(转)C#在父窗口中调用子窗口的过程(无法访问已释放的对象)

private void OpenChildFrmToolStripMenuItem_Click(object sender, EventArgs e)
{
    if(myChildFrm != null)
    {
        myChildFrm.Show();//显示子窗口
        myChildFrm.Focus();//使子窗口获得焦点
    }
    else
    {
        myChildFrm = new ChildFrm();//创建子窗口对象
        myChildFrm.Show();//显示子窗口
        myChildFrm.Focus();//使子窗口获得焦点
    }
}

private void OpenChildFrmToolStripMenuItem_Click(object sender, EventArgs e)
{
    if(myChildFrm != null)
    {
          if(myChildFrm.IsDisposed)
                  myChildFrm = new ChildFrm();//如果已经销毁，则重新创建子窗口对象
          myChildFrm.Show();
          myChildFrm.Focus();
     }
    else
    {
        myChildFrm = new ChildFrm();
        myChildFrm.Show();
        myChildFrm.Focus();
    }
}

private void OpenChildFrmToolStripMenuItem_Click(object sender, EventArgs e)
{
    if(myChildFrm == null || myChildFrm.IsDisposed)
    {  
        myChildFrm = new ChildFrm();
    }
    myChild..MdiParent = this; //建立父子关系

        myChildFrm.Show(); //显示子窗口
    myChildFrm.Focus();  //子窗口获得焦点
}