Populate Columns in DropDownList and Filter GridView Data on SelectedIndexChanged.

 <asp:TextBox ID="txtValue" runat="server"></asp:TextBox>
    <asp:DropDownList ID="ddlColumns" runat="server" AutoPostBack = "true">
        <asp:ListItem Text="All" Value="All"></asp:ListItem>
        <asp:ListItem Text="City" Value="City"></asp:ListItem>
        <asp:ListItem Text="Country" Value="Country"></asp:ListItem>
    </asp:DropDownList><br />
    <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns = "false">
    <Columns>
    <asp:BoundField DataField = "CustomerId" HeaderText = "Customer Id" />
    <asp:BoundField DataField = "ContactName" HeaderText = "Contact Name"  />
    <asp:BoundField DataField = "City" HeaderText = "City"  />
    <asp:BoundField DataField = "Country" HeaderText = "Country"  />
    </Columns>
    </asp:GridView>

String conString = System.Configuration.ConfigurationManager.ConnectionStrings["conStr"].ConnectionString;
string query = "select * from customers";
SqlCommand cmd = new SqlCommand();

if (ddlColumns.SelectedIndex > 0 && !string.IsNullOrEmpty(txtValue.Text.Trim()))
{
    query = query + " where " + ddlColumns.SelectedItem.Value + "=@Param";
    cmd.Parameters.AddWithValue("@Param", txtValue.Text.Trim());
}
cmd.CommandText = query;
SqlConnection con = new SqlConnection(conString);
cmd.CommandType = CommandType.Text;
cmd.Connection = con;
SqlDataAdapter sda = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
sda.Fill(ds);
GridView1.DataSource = ds;
GridView1.DataBind();