c# 判断窗体已打开则显示为焦点

MDI编程模式

private void menuItem3_Click(object sender, System.EventArgs e) //菜单的CLICK事件.
  {
   FrmClass MyClass = null; //生成自己的表单
   foreach( Form ftemp in this.MdiChildren) //查找当前父表单所有子表单
   {
    if(ftemp is FrmClass)
    {
     MyClass = (FrmClass)ftemp;
     break;
    }
   }

   if(MyClass==null || MyClass.IsDisposed)
   {
    MyClass = new FrmClass();
    MyClass.MdiParent = this;
   }
   MyClass.Show();
   MyClass.Activate();

  }

普通显示模式：

private void lblCopyRight_Click(object sender, EventArgs e)
        {
            Form2 frm = null;
            foreach (Form frmt in Application.OpenForms)
            {
                if (frmt is Form2)
                {
                    frm = (Form2)frmt;
                    break;
                }
            }
            if (frm == null)
                (new Form2()).Show();
            else
            {
                frm.Show();
                frm.Activate();
            }
        }