public Form1()
{
  InitializeComponent();
  //将Form1设置为MDI窗体，当然在Form1的IsMdiContainer属性页可以设置
  this.IsMdiContainer = true;
}

private void tsmiOpenWindow_Click(object sender, EventArgs e)
{  
  FrmChildren child = FrmChildren.GetWindow();//调用方法
  child.MdiParent = this;//设置child的父窗体为当前窗体
  child.Show();
  
}

 public partial class FrmChildren : Form
  {
    private FrmChildren() //由 public FrmChildren改为 private FrmChildren
    {
      InitializeComponent();
    }
    static FrmChildren fc = null; 创建一个静态对象
    public static FrmChildren GetWindow()
    {  //当子窗体没有开启或者已经释放。就可以创建新窗体
      if (fc==null||fc.IsDisposed)
      {
        fc = new FrmChildren();
      }
      return fc;
    }
  }

private void tsmiOpenWindow_Click(object sender, EventArgs e)
{ 

#region 方法二Application收集打开的窗体，用索引器来寻找，就是窗体的Name属性
//方法二.如果没有Name为FrmChildren的子船体，实例化创建。和之前的正规做法没有什么差别，只是多了判断。
if (Application.OpenForms["FrmChildren"] == null)
{
FrmChildren child = new FrmChildren();
child.MdiParent = this;
child.Show();
}
else//有Name为FrmChildren的子船体，就直接show()
{
Application.OpenForms["FrmChildren"].Show();
}
#endregion
}