If TextBox3.Text <> "*" Or TextBox3.Text <> "/" Or TextBox3.Text <> "+" Or TextBox3.Text <> "-" Then
MsgBox("请在第三个框中输入运算符!")
TextBox3.Text = ""
TextBox3.Focus()
If TextBox3.Text <> "*" Or TextBox3.Text <> "/" Or TextBox3.Text <> "+" Or TextBox3.Text <> "-" Then
MsgBox("请在第三个框中输入运算符!")
TextBox3.Text = ""
TextBox3.Focus()