I do not know what I did but the error:
Error 4 A using namespace directive can only be applied to namespaces; 'System.Windows.Forms.MessageBoxIcon' is a type not a namespace C: \ Users \ Guilherme \ Documents \ PanelX \ PanelX \ Form1.cs 25 7 PanelX
error print: link
public void button9_Click(object sender, EventArgs e)
{
if (MessageBox.Show("mensagem", "titulo", MessageBoxIcon.Warning, MessageBoxButtons.YesNo) == DialogResult.Yes)
{
}
}