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限定符运算 (Visual Basic)

  • 项目

限定符运算返回一个 Boolean 值，该值指示序列中是否有一些元素满足条件或是否所有元素都满足条件。

下图描述了两个不同源序列上的两个不同限定符运算。 第一个运算询问是否有任何元素为字符“A”。 第二个运算询问是否所有元素都为字符“A”。 这两种方法在此示例中都返回 true。

LINQ Quantifier Operations

下节列出了执行限定符运算的标准查询运算符方法。

方法

方法名 描述 Visual Basic 查询表达式语法 更多信息
全部 确定是否序列中的所有元素都满足条件。 Aggregate … In … Into All(…) Enumerable.All

Queryable.All
任意 确定序列中是否有元素满足条件。 Aggregate … In … Into Any() Enumerable.Any

Queryable.Any
包含 确定序列是否包含指定的元素。 不适用。 Enumerable.Contains

Queryable.Contains

查询表达式语法示例

这些示例使用 Visual Basic 中的 Aggregate 子句作为 LINQ 查询中的筛选条件的一部分。

下面的示例使用 Aggregate 子句和 All 扩展方法从集合中返回其宠物都超过指定年龄的人员。

Class Person
    Public Property Name As String
    Public Property Pets As Pet()
End Class

Class Pet
    Public Property Name As String
    Public Property Age As Integer
End Class

Sub All()
    Dim barley As New Pet With {.Name = "Barley", .Age = 4}
    Dim boots As New Pet With {.Name = "Boots", .Age = 1}
    Dim whiskers As New Pet With {.Name = "Whiskers", .Age = 6}
    Dim bluemoon As New Pet With {.Name = "Blue Moon", .Age = 9}
    Dim daisy As New Pet With {.Name = "Daisy", .Age = 3}

    Dim charlotte As New Person With {.Name = "Charlotte", .Pets = New Pet() {barley, boots}}
    Dim arlene As New Person With {.Name = "Arlene", .Pets = New Pet() {whiskers}}
    Dim rui As New Person With {.Name = "Rui", .Pets = New Pet() {bluemoon, daisy}}

    ' Create the list of Person objects that will be queried.
    Dim people As New System.Collections.Generic.List(Of Person)(New Person() {charlotte, arlene, rui})

    Dim query = From pers In people
                Where (Aggregate pt In pers.Pets Into All(pt.Age > 2))
                Select pers.Name

    Dim sb As New System.Text.StringBuilder()
    For Each name As String In query
        sb.AppendLine(name)
    Next

    ' Display the results.
    MsgBox(sb.ToString())

    ' This code produces the following output:

    ' Arlene
    ' Rui

End Sub

下一个示例使用 Aggregate 子句和 Any 扩展方法从集合中返回至少有一只宠物超过指定年龄的人员。

Class Person
    Public Property Name As String
    Public Property Pets As Pet()
End Class

Class Pet
    Public Property Name As String
    Public Property Age As Integer
End Class

Sub Any()
    Dim barley As New Pet With {.Name = "Barley", .Age = 4}
    Dim boots As New Pet With {.Name = "Boots", .Age = 1}
    Dim whiskers As New Pet With {.Name = "Whiskers", .Age = 6}
    Dim bluemoon As New Pet With {.Name = "Blue Moon", .Age = 9}
    Dim daisy As New Pet With {.Name = "Daisy", .Age = 3}

    Dim charlotte As New Person With {.Name = "Charlotte", .Pets = New Pet() {barley, boots}}
    Dim arlene As New Person With {.Name = "Arlene", .Pets = New Pet() {whiskers}}
    Dim rui As New Person With {.Name = "Rui", .Pets = New Pet() {bluemoon, daisy}}

    ' Create the list of Person objects that will be queried.
    Dim people As New System.Collections.Generic.List(Of Person)(New Person() {charlotte, arlene, rui})

    Dim query = From pers In people
                Where (Aggregate pt In pers.Pets Into Any(pt.Age > 7))
                Select pers.Name

    Dim sb As New System.Text.StringBuilder()
    For Each name As String In query
        sb.AppendLine(name)
    Next

    ' Display the results.
    MsgBox(sb.ToString())

    ' This code produces the following output:

    ' Rui

End Sub

另请参阅