C#/.NET应用程序编程开发中如何从一个集合List<T>中删除在另一个集合List<T>中的所有元素？

问题描述

在C#/.NET应用程序编程开发中，如何从一个集合List<T>中删除在另一个集合List<T>中的所有元素？

现在有一个业务场景为：某系统有一个客户表，其中存储着所有客户的数据，现在需要从这个客户表中提取符合条件的客户列表，并发送促销邮件，这里的条件为排除退订的客户。

假设我们把所有客户集合命名为allCustomers，将需要排除的客户集合命名为exceptCustomers。

这里假设是用C#程序来作数据筛选，请暂时忘掉数据库的条件筛选功能，OK?

客户类Customer.cs:

public class Customer
{
  public int Id{get;set;}
  public string Name{get;set;}
}

所有客户集合allCustomers:

var allCustomers = new List<Customer>
{
    new Customer { Id = 1, Name = "Customer 1"},
    new Customer { Id = 2, Name = "Customer 2"},
    new Customer { Id = 3, Name = "Customer 3"},
    new Customer { Id = 4, Name = "Customer 4"},
    new Customer { Id = 5, Name = "Customer 5"}
};

需要排除的客户集合exceptCustomers:

var exceptCustomers = new List<Customer>
{
    new Customer { Id = 1, Name = "Customer 1"},
    new Customer { Id = 3, Name = "Customer 3"}
};

排除后的结果应该是allCustomers集合中只剩下用户ID为:2,4,5。

方案一

如果是对象集合，则使用LINQ的Where()和Any()扩展方法筛选，完整的测试代码如下(你可以复制以下代码，然后在https://try.dot.net/运行)：

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

public class Program
{
  public static void Main()
  {
    DoWork();
  }

  private static void DoWork()
  {
    var allCustomers = new List<Customer>
    {
      new Customer { Id = 1, Name = "Customer 1"},
      new Customer { Id = 2, Name = "Customer 2"},
      new Customer { Id = 3, Name = "Customer 3"},
      new Customer { Id = 4, Name = "Customer 4"},
      new Customer { Id = 5, Name = "Customer 5"}
    };

    var exceptCustomers = new List<Customer>
    {
      new Customer { Id = 2, Name = "Customer 2"},
      new Customer { Id = 4, Name = "Customer 4"}
    };

    var result = allCustomers.Where(x=>!exceptCustomers.Any(e=>e.Id==x.Id)).ToList();

    foreach(var customer in result){
      Console.WriteLine($"Customer id:{customer.Id}");
    }
  }
}

public class Customer
{
  public int Id{get;set;}
  public string Name{get;set;}
}

输出结果为:
Customer id:1
Customer id:3
Customer id:5

方案二

我们知道，LINQ还有一个扩展方法为Except()，它是用作从一个集合中排除另一个集合的。在上面的示例中，如果你直接使用Except()方法是行不通的，这时的Customer类要继承IEquatable接口并实现Equals()方法，如下：

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

public class Program
{
  public static void Main()
  {
    DoWork();
  }

  private static void DoWork()
  {
    var allCustomers = new List<Customer>
    {
      new Customer { Id = 1, Name = "Customer 1"},
      new Customer { Id = 2, Name = "Customer 2"},
      new Customer { Id = 3, Name = "Customer 3"},
      new Customer { Id = 4, Name = "Customer 4"},
      new Customer { Id = 5, Name = "Customer 5"}
    };

    var exceptCustomers = new List<Customer>
    {
      new Customer { Id = 2, Name = "Customer 2"},
      new Customer { Id = 4, Name = "Customer 4"}
    };

    var result = allCustomers.Except(exceptCustomers).ToList();

    foreach(var customer in result){
      Console.WriteLine($"Customer id:{customer.Id}");
    }
  }
}

public class Customer : IEquatable<Customer>
{
  public int Id{get;set;}
  public string Name{get;set;}

  public bool Equals(Customer other)
  {
      if (other is null)
          return false;

      return this.Id == other.Id;
  }

  public override bool Equals(object obj) => Equals(obj as Customer);
  public override int GetHashCode() => (Id, Name).GetHashCode();
}

输出结果为:
Customer id:1
Customer id:3
Customer id:5

方案三

如果两个集合为简单数据类型集合，如：

var allCustomers = new List<int>{1,2,3,4,5,6};
var exceptCustomers = new List<int>{1,3,5};

这时，我们可以使用LINQ的Except()扩展方法，如下：

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

public class Program
{
  public static void Main()
  {
    DoWork();
  }

  private static void DoWork()
  {
    var allCustomers = new List<int>{1,2,3,4,5,6};
    var exceptCustomers = new List<int>{1,3,5};
    var result = allCustomers.Except(exceptCustomers);
    foreach(var customer in result){
      Console.WriteLine($"Customer id:{customer}");
    }
  }
}

输出结果为:
Customer id:2
Customer id:4
Customer id:6

方案四

最传统的循环的方式，使用LINQ的Remove()方法，如下：

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

public class Program
{
  public static void Main()
  {
    DoWork();
  }

  private static void DoWork()
  {
    var allCustomers = new List<Customer>
    {
      new Customer { Id = 1, Name = "Customer 1"},
      new Customer { Id = 2, Name = "Customer 2"},
      new Customer { Id = 3, Name = "Customer 3"},
      new Customer { Id = 4, Name = "Customer 4"},
      new Customer { Id = 5, Name = "Customer 5"}
    };

    var exceptCustomers = new List<Customer>
    {
      new Customer { Id = 2, Name = "Customer 2"},
      new Customer { Id = 4, Name = "Customer 4"}
    };

    foreach(var c in exceptCustomers){
      foreach(var cus in allCustomers){
        if(c.Id == cus.Id){
          allCustomers.Remove(cus);
          break;
        }
      }
    }

    foreach(var customer in allCustomers){
      Console.WriteLine($"Customer id:{customer.Id}");
    }
  }
}

public class Customer : IEquatable<Customer>
{
  public int Id{get;set;}
  public string Name{get;set;}

  public bool Equals(Customer other)
  {
      if (other is null)
          return false;

      return this.Id == other.Id;
  }

  public override bool Equals(object obj) => Equals(obj as Customer);
  public override int GetHashCode() => (Id, Name).GetHashCode();
}

输出结果为:
Customer id:1
Customer id:3
Customer id:5