带有list<T>多元素的“包含”的Linq查询筛选器

Question

我想通过Linq查询一个列表，但在我的示例中通过包含两个元素(名称、状态)的另一个列表进行筛选。这是受一个老问题的启发，我已经适应了我的问题。LINQ: "contains" and a Lambda query (在这个答案中，它只工作于一个元素，即状态)

我尝试使用“包含”方法，但没有成功地过滤我的列表。

我应该得到的结果只有两栋楼(两栋，五座)，有人知道我在哪里停下来了吗？

谢谢

区块报价

    public class Building
    {
        public enum StatusType
        {
            open,
            closed,
            weird,
        };

        public string Name { get; set; }
        public StatusType Status { get; set; }
    }

    private static readonly List<Building> BuildingList = new List<Building>()
    {
        new Building() {Name = "one", Status = Building.StatusType.open},
        new Building() {Name = "two", Status = Building.StatusType.closed},
        new Building() {Name = "three", Status = Building.StatusType.weird},
        new Building() {Name = "four", Status = Building.StatusType.open},
        new Building() {Name = "five", Status = Building.StatusType.closed},
        new Building() {Name = "six", Status = Building.StatusType.weird},
    };

    private  void GetResult()
    {
        var buildingSelect = new List<Building>
        {
            new Building() {Name = "two", Status = Building.StatusType.closed},
            new Building() {Name = "five", Status = Building.StatusType.closed}
        };

        var q = (from building in BuildingList
            where buildingSelect.Contains(building.Name, building.Status)
            select building).ToList();

        dataGridView1.DataSource = q;
    }

Answer 1

LINQ的主要问题是您正在尝试比较两个Buildings的相等性，因为Building没有实现IEquatable<Building>或覆盖object.Equals，所以LINQ只能通过它们的引用进行比较。

解决这个问题的一种方法是手动指定哪些属性可以按照@Wayne的答案进行相等的比较。

另一种方法是，如果Building实例要被值而不是引用等同，那么实现IEquatable<Building>并覆盖object.Equals。

public class Building : IEquatable<Building>
{
    public Building(string name, StatusType status)
    {
        Name = name;
        Status = status;
    }

    public enum StatusType
    {
        open,
        closed,
        weird,
    };

    public string Name { get; }

    public StatusType Status { get; }

    public static bool operator ==(Building left, Building right)
        => Equals(left, right);

    public static bool operator !=(Building left, Building right)
        => !Equals(left, right);

    public override bool Equals(object obj) => Equals(obj as Building);

    public bool Equals(Building other)
    {
        if (ReferenceEquals(this, other))
        {
            return true;
        }

        if (ReferenceEquals(other, null) || GetType() != other.GetType())
        {
            return false;
        }

        return Name == other.Name && Status == other.Status;
    }

    public override int GetHashCode()
    {
        unchecked
        {
            int hash = 17;
            hash = hash * 23 + Name?.GetHashCode() ?? 0;
            hash = hash * 23 + Status.GetHashCode();
            return hash;
        }
    }
}

这样，您的原始代码就可以工作了，因为List.Contains现在将使用您的IEquatable<Building>实现来检查是否相等。

Answer 2

你是说像这样的事？

    var q = from b in BuildingList
        from bs in buildingSelect
        where b.Name == bs.Name && b.Status == bs.Status
        select b;

或者也许：

    var q = from b in BuildingList
        join bs in buildingSelect
        on new { b.Name, b.Status } equals new { bs.Name, bs.Status }
        select b;

Answer 3

如果这样做有意义的话，可以重写类本身中的等式。

或者通常使用Any()进行检查，如下所示：

    var q = (from building in BuildingList
        where buildingSelect.Any(b => b.Name == building.Name 
                                      && b.Status == building.Status)
        select building).ToList();