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左外部結合の実行に載っていた次のコードに対しての質問です。
https://docs.microsoft.com/ja-jp/dotnet/csharp/linq/perform-left-outer-joins

(質問１）
コメント①のKeyは、必要でしょうか？削除しても、同じ動作をしますが、動作を明示するためにわざわざつけた方がよいという意味でしょうか？

(質問２)
コメント②にある二項演算子で使われているものですが、VBに、こんな演算子ってありましたけ？subpet?.Name
ビルドすると問題なく動作します。

注：このコードは、C#→VBの変換をWebで行ったものを整理したものです。

詳しい方いらっしゃいましたら、教えて頂けないでしょうか？

Public Class Form1
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim magnus As Person =
            New Person With {.FirstName = "Magnus", .LastName = "Hedlund"}
        Dim terry As Person =
            New Person With {.FirstName = "Terry", .LastName = "Adams"}
        Dim charlotte As Person =
            New Person With {.FirstName = "Charlotte", .LastName = "Weiss"}
        Dim arlene As Person =
            New Person With {.FirstName = "Arlene", .LastName = "Huff"}

        Dim barley As Pet =
            New Pet With {.Name = "Barley", .Owner =
                terry}
        Dim boots As Pet =
            New Pet With {.Name = "Boots", .Owner =
                terry}
        Dim whiskers As Pet =
            New Pet With {.Name = "Whiskers", .Owner =
                charlotte}
        Dim bluemoon As Pet =
            New Pet With {.Name = "Blue Moon", .Owner =
                terry}
        Dim daisy As Pet =
            New Pet With {.Name = "Daisy", .Owner =
                magnus}

        Dim people2 As List(Of Person) =
            New List(Of Person) From {
                magnus,
                terry,
                charlotte,
                arlene}
        Dim pets2 As List(Of Pet) =
            New List(Of Pet) From {
                barley,
                boots,
                whiskers,
                bluemoon,
                daisy}

        Dim query = From person In people2
                    Group Join pet In pets2
                        On person Equals pet.Owner
                        Into gj = Group
                    From subpet In gj.DefaultIfEmpty()
                    Select New With {
                        person.FirstName,
                       Key .PetName =　'①
                        If(subpet?.Name, String.Empty)}　'②

        For Each v In query
            Console.WriteLine($"{v.FirstName & ":"}{v.PetName}")
        Next
        'OutPut:
        'Magnus: daisy
        'Terry:  barley
        'Terry:  boots
        'Terry:  Blue Moon
        'Charlotte: whiskers
        'Arlene:
    End Sub

    Class Person
        Public Property FirstName As String
        Public Property LastName As String
    End Class

    Class Pet
        Public Property Name As String
        Public Property Owner As Person
    End Class

End Class