C#中 在父窗体中新建子窗体的时候怎么关闭子窗体

比如父A建个子B 我要再见个子C（前提是要在建C的同时关了B）
在父窗体(form1)中新建子窗体(form2)点的是同一个按钮，在运行前，B和C都还没有生成 呢。要得效果是运行时，点一下生成B再点一下就要删除B生成C

要考虑B被认为关掉的情况
Form f2 = new Form();
Form f3 = new Form();
bool IsForm2Showed = false;
bool IsForm3Showed = false;
bool IsClosedByButton = true;
private void button1_Click(object sender, EventArgs e)
{

if (!IsForm2Showed)
{
if (IsClosedByButton)
{
f2 = new Form();
f2.Text = "f2";
f2.FormClosed += new FormClosedEventHandler(f2_FormClosed);
f2.Show();
IsForm2Showed = true;
}
else
{
f3 = new Form();
f3.Text = "f3";
f3.FormClosed += new FormClosedEventHandler(f3_FormClosed);