列表包含列表

Question

您好，我有以下列表：

 var CustomerName = new List<Customer>();
 var DummyData = new List<Customer>();

如何快速检查 DummyData 是否包含在 CustomerName 中？性能也是关键，因为这些列表可能包含数千个值。

Answer 1

蛮力法

对 DummyData 变量 O(N*K) 使用 linq all 方法

// If you override Equals and GetHashCode or are comparing by reference
DummyData.All(a=>CustomerName.Contains(a))

//If you compare by property
DummyData.All(a=>
               CustomerName.Any(b=>
                   a.FirstName==b.FirstName && 
                   a.LastName == b.LastName
                   //repeat to include checks for all properties
              )
          );

使用哈希集

将你的结果放入一个hashset，再次使用linq的All方法检查hashset是否包含项目，构建hashset需要N步，检查k步，复杂度为O(N+K)

var hs = new HashSet<Customer>(CustomerName);
DummyData.All(a=>hs.Contains(a));

您需要覆盖 Equals 和 GetHashCode

如果你还没有覆盖这两个，除非你想比较属性，否则你需要这样做，这也会阻止你使用哈希集方法

public class Customer
{
    public string FirstName { get; set; }
    public string LastName { get; set; }

    public override bool Equals(object obj)
    {
        var customer = obj as Customer;
        return customer != null && Equals(customer);
    }

    protected bool Equals(Customer other)
    {
        return string.Equals(FirstName, other.FirstName) && string.Equals(LastName, other.LastName);
    }

    public override int GetHashCode()
    {
        unchecked
        {
            return ((FirstName?.GetHashCode() ?? 0)*397) ^ (LastName?.GetHashCode() ?? 0);
        }
    }
}