列表(的 T)。找不到值时 find() 不起作用，循环更好？

Question

我将不得不离开 Find（）， .FirstOrDefault（） 等并将其替换为循环，除非有人可以解释如何处理不会返回值的列表？

Public Class Form1
    Public Class TestClass
        Public Property item1 As String
        Public Property item2 As String
    End Class
    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
        Dim myLst As New List(Of TestClass)
        myLst.Add(New TestClass With {.item1 = "A", .item2 = "B"})
        myLst.Add(New TestClass With {.item1 = "C", .item2 = "D"})
        Debug.WriteLine(myLst.Find(Function(n) n.item1 = "X").item1) ' nullreferenceexception, what?
    End Sub
End Class

Answer 1

您只需要处理Find或FirstOrDefault不返回任何内容的情况。如果不返回任何内容，则正在访问 item1 属性，这会导致 NullReferenceException。

Public Sub Button2_Click(sender As Object, e As EventArgs)
    Dim myLst As New List(Of TestClass)
    myLst.Add(New TestClass With {.item1 = "A", .item2 = "B"})
    myLst.Add(New TestClass With {.item1 = "C", .item2 = "D"})
    Dim tryFindItem = myLst.Find(Function(n) n.item1 = "X")
    If tryFindItem IsNot Nothing Then
        Debug.WriteLine(tryFindItem.item1)
    Else
        Debug.WriteLine("Nothing was found!")
    End If
End Sub