首先创建一个判断函数,判断窗体是否已经打开:

public bool openonece(Form myform)

{

foreach (Form frm in this.MdiChildren)

{

if (frm.Name == myform.Name)

{

frm.Activate();

frm.WindowState = FormWindowState.Maximized;

return true;

}

}

return false;

}

然后点击MDI窗体中的菜单按钮时编写代码:

private void tool_Print_Label_Click(object sender, EventArgs e)

{

Form frm = new Form_Name();//Form_Name为要打开窗体的NAME属性中的名称

if (openonece(frm) == false)

{

frm.MdiParent = this;

frm.WindowState = FormWindowState.Maximized;//最大化窗体

frm.Show();

}

}