带有“包含”列表<T>多个元素的Linq查询过滤器

Question

我想通过 Linq 查询一个列表，但在我的示例中通过另一个包含两个元素（名称、状态）的列表进行过滤。 这是受到我已经适应我的问题的一个老问题的启发。 LINQ: "contains" and a Lambda query （在这个答案中，它只适用于一个元素，即状态）

我尝试使用“包含”方法，但未能成功过滤我的列表。

我应该得到一个只有两座建筑物（二、五）的结果 有人知道我在哪里停下来吗？

谢谢

块引用

    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

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

解决此问题的一种方法是根据@Wayne 的回答手动指定要比较哪些属性是否相等。

另一种方法是，如果Building 实例的值与它们的引用相等，则实现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，您的原始代码将起作用，因为 List.Contains 现在将使用您的 IEquatable<Building> 实现来检查是否相等。

Answer 2

你的意思是这样的？

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

或许：

    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

如果有意义的话，您可以覆盖类本身中的相等性。

或者直接用Any()进行正常检查，像这样：

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