Private Sub btnOk_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnOk.Click
if cmbDay.SelectedIndex = "" Then MessageBox.Show("Select a Day which you want to make a book.") else if cmbMonth.SelectedIndex = "" then MessageBox.Show("Select a Month which you want to make a book.") else if cmbCourse.SelectedIndex = "" then MessageBox.Show("Select a Course which you want to make a book.") else if cmbRoom.SelectedIndex = "" then MessageBox.Show("Select a Room which you want to make a book.")
馬鹿のような質問なのですが ---------------------------------------------- If cmbDay.SelectedIndex = "" Then MessageBox.Show("Select a Day which you want to make a book.") ---------------------------------------------- のような場合、「SelectedIndex = ""」でよいのでしょうか？以下エラーが発生します。