ボタンで閉じられる以外の場合、ダイアログを表示して、閉じさせないようにする

BaseForm.vb

Public Class BaseForm

    ''' <summary>ボタンで閉じられたかどうか</summary>
    Protected isClosedBtn As Boolean = False

    Private Sub BaseForm_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        isClosedBtn = False
    End Sub

    Private Sub BaseForm_Closed(sender As Object, e As FormClosingEventArgs) Handles Me.FormClosing
        If Not isClosedBtn Then
            MsgBox("ボタンで閉じて！")
            e.Cancel = True
        End If
    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        isClosedBtn = True
        Me.Close()
    End Sub
End Class