LINQ: "contains"和Lambda查询

Question

我有一个名为buildingStatus的List<BuildingStatus>。我想检查它是否包含一个状态，其字符代码（由GetCharCode()返回）等于某个变量v.Status。

是否有一种方法可以实现这一点，类似于以下（不能编译的）代码？

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

- mark smith

Answer 1

使用Any()代替Contains()：

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

- Rex M

Answer 2

Linq 扩展方法 Any 可以帮助你...

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

- flq

Answer 3

以下是如何使用Contains来实现你想要的功能：

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

- Soft-Brain

Answer 4

我不确定您具体需要什么，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);
    }

能够产生预期的输出结果：

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

---

这个程序比较枚举类型的字符串表示，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转换成另一个，但是我不确定它的效率如何；它可能只是在幕后创建了一个列表。

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

然后你可以将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<string>。

- XXXXX

---

谢谢Larry，它起作用了，这是我通过参考您的代码所做的...但如果可能的话，不创建新列表会更好吗？//使用ToList，因为它是一个ILIST并运行我的GetCharValue //这将产生一个“NEW”列表与我的字符 var statusStringList = building.ToList().ConvertAll<char>(st => st.GetCharValue()); var test = from v in qry where statusStringList.Contains(v.Status) select v;所有工作都正常，如我所说，不必做一个新列表或在Contains中使用lambda会更好，但似乎不可能？ - mark smith

我假设状态属性是一个字符串；因此，你需要将状态枚举转换为字符串以进行每次比较。最好在开始时将它们转换一次，然后就可以了。 - XXXXX

我已经编辑了问题，使其大大简化，但同时也使这个回答有点失效。对此我感到抱歉，但我认为这样做对整体有利。 - Mark Amery

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) });

- mohit vijay chaudhari

---

在这里，我正在尝试将用户类型表中的userType与用户表匹配，以收集部门负责人姓名。希望成功。 - mohit vijay chaudhari

Answer 6

如果我理解正确，您需要将存储在 Building 列表中的类型（char 值）转换为存储在 buildingStatus 列表中的类型（枚举）。

（对于 Building 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;
}

- psabela

---

-1；虽然我对问题进行的编辑已经将所有关于“Building”的引用从问题中删除，但这已经是非常错误的。foreach(Enum value in Enum.GetValues(typeof(buildingStatus)))是无意义的。 - Mark Amery