C# と VB.NET の質問掲示板

EventではなくAction型で定義すればNothingの判断が出来るので可能かと思います。

'--- Button1, 2を配置
Public Class Form1

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Using fm2 As New Form2()
            fm2.ShowDialog(Me)
        End Using
    End Sub

    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
        Using fm2 As New Form2()
            fm2.TestEvent = Sub()
                                fm2.Label1.Text = "Form1側で設定処理を実行"
                            End Sub
            fm2.ShowDialog(Me)
        End Using
    End Sub
End Class


'--- Button1とLabel1を配置
Public Class Form2

    Public Property TestEvent As Action = Nothing

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        If TestEvent Is Nothing Then
            Label1.Text = "Form2側で設定処理を実行"
        Else
            TestEvent.Invoke()
        End If
    End Sub
End Class