error in Change Password without changepassword control

HELLO,
i use asp.net membership
now i created a page for user ,for chage password ,
i don't want to use <asp:ChangePassword>,i want create this custom.
my problem is: i want check user oldpassword from oldpassword textbox ,if currect, password change, and display "success" message, or wrong, display "oldpassword wrong" message.
my html code is:

oldpassword <asp:TextBox runat="server" ID="oldpassword"></asp:TextBox><br /> newpassword <asp:TextBox runat="server" ID="newpassword"></asp:TextBox><br /> confirmpassword <asp:TextBox runat="server" ID="confirmpassword"></asp:TextBox><br /> <asp:Label runat="server" ID="errorlabel"></as ...