不能使用新的 HashCode 覆盖类中的 GetHashCode

如何解决不能使用新的 HashCode 覆盖类中的 GetHashCode

我已经复制了教授给我的代码，但出现错误“名称'HashCode 在当前上下文中不存在”。我读了一些关于它的东西，我认为它应该有效。我使用的是 VisualStudio 2019。该行在下方标记。

Visual 给我的潜在修复之一是安装包 Microsoft.Bcl.HashCode，但正如 Microsoft 文档所说，它应该已经在系统中。

没有发现任何关于此的信息，因为它是最近添加的。有一些用途（和我的一样），但不知道为什么我的不起作用。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace LV6.Zad3 {
    public class Car : IEquatable<Car>
        {
        public string Make { get; private set; }
        public string Model { get; private set; }
        public int Km { get; private set; }
        public int Year { get; private set; }
        
        public Car(string brand,string type,int km,int year)
        {
            Make = brand;
            Model = type;
            Km = km;
            Year = year;
        }
        
        public override string ToString() => $"{Make} - {Model} - {Km} - {Year}";
        
        public override int GetHashCode() => HashCode.Combine(Make,Model,Km,Year);  // I this line <--
        
        public override bool Equals(object obj){
            if (obj is Car == false) return false;
            return this.Equals((Car) obj);
        }
        
        public bool Equals(Car other) => this.Make == other.Make &&
            this.Model == other.Model &&
            this.Year == other.Year &&
            this.Km == other.Km;
        
    }

}

解决方法

如果 HashCode 不可用，因为这不是 .NET Core 应用，那么您可以使用自定义实现。下面的示例基于 on this accepted answer。

public class Car : IEquatable<Car>
{
    public string Make { get; private set; }
    public string Model { get; private set; }
    public int Km { get; private set; }
    public int Year { get; private set; }

    public Car(string brand,string type,int km,int year)
    {
        Make = brand;
        Model = type;
        Km = km;
        Year = year;
    }

    public override string ToString() => $"{Make} - {Model} - {Km} - {Year}";


    #region IEquatable Members
    /// <summary>
    /// Equality overrides from <see cref="System.Object"/>
    /// </summary>
    /// <param name="obj">The object to compare this with</param>
    /// <returns>False if object is a different type,otherwise it calls <code>Equals(Car)</code></returns>
    public override bool Equals(object obj)
    {
        if (obj is Car other)
        {
            return Equals(other);
        }
        return false;
    }

    /// <summary>
    /// Checks for equality among <see cref="Car"/> classes
    /// </summary>
    /// <param name="other">The other <see cref="Car"/> to compare it to</param>
    /// <returns>True if equal</returns>
    public virtual bool Equals(Car other)        
        => this.Make == other.Make &&
        this.Model == other.Model &&
        this.Year == other.Year &&
        this.Km == other.Km;
    

    /// <summary>
    /// Calculates the hash code for the <see cref="Car"/>
    /// </summary>
    /// <returns>The int hash value</returns>
    public override int GetHashCode()
    {
        unchecked
        {
            int hc = -1817952719;
            hc = (-1521134295)*hc + Make.GetHashCode();
            hc = (-1521134295)*hc + Model.GetHashCode();
            hc = (-1521134295)*hc + Year.GetHashCode();
            hc = (-1521134295)*hc + Km.GetHashCode();
            return hc;
        }
    }

    #endregion

}

或者使用Tuple<>使用的哈希码组合

// System.Tuple
internal static int CombineHashCodes(int h1,int h2)
{
    return ((h1 << 5) + h1) ^ h2;
}