VB.NET值类型的“类”被转换为“类型”

Question

我有一个接口，像这样：

Public Interface IDescriptionIdentityPair 
    Property Id() As String 
    Property Description() As String 
End Interface 

我有一个类，如下所示：

Public Class DescriptionIdentityPair : Implements IDescriptionIdentityPair 

    Public Property Id As String Implements IDescriptionIdentityPair.Id 
    Public Property Description As String Implements IDescriptionIdentityPair.Description 

    Public Overrides Function ToString() As String 
     Return Description 
    End Function 
End Class 

我有一种方法像这样：

Public Shared Function AddEmptyRow(Of T As IDescriptionIdentityPair)(ByVal collection As IList(Of T)) As IList(Of T) 
    Dim AddEmptyRowToCollection = collection 
    collection.Insert(0, New DescriptionIdentityPair With {.Id = "", .Description = FxApplication.DefaultText}) 
    Return collection 
End Function 

我正在试图插入DescriptionIdentityPair类的新实例时，编译时错误，这种类型的价值。“ DescriptionIdentityPair“不能转换为T.我甚至尝试将DescriptionIdentityPair转换为它的接口（绝望），但这并没有解决问题。

Answer 1

错误是正确的。考虑一个情况，当你有另一个类实现的接口：

Public Class AnotherDescriptionIdentityPair : Implements IDescriptionIdentityPair 

而你打电话给你的通用方法与T = AnotherDescriptionIdentityPair：当你尝试添加的DescriptionIdentityPair实例List(Of AnotherDescriptionIdentityPair)

Dim list As New List(Of AnotherDescriptionIdentityPair) 
AddEmptyRow(list) 

会发生什么？

您可以修复addint New通用的限制，以你的方法和初始化New T，而不是New DescriptionIdentityPair：

Public Shared Function AddEmptyRow(Of T As {New, IDescriptionIdentityPair})(ByVal collection As IList(Of T)) As IList(Of T) 
    Dim AddEmptyRowToCollection = collection 
    collection.Insert(0, New T With {.Id = "", .Description = "DefaultDescription"}) 
    Return collection 
End Function