<asp:DropDownList ID="DropDownDirPayService" runat="server" 
                        DataSourceID="SqlDataSourceDirPayService" 
                        DataTextField="DirPayServiceName" DataValueField="DirPayServiceID"
                        Font-Size="Medium" Height="30px" Width="500px">
                    </asp:DropDownList>
                    
                    <asp:SqlDataSource ID="SqlDataSourceDirPayService" runat="server"></asp:SqlDataSource>

            SqlDataSourceDirPayService.ConnectionString = System.Configuration.ConfigurationManager.ConnectionStrings["ConStrMSSQL"].ConnectionString;
            SqlDataSourceDirPayService.SelectCommand = "SELECT DirPayServiceID, DirPayServiceName FROM DirPayService";

                    <asp:Button ID="ButtonPay" runat="server" Text="Сохранить" Height="37px" 
                        Width="150px" Font-Bold="True" Font-Size="Large" onclick="ButtonPay_Click"/>

string sss1 = DropDownDirPayService.SelectedValue; // == ""
string sss2 = DropDownDirPayService.SelectedIndex; // == -1

<form id="form1" runat="server" enableviewstate="False">