1.
private Form2 f2 = new Form2();
private void button1_Click(object sender, EventArgs e)
{
if (f2==null || f2.IsDisposed)
{
f2 = new Form2();
}
f2.Show();
}
2.
private Form2 f2 = new Form2();
#region 打开子窗体
void showmdichild(Form childform)
{
childform.MdiParent = this;
childform.Show();
}
#endregion
private void openToolStripMenuItem_Click(object sender, EventArgs e)
{
if (f2.IsDisposed == false)
{
showmdichild(f2);
}
else
{
f2 = new Form2();
showmdichild(f2);
}
}