<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
!<asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="=" />
<asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>

protected void Button1_Click(object sender, EventArgs e)
{
int data = Convert.ToInt32(this.TextBox1.Text.Trim());//data為大于等于0的整數(shù)
this.TextBox2.Text = jieCheng(data).ToString();
}
private static int jieCheng(int data)
{
if (data == 0) //在這里需要考慮0和1的階乘都為1，所以data==0的時候要返回1.
{
return 1;
}
else
{
return data * jieCheng(data - 1);
}
}