ASP.NET - Validar sólo números en un texbox

<asp:TextBox runat="server" ID=""  CausesValidation="true" ValidationGroup="okButton" Width="132px"></asp:TextBox>

<script>

        function solonumeros(e) {

            var key;

            if (window.event) // IE

            {

                key = e.keyCode;

            }

            else if (e.which) // Netscape/Firefox/Opera

            {

                key = e.which;

            }

            if (key < 48 || key > 57) {

                return false;

            }

            return true;

        }

    </script>

<asp:TextBox runat="server" ID=""  CausesValidation="true" ValidationGroup="okButton" Width="132px" onkeypress="javascript:return solonumeros(event)"></asp:TextBox>

<asp:TextBox runat="server" ID="TextBox1" type="number"  CausesValidation="true" ValidationGroup="okButton" Width="132px"></asp:TextBox>

<asp:TextBox runat="server" ID="TextBox1" type="number" min="1" max="20" CausesValidation="true" ValidationGroup="okButton" Width="132px"></asp:TextBox>