LINQ：“包含”和 Lambda 查询

Question

我有一个名为 buildingStatus 的 List。我想检查它是否包含其字符代码（由 GetCharCode() 返回）等于某个变量 v.Status 的状态。

是否有某种方法可以按照下面的（非编译）代码来做到这一点？

buildingStatus.Contains(item => item.GetCharValue() == v.Status)

Answer 1

使用 Any() 而不是 Contains()：

buildingStatus.Any(item => item.GetCharValue() == v.Status)

Answer 2

Linq 扩展方法 Any 可以为您工作......

buildingStatus.Any(item => item.GetCharValue() == v.Status)

Answer 3

以下是如何使用 Contains 来实现您想要的：

buildingStatus.Select(item => item.GetCharValue()).Contains(v.Status) 这将返回一个布尔值。

Answer 4

我不确定您到底在寻找什么，但是这个程序：

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

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

    public static 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 },
    };

    static void Main (string [] args)
    {
        var statusList = new List<Building.StatusType> () { Building.StatusType.open, Building.StatusType.closed };

        var q = from building in buildingList
                where statusList.Contains (building.Status)
                select building;

        foreach ( var b in q )
            Console.WriteLine ("{0}: {1}", b.Name, b.Status);
    }

产生预期的输出：

one: open
two: closed
four: open
five: closed

---

这个程序比较枚举的字符串表示形式并产生相同的输出：

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

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

    public static List <Building> buildingList = new List<Building> ()
    {
        new Building () { Name = "one", Status = "open" },
        new Building () { Name = "two", Status = "closed" },
        new Building () { Name = "three", Status = "weird" },

        new Building () { Name = "four", Status = "open" },
        new Building () { Name = "five", Status = "closed" },
        new Building () { Name = "six", Status = "weird" },
    };

    static void Main (string [] args)
    {
        var statusList = new List<Building.StatusType> () { Building.StatusType.open, Building.StatusType.closed };
        var statusStringList = statusList.ConvertAll <string> (st => st.ToString ());

        var q = from building in buildingList
                where statusStringList.Contains (building.Status)
                select building;

        foreach ( var b in q )
            Console.WriteLine ("{0}: {1}", b.Name, b.Status);

        Console.ReadKey ();
    }

---

我创建了这个扩展方法来将一个 IEnumerable 转换为另一个 IEnumerable，但是我不确定它的效率如何；它可能只是在幕后创建一个列表。

public static IEnumerable <TResult> ConvertEach (IEnumerable <TSource> sources, Func <TSource,TResult> convert)
{
    foreach ( TSource source in sources )
        yield return convert (source);
}

然后，您可以将 where 子句更改为：

where statusList.ConvertEach <string> (status => status.GetCharValue()).
    Contains (v.Status)

并跳过在开头使用 ConvertAll () 创建 List 的过程。

Answer 5

var depthead = (from s in db.M_Users
                  join m in db.M_User_Types on s.F_User_Type equals m.UserType_Id
                  where m.UserType_Name.ToUpper().Trim().Contains("DEPARTMENT HEAD")
                  select new {s.FullName,s.F_User_Type,s.userId,s.UserCode } 
               ).OrderBy(d => d.userId).ToList();

Model.AvailableDeptHead.Add(new SelectListItem { Text = "Select", Value = "0" });
for (int i = 0; i < depthead.Count; i++)
    Model.AvailableDeptHead.Add(new SelectListItem { Text = depthead[i].UserCode + " - " + depthead[i].FullName, Value = Convert.ToString(depthead[i].userId) });

Answer 6

如果我理解正确，您需要转换您存储的类型（char 值）
将 Building list 中的类型更改为您存储在buildingStatus 列表中的类型（枚举）。

（对于建筑物列表中的每个状态//字符值//，
该状态是否存在于buildingStatus列表中//枚举值//)

public static IQueryable<Building> WithStatus(this IQueryable<Building> qry,  
IList<BuildingStatuses> buildingStatus) 
{ 
    return from v in qry
           where ContainsStatus(v.Status)
           select v;
} 


private bool ContainsStatus(v.Status)
{
    foreach(Enum value in Enum.GetValues(typeof(buildingStatus)))
    {
        If v.Status == value.GetCharValue();
            return true;
    }

    return false;
}