Question

请您查看下面的代码，注意返回错误消息的3个位置并建议修复？注释掉的错误位于违规行的右侧

谢谢！

Public Interface IIdentity

    Property Identity As Integer

End Interface


Public Class IdentityCollection(Of T As IIdentity)

    'Inherits System.Collections.ObjectModel.Collection(Of T)
    Inherits System.Collections.Generic.Dictionary(Of Integer, T)

    Public Function ItemByPrimaryKey(identity As Integer) As T

        For Each currentItem As T In Me 'Error  1   Value of type 'System.Collections.Generic.KeyValuePair(Of Integer, T)' cannot be converted to 'T'.  d:\users\chad\documents\visual studio 2010\Projects\WindowsApplication4\WindowsApplication4\Class2.vb   18  38  WindowsApplication4
            If currentItem.Key = identity Then 'Error   2   'Key' is not a member of 'T'.   d:\users\chad\documents\visual studio 2010\Projects\WindowsApplication4\WindowsApplication4\Class2.vb   19  16  WindowsApplication4
                Return T
            End If
        Next
        Return Nothing
    End Function

    Public Function ItemByPrimaryKeySecondApproach(identity As Integer) As T
        Return (From X In Me Select X Where X.Key = identity).FirstOrDefault 'Error 4   Value of type 'System.Collections.Generic.KeyValuePair(Of Integer, T)' cannot be converted to 'T'.  d:\users\chad\documents\visual studio 2010\Projects\WindowsApplication4\WindowsApplication4\Class2.vb   27  16  WindowsApplication4
    End Function


    Public Shadows Sub Add(item As T)
        MyBase.Add(item.Identity, item)
    End Sub

End Class

Public Class Customer

    Implements IIdentity

    Public Sub New(customerKey As Integer, lastName As String, firstname As String)
        _CustomerKey = customerKey
        _LastName = lastName
        _FirstName = firstname
    End Sub

    Private _CustomerKey As Integer

    Public Property CustomerKey As Integer Implements IIdentity.Identity
        Get
            Return _CustomerKey
        End Get
        Set(value As Integer)
            _CustomerKey = value
        End Set
    End Property

    Private _LastName As String
    Public Property LastName() As String
        Get
            Return _LastName
        End Get
        Set(ByVal value As String)
            _LastName = value
        End Set
    End Property

    Private _FirstName As String
    Public Property FirstName() As String
        Get
            Return _FirstName
        End Get
        Set(ByVal value As String)
            _FirstName = value
        End Set
    End Property

End Class

Public Class CustomerCollection

    Inherits IdentityCollection(Of Customer)

End Class

更新

我看到Dictionary集合对象上的Item函数做了我想要的，但我仍然想知道为什么我得到上面提到的错误。

Answer 1

错误1：当您通过词典进行枚举时，每个项目都是KeyValuePair，其中包含Key（在您的情况下为Integer类型）和值（在您的情况下为T类型）。所以你应该：

For Each currentItem As KeyValuePair(Of Integer, T) In Me

错误2：我认为错误信息非常清楚，但如果您执行上述更改，此行就可以了。 Key属性是KeyValuePair类的一部分，并获取字典键。

错误3：与错误1相同，查询字典时返回类型为KeyValuePair（Of Integer，T），而不是T.要检索实际值（类型为T），请使用KeyValuePair的Value属性：

Return (From X In Me Where X.Key = identity Select X.Value).FirstOrDefault

Noob需要帮助泛型

1 个答案: