我们可以使用 IEqualityComparer 接口使用 LINQ 扩展方法 SequenceEqual 逐字段比较两个复杂集合吗

Question

我正在尝试使用 IEqualityComparer 逐个字段比较 2 个集合中的 2 个字段。 IEqualityComparer 仅比较 1 个字段“名称”。我也想比较“mark”。

在Java中，我们有comparator接口来比较Equals方法中的多个字段。

using System;
using System.Linq;
using System.Collections.Generic;

    public class Program
    {
        public static void Main()
        {
            IList<Student> studentList1 = new List<Student>()
            {
                new Student(){ name="aaaaa", mark = 95, },
                new Student(){ name="bbbb", mark = 25, },
                new Student(){ name="ccc",  mark = 80 }
            };

            IList<Student> studentList2 = new List<Student>()
            {
                new Student(){ name="aaaaa", mark = 95, },
                new Student(){ name="bbbb", mark = 5, },
                new Student(){ name="ccc",  mark = 80 }
            };

            bool isEqual = studentList1.SequenceEqual(studentList2, new StudentComparer());
            Console.WriteLine("Names in 2 collections are {0}", isEqual?"equal":"not equal");   
        }
    }

    public class Student
    {
        public string name { get; set; }
        public int mark { get; set; }
    }

    public class StudentComparer : IEqualityComparer<Student>
    {
        public bool Equals(Student x, Student y)
        {
            if (x.name == y.name)
                return true;
            return false;
        }

        public int GetHashCode(Student obj)
        {
            return obj.GetHashCode();
        }
    }

实际结果： 2 个集合中的名称相等 预期结果： 2 个集合中的名称相等 2个集合中的分数不相等

Answer 1

您的问题是 Object.GetHashCode() 在对象之间必然是唯一的。这意味着当SequenceEqual(…) 调用comparer.GetHashCode(…) 时，尽管您在StudentComparer.Equals(…) 中声明它们相等，但它会为不同的对象实例获得不同的值。

所有这一切的意思是，您应该返回唯一的哈希码，仅在根据 IEqualityComparer 的 Student 实例是唯一的情况下。例如，您可以将 StudentComparer.GetHashCode(Student obj) 的实现更改为： return obj.name.GetHashCode()

Answer 2

需要像这样正确实现相等比较器（此代码已由R#生成）：

public sealed class StudentComparer  : IEqualityComparer<Student>
{
    public bool Equals(Student x, Student y)
    {
        if (ReferenceEquals(x, y)) return true;
        if (ReferenceEquals(x, null)) return false;
        if (ReferenceEquals(y, null)) return false;
        if (x.GetType() != y.GetType()) return false;
        return string.Equals(x.name, y.name) && x.mark == y.mark;
    }

    public int GetHashCode(Student obj)
    {
        unchecked
        {
            return ((obj.name != null ? obj.name.GetHashCode() : 0) * 397) ^ obj.mark;
        }
    }
}

PSWhy is '397' used for ReSharper GetHashCode override?.

Answer 3

基本上您需要检查Equals 方法中的名称和标记是否相等，并分别获取name 属性的哈希码以及mark 属性以使IEqualityComparer 正常工作。

将您的代码修改为：

public class Program
{
    public static void Main()
    {
        IList<Student> studentList1 = new List<Student>()
        {
            new Student { name = "aaaaa", mark = 95, },
            new Student { name = "bbbb", mark = 25, },
            new Student { name = "ccc",  mark = 80 }
        };

        IList<Student> studentList2 = new List<Student>()
        {
            new Student { name = "aaaaa", mark = 95, },
            new Student { name = "bbbb", mark = 5, },
            new Student { name = "ccc",  mark = 80 }
        };

        bool isEqual = studentList1.SequenceEqual(studentList2, new StudentComparer());
        Console.WriteLine("Contents in 2 collections are {0}", isEqual ? "equal" : "not equal");
    }
}

public class Student
{
    public string name { get; set; }
    public int mark { get; set; }
}

public class StudentComparer : IEqualityComparer<Student>
{
    public bool Equals(Student x, Student y)
    {
        //Check whether the compared objects reference the same data. 
        if (object.ReferenceEquals(x, y))
            return true;

        //Check whether any of the compared objects is null. 
        if (object.ReferenceEquals(x, null) || object.ReferenceEquals(y, null))
            return false;

        return string.Equals(x.name, y.name, StringComparison.OrdinalIgnoreCase) && x.mark == y.mark;
    }

    public int GetHashCode(Student student)
    {
        //Check whether the object is null 
        if (object.ReferenceEquals(student, null))
            return 0;

        //Get hash code for the name field if it is not null
        int nameHashCode = !string.IsNullOrEmpty(student.name) ? 0 : student.name.GetHashCode();

        // Get hash code for marks also if its not 0
        int marksHashCode = student.mark == 0 ? 0 : student.mark.GetHashCode();

        return nameHashCode ^ marksHashCode;
    }
}

输出：

 Contents in 2 collections are not equal

Answer 4

我建议您将 IEquatable 接口添加到您的 Student 类，因为它的代码更少，并且更清楚您想要实现的目标：

public class Student : IEquatable<Student>
{
    public string name { get; set; }
    public int mark { get; set; }

public bool Equals(Student other)
    {
        if (string.Equals(name, other.name) && mark == other.mark)
        {
            return true;
        }

        return false;
    }
}

您可以省略额外的 StudentComparer 实现。