vb.net - 通过值 x、y 和 z 在 List(Of T) 中查找项目

Question

我有以下设置:

Class A
      property x as string
      property y as int
      property z as String

End Class

Class CollOfA
    inherits List(Of A)

End Class

我想要的是集合中的一个 Item 属性，我可以说:

dim c as new CollOfA
c.item("this", 2, "that")

我尝试在 CollOfA 中实现以下内容:

Class CollOfA
    inherits List(Of A)

    Default Public Overridable Shadows ReadOnly Property Item(ByVal x As String, ByVal y As Integer, byval z as string)
        Get
            ' I want to do something like:
            ' ForEach item in me see if anything matches these three things

        End Get
    End Property
End Class

我知道谓词，但我正在为如何设置谓词的条件(即传递 x、y 和 z)而苦苦挣扎。

有没有人实现过类似的东西？

Answer 1

这是我想出的两种方法来完成您的问题。一种方法是使用 LINQ 查询语法进行过滤；第二个使用自定义对象来保存谓词参数，然后使用该对象执行过滤器。

在 Item 属性中使用 LINQ 语法:

Default Public Overridable Shadows ReadOnly Property Item(ByVal x As String, ByVal y As Integer, ByVal z As String) As IEnumerable(Of A)
    Get
        Return (From theA In Me
                Where (theA.x = x And theA.y = y And theA.z = z)
                Select theA)

    End Get
End Property

另一种方法是创建一个 PredicateParameter 类来保存您的参数以及一个用于执行过滤器的委托(delegate)方法。我在 MSDN 评论中看到了这个 - 这是 link .这是类(class):

Class PredicateParams

    Public Sub New(ByVal theA As A)
        Criteria = theA
    End Sub

    Public Property Criteria As A

    Public Function IsMatch(ByVal theA As A) As Boolean
        Return (theA.x = Criteria.x And theA.y = Criteria.y And theA.z = Criteria.z)
    End Function

End Class

这是使用它的 CollOfA 类中的属性:

Public Overridable Shadows ReadOnly Property ItemPred(ByVal x As String, ByVal y As Integer, ByVal z As String) As IEnumerable(Of A)
    Get
        Dim predA As New A
        predA.x = x
        predA.y = y
        predA.z = z

        Dim pred As New PredicateParams(predA)

        Return Me.FindAll(AddressOf pred.IsMatch)
    End Get

End Property

最后，这里有一个控制台运行器来测试它。

Sub Main()
    Dim mycoll As New CollOfA()


    For index = 1 To 100
        Dim anA As New A()
        anA.x = (index Mod 2).ToString()
        anA.y = index Mod 4
        anA.z = (index Mod 3).ToString()
        mycoll.Add(anA)
    Next

    Dim matched As IEnumerable(Of A) = mycoll.Item("1", 3, "2")
    Dim matched2 As IEnumerable(Of A) = mycoll.ItemPred("1", 3, "2")

    Console.WriteLine(matched.Count.ToString())  'output from first search
    Console.WriteLine(matched2.Count.ToString()) 'output from second search (s/b same)
    Console.ReadLine()

End Sub

希望这对您有所帮助。可能有更优雅的方法来执行此操作，但我没有看到。 (顺便说一句，我通常使用 C#，所以我的 VB.NET 有点生疏。)