        public double kinid { get; set; }

        public override int GetHashCode() 
        { 
          return 17 * managername.GetHashCode() + kinid.GetHashCode();
        }
    }
    public class program
    {
        public static void Main()
        {
            employee emp = new employee();
            employee emp2 = new employee();
            manager mng = new manager();
            manager mng2 = new manager();

            emp.empname = "Deepak";
            emp.location = "Pune";
            emp.kinid = 36885;
            emp.managerKin = 007;


            emp2.empname = "Astha";
            emp2.location = "Pune";
            emp2.kinid = 30000;
            emp2.managerKin = 007;

            mng.kinid = 007;
            mng.managername = "Gaurav";
            mng2.kinid = 001;
            mng2.managername = "Surya";

            Dictionary<employee, manager> relations = new Dictionary<employee, manager>();
            relations.Add(emp, mng);
            relations.Add(emp2, mng2);

            Console.ForegroundColor = ConsoleColor.Yellow;
            Console.WriteLine("The Manager details are :");
            foreach (var element in relations)
            Console.WriteLine(" \n KINID : {0} \n  Manager'sName :                    {1}",element.Value.kinid, element.Value.managername);
            Console.WriteLine("Enter the details of the manager..");
            Console.ForegroundColor = ConsoleColor.Gray;
            Console.Write("\nManager's Kin : ");
            double mkin = Convert.ToDouble(Console.ReadLine());

            Console.Write("Manager's Name : ");
            string mname = Console.ReadLine();

            manager mng1 = new manager();
            mng1.kinid = mkin;
            mng1.managername = mname;
            int hashvalue = 17 * mname.GetHashCode() + mkin.GetHashCode();



            #region BY USING FOREACH LOOP
            int i = 0;
            foreach (var element in relations)
            {
                if (element.Value.GetHashCode() == hashvalue)
                {
                    i += 1;
                    if (i == 1)
                    {
                        Console.WriteLine("The Following employees report to the Manager : {0}", mname);

                    }
                    Console.WriteLine(element.Key.empname + " " + element.Key.kinid + " " + element.Key.location + " " + element.Key.managerKin);

                }
            }
            if (i == 0)
            {
                Console.WriteLine("sorry the manager's details you entered \"{0}\" \"{1}\" does not exist in our database..", mng1.managername, mng1.kinid);

            }
            #endregion

            Console.ReadLine();
        }

    }

Answer 1

为了使用ContainsKey或ContainsValue关键字在字典中搜索对象，编译器使用两个隐式函数，即GetHashCode（）和Equals（）。 所以当我们有一个比较对象时，我们需要覆盖这两种方法!!

这是代码

#region USING DICTIONARY TO STORE CLASS OBJECTS (check employee existence and print manager's name)
public class employee
{
    public string empname { get; set; }
    public string location { get; set; }
    public double kinid { get; set; }
    public double managerKin { get; set; }

    //public override bool Equals(object obj) // ANY OF THE TWO EQUALS METHOD WORKS.
    //{
    //    employee otheremployee;
    //    otheremployee = (employee)obj;
    //    return (otheremployee.kinid == this.kinid && otheremployee.location == this.location && otheremployee.empname == this.empname && otheremployee.managerKin == this.managerKin);

    //}
    public override bool Equals(object obj)   //When Running this entire code, put a break-point on both the Equals() and GetHashCode() methods, and see the execution flow.
    {
        employee otheremployee;
        otheremployee = (employee)obj;
        return (obj.GetHashCode() == otheremployee.GetHashCode());

    }
    public override int GetHashCode()    //When Running this entire code, put a break-point on both the Equals() and GetHashCode() methods, and see the execution flow.
    {
        //int temp = base.GetHashCode(); // DONT USE THIS
        //return base.GetHashCode();
        int temp = empname.GetHashCode() + location.GetHashCode() + kinid.GetHashCode() + managerKin.GetHashCode();
        return temp;
    }
}

public class manager
{
    public string managername { get; set; }
    public double kinid { get; set; }



    public override int GetHashCode()
    {
        return base.GetHashCode();
    }
    public override bool Equals(object obj)
    {
        return base.Equals(obj);
    }
}
public class program
{
    public static void Main()
    {
        employee emp = new employee();
        employee emp2 = new employee();
        manager mng = new manager();
        manager mng2 = new manager();

        emp.empname = "Deepak";
        emp.location = "Pune";
        emp.kinid = 36885;
        emp.managerKin = 007;


        emp2.empname = "Astha";
        emp2.location = "Pune";
        emp2.kinid = 30000;
        emp2.managerKin = 001;

        mng.kinid = 007;
        mng.managername = "Gaurav";
        mng2.kinid = 001;
        mng2.managername = "Surya";

        Dictionary<employee, manager> relations = new Dictionary<employee, manager>();
        relations.Add(emp, mng); // put a BreakPoint here and see the execution flow
        relations.Add(emp2, mng2);// put a BreakPoint here and see the execution flow

        Console.ForegroundColor = ConsoleColor.Yellow;
        Console.WriteLine("The Employee details are :");
        foreach (var element in relations)
            Console.WriteLine(" \n Employee Name : {0} \n Location : {1} \n Employee KinId : {2} \n Manager's KinId : {3} ",
                element.Key.empname, element.Key.location, element.Key.kinid, element.Key.managerKin);

        Console.WriteLine("Enter the details of the Employee..");
        Console.ForegroundColor = ConsoleColor.Gray;
        Console.Write("\nEmployee Name : "); string ename = Console.ReadLine();
        Console.Write("Location : "); string elocn = Console.ReadLine();
        Console.Write("Employee KinId : "); double ekinid = Convert.ToDouble(Console.ReadLine());
        Console.Write("Manager's ID : "); double emngr = Convert.ToDouble(Console.ReadLine());
        employee emp1 = new employee();
        emp1.empname = ename;
        emp1.location = elocn;
        emp1.kinid = ekinid;
        emp1.managerKin = emngr;


        int i = 0; // This variable acts as a indicator to find whether the Employee Key exists or not.
        if (relations.ContainsKey(emp1)) //Put a break point here and see the execution flow.
        {
            Console.WriteLine("the Employee : {0} exists..", emp1.empname);
            Console.WriteLine("the Employee reports to the following manager : {0} \n and the Manager's KinId is {1}.", (relations[emp1]).managername, relations[emp1].kinid);
            i = 1;
            Console.ReadLine();
        }

        if (i == 0)
        {
            Console.WriteLine("the details of the employee named {0} does not exist !!", emp1.empname);
            Console.ReadLine();
        }

#endregion

Answer 2

要在字典中搜索元素，您可以使用ContainsKey，ContainsValue方法或只编写LINQ查询

var dict = (from pair in relations
where pair.Value.Equals(mng1)
select pair).ToDictionary<employee,manager>();

Answer 3

Dictionary.ContainsKey(employee)在这里没有帮助，因为员工是“未知”值，而Contains将无济于事，因为它需要一个KeyValuePair<employee,manager>和......再次......没有员工知道。 ContainsValue(manager)将无法帮助，因为它不返回任何键，因为它不是键，它是O(n)操作，而不是像ContainsKey这样的O(1) ！

使用当前结构唯一的方法是使用某种形式的循环，虽然我会这样写：

// Key is Employee, Value is Manager
// This is O(n)
var theEmployees = relations
  .Where(rel => rel.Value.Equals(theManager))
  .Select(rel => rel.Key);

只有在manager获得有效的Equals实施后，这才有效。 请注意， 根本不使用哈希码。 （ 因为不同的对象可能共享相同的哈希代码，所以只比较哈希代码不能替代Equals ， ==或CompareTo ！ - 取决于哪一个是合适的。）

如果存在许多这样的查询，则初始结构可以“反转”。

// Build a reverse lookup-up
var employeesForManager = relations
  .GroupBy(rel => rel.Value)            // group on Manager
  .ToDictionary(g => g.Key, g => g);    // Key is the group's Manager

// This is O(1), but only valid AFTER employeesForManager is [re-]generated
var theEmployees = employeesForManager[theManager]

这仅在manager具有有效的Equals和GetHashCode实现时才有效。 （GetHashCode是必需的，因为manager对象被用作新词典的键。）

至于哪个是“更好” - 嗯，这取决于。 例如，创建反向查找仅使用一次是愚蠢的。 在出现性能问题之前没有性能问题：编写干净的代码和配置文件。

快乐的编码。

Answer 4

为了能够比较2个实例的相等性，您应该重写Equals方法，并且实现IEquatable<T>也是一种好习惯。 当您重写Equals时，您还应该覆盖GetHashcode（当您将实例放入字典中以计算存储桶时使用此方法）。

你不应该使用GetHashcode来比较对象的2个实例是否相等; 相反，你应该使用Equals （或EqualityComparer ，它也将使用Equals方法）。

如果您已经很好地实现了GetHashCode和Equals，那么您可以通过执行以下操作来确定字典是否包含特定实例：

var myDictionary<int, Manager> = new Dictionary<int,Manager>();

myDictionary.ContainsKey (someKey)

要么

var mySet = new HashSet<Manager>();
mySet.Contains(someManagerObject);

Answer 5

我相信你的最终回复中有一个错误。

这条线

return（obj.GetHashCode（）== otheremployee.GetHashCode（））;

可能应该是

return（this.GetHashCode（）== otheremployee.GetHashCode（））;

这样您就可以比较此对象和其他对象的哈希码。 正如您在回复中所写的那样，您似乎正在将另一个对象与自身进行比较。