asp:dropdownlist issue for repeater control

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C#

ASP.NET

<asp:DropDownList ID="mainsubject" runat="server" Width="200px">
                                    <asp:ListItem>Select Topic</asp:ListItem>
                                    <asp:ListItem>Music</asp:ListItem>
                                    <asp:ListItem>Entertainment</asp:ListItem>
                                    <asp:ListItem>Sports</asp:ListItem>
                                    <asp:ListItem>Politics</asp:ListItem>
                                    <asp:ListItem>General</asp:ListItem>
                                    </asp:DropDownList>

In the above code i am using this dropdownlist as the main subject which on selection should display the values of the data repeaters based on the selected values.

string quryAddBlog = "select sno,title, image, body, date, time from Blog_tb where subject='" + mainsubject.SelectedItem.Text.Trim() + "' order by sno desc";
        adp1 = new SqlDataAdapter(quryAddBlog, con);
        adp1.Fill(dt1);
        Repeater1.DataSource = dt1;
        Repeater1.DataBind();

The selected value of the dropdownlist is not selecting the subject column value from the database.

any suggestions will help me a lot.
thanks in advance