一个关于onclick的问题

<div>
        <asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" />
        </div>
csprotected void Page_Load(object sender, EventArgs e)
    {
        Button1.Attributes.Add("onclick","return confirm('asdfas');");
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        Response.Write("asdfasdf");
    }为什么我点击取消的时候Button1_Click还是会执行呢?