C#中如何使用.Except方法移除两列表间的指定重复项？

你的问题核心在于：Except方法默认的比较逻辑不满足需求——它对于自定义引用类型（比如MyObject）默认按对象引用而非OperationId属性值判断是否相等，所以即使两个对象的OperationId相同，也会被判定为不同元素，导致Except没达到预期效果。

下面提供几种正确使用Except实现需求的方法：

方法1：为MyObject重写Equals和GetHashCode #

让MyObject基于OperationId定义相等逻辑，这样Except就能正确识别出OperationId相同的元素：

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

public class Program
{
    public static void Main()
    {
        var listA = new List<MyObject>{
            new() {OperationId = 123, Mode = "In-Transit", Location = "ATL"},
            new() {OperationId = 234, Mode = "Delivered", Location = "PHX"},
            new() {OperationId = 345, Mode = "Delivered", Location = "COL"}
        };
        
        var listB = new List<MyObject>{
            new() {OperationId = 123, Mode = "In-Transit", Location = "ATL"}
        };
        
        var usingExcept = listA.Except(listB);
        // 输出2，符合预期
        Console.WriteLine(usingExcept.Count());
    }
}

public class MyObject
{
    public int OperationId { get; set; }
    public string Mode { get; set; }
    public string Location { get; set; }

    // 重写Equals，基于OperationId判断相等
    public override bool Equals(object obj)
    {
        return obj is MyObject other && OperationId == other.OperationId;
    }

    // 重写GetHashCode，确保相等的对象返回相同哈希值
    public override int GetHashCode()
    {
        return HashCode.Combine(OperationId);
    }
}

方法2：使用Except的重载，传入自定义IEqualityComparer #

如果不想修改MyObject的类定义，可以创建自定义比较器，让Except用该比较器判断元素是否相等：

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

public class Program
{
    public static void Main()
    {
        var listA = new List<MyObject>{
            new() {OperationId = 123, Mode = "In-Transit", Location = "ATL"},
            new() {OperationId = 234, Mode = "Delivered", Location = "PHX"},
            new() {OperationId = 345, Mode = "Delivered", Location = "COL"}
        };
        
        var listB = new List<MyObject>{
            new() {OperationId = 123, Mode = "In-Transit", Location = "ATL"}
        };
        
        var usingExcept = listA.Except(listB, new MyObjectEqualityComparer());
        // 输出2，符合预期
        Console.WriteLine(usingExcept.Count());
    }
}

public class MyObject
{
    public int OperationId { get; set; }
    public string Mode { get; set; }
    public string Location { get; set; }
}

// 自定义比较器，基于OperationId比较
public class MyObjectEqualityComparer : IEqualityComparer<MyObject>
{
    public bool Equals(MyObject x, MyObject y)
    {
        if (x == null || y == null)
            return x == y;
        return x.OperationId == y.OperationId;
    }

    public int GetHashCode(MyObject obj)
    {
        return obj?.OperationId.GetHashCode() ?? 0;
    }
}

方法3：先提取OperationId集合再筛选 #

如果仅需基于OperationId过滤，可先获取listB的OperationId集合，再从listA中筛选出不在该集合中的元素，效果和Except一致：

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

public class Program
{
    public static void Main()
    {
        var listA = new List<MyObject>{
            new() {OperationId = 123, Mode = "In-Transit", Location = "ATL"},
            new() {OperationId = 234, Mode = "Delivered", Location = "PHX"},
            new() {OperationId = 345, Mode = "Delivered", Location = "COL"}
        };
        
        var listB = new List<MyObject>{
            new() {OperationId = 123, Mode = "In-Transit", Location = "ATL"}
        };
        
        var bOperationIds = listB.Select(b => b.OperationId).ToHashSet();
        var filteredList = listA.Where(a => !bOperationIds.Contains(a.OperationId));
        // 输出2，符合预期
        Console.WriteLine(filteredList.Count());
    }
}

public class MyObject
{
    public int OperationId { get; set; }
    public string Mode { get; set; }
    public string Location { get; set; }
}

补充说明 #

你原来的RemoveAll能生效，是因为显式指定了用OperationId匹配判断；而Except默认没有这个逻辑，所以需要通过上述方法给Except指定基于OperationId的相等判断规则。

内容的提问来源于stack exchange，提问作者GThree