如何为字典键使用自定义类型？

Question

我有一个使用泛型的自定义 class。

我需要使用这个 class 作为字典的键，如下面的代码示例所示：

我可以使用覆盖的 Object.GetHashCode 方法，但我不确定如何从那里继续。请帮忙，谢谢。

    Module Module2
    Dim myStore As New Dictionary(Of Pair(Of Long, Integer), String)

    Public Function ContainsItem(id As Long, code As Integer) As Boolean
        Return myStore.ContainsKey(New Pair(Of Long, Integer)(id, code))
    End Function

    Public Class Pair(Of T1, T2)
        Implements IEquatable(Of Pair(Of T1, T2))

        Private v1 As T1
        Private v2 As T2

        Public Sub New(ByVal v1 As T1, ByVal v2 As T2)
            Me.v1 = v1
            Me.v2 = v2
        End Sub

        Public Function first() As T1
            Return v1
        End Function

        Public Function second() As T2
            Return v2
        End Function

        Public Overrides Function GetHashCode() As Integer
            'i hit this break point, but ... 
            'how do i compute an integer hashcode from a long and an Integer?
            Return MyBase.GetHashCode()
        End Function

        Public Overrides Function Equals(obj As Object) As Boolean
            Return MyBase.Equals(obj)
        End Function

        Public Function Equals1(other As Pair(Of T1, T2)) As Boolean Implements IEquatable(Of Pair(Of T1, T2)).Equals
            'just as a test, but the code never gets here.
            Return True
        End Function
    End Class

    Public Sub TestCase()
        Dim a = New Pair(Of Long, Integer)(10, 10)
        myStore.Add(a, "Item 1")

        Dim b = ContainsItem(10, 10)
        'b is always false 
    End Sub
End Module

Answer 1

这实现了 IEquatable 并覆盖了 GetHashCode 和 Equals:

' an assumption about Pair(Of... :
Public Class Pair(Of T, TT)
    Implements IEquatable(Of Pair(Of T, TT))

    Public Property ValueT As T
    Public Property ValueTT As TT

然后，方法：

' basic Equals for this Type
Public Overrides Function Equals(obj As Object) As Boolean
    If obj.GetType Is GetType(Pair(Of Long, Integer)) Then
        Return Equals1(CType(obj, Pair(Of T, TT)))
    Else
        Return False
    End If
End Function

' used by the Dictionary 
Public Function Equals1(obj As Pair(Of T, 
                 TT)) As Boolean Implements IEquatable(Of Pair(Of T, TT)).Equals
    ' the other thing is Something Else
    If obj.GetType <> GetType(Pair(Of Long, Integer)) Then
        Return False
    End If

    'prefer T over TT, testing first
    If Integer.Equals(obj.ValueT, ValueT) = False Then
        Return False
    End If

    'T is equal, what about TT:
    Return Long.Equals(obj.ValueTT, ValueTT)
End Function

 ' dictionary will use the hashcode for ContainsKey, Add
Public Overrides Function GetHashCode() As Integer
    ' 
    ' marc gravell:
    Dim hash As Integer = 13
    hash = (hash * 7) + ValueT.GetHashCode()
    hash = (hash * 7) + ValueTT.GetHashCode()

    Return hash

    ''msdn (non generic value types):
    'Dim hCode As Long = ValueT Xor ValueTT
    'Return hCode.GetHashCode()
End Function

测试：

Dim a = New Pair(Of Long, Integer)(10, 10)
Dim b = New Pair(Of Long, Integer)(5, 5)
' different object, same values:
Dim c = New Pair(Of Long, Integer)(10, 10)

Dim mydict As New Dictionary(Of Pair(Of Long, Int32), String)
mydict.Add(a, "ziggy")
mydict.Add(b, "zoey")

Console.WriteLine("a==b? {0}", a.Equals(b).ToString)
Console.WriteLine("a==c? {0}", a.Equals(c).ToString)
Console.WriteLine("b==c? {0}", b.Equals(c).ToString)

Console.WriteLine("Contains a? {0}", mydict.ContainsKey(a).ToString)
Console.WriteLine("Contains b? {0}", mydict.ContainsKey(b).ToString)
' since the c OBJECT is not in the collection, it SHOULD report false
 ' but since the values are, and thats all that seems to matter:
Console.WriteLine("Contains c? {0}", mydict.ContainsKey(c).ToString)

结果：

a==b? False
a==c? True
b==c? False
Contains a? True
Contains b? True
Contains c? True

使用c的测试实际上是错误的。对象 'c' 从未添加到词典中，并且 'c' 与 a 是不同的对象。但是使用的覆盖只是根据相同的 2 个值测试对象。

我不认为我会这样做，因为 a<>c。相反，可以使用某种集合 class 并避免重新定义 Equals。确切的实现将取决于集合中的内容。对我来说，这是最后的手段。

另请参阅：

IEquatable
Why is it important to override GetHashCode when Equals method is overridden?
Dictionary Reference Source

如何为字典键使用自定义类型？

How do you use a custom type for a dictionary key?

vb.net

generics

dictionary

icomparable

icomparer