How to insert LINQ where parameter in cde behind - asp.net

Question

I want to add a dropdownlist fro user
to select All records, coming 2 months, etc.
and then refresh the gridview.
How can i add the where parameter to linqdatasource by code behind ?
<telerik:RadDropDownList ID="rdl_filter_date" runat="server" OnSelectedIndexChanged="rdl_filter_date_SelectedIndexChanged">
<Items>
<telerik:DropDownListItem Text="All" Value="all" />
<telerik:DropDownListItem Text="Coming 2 months" Value="2mths" />
</Items>
</telerik:RadDropDownList>
<telerik:RadGrid ID="RadGrid1" runat="server" CellSpacing="0" AutoGenerateColumns="False"
DataSourceID="LDS_UpcomingTraining" GridLines="None" Width="700">
<MasterTableView AutoGenerateColumns="False" DataSourceID="LDS_UpcomingTraining">
<CommandItemSettings ExportToPdfText="Export to PDF"></CommandItemSettings>
<RowIndicatorColumn Visible="True" FilterControlAltText="Filter RowIndicator column">
<HeaderStyle Width="20px"></HeaderStyle>
</RowIndicatorColumn>
<ExpandCollapseColumn Visible="True" FilterControlAltText="Filter ExpandColumn column">
<HeaderStyle Width="20px"></HeaderStyle>
</ExpandCollapseColumn>
<Columns>
<telerik:GridBoundColumn DataField="db_course.Course_Name"
FilterControlAltText="Filter Course Name"
HeaderText="Course Name" SortExpression="db_course.Course_Name"
UniqueName="db_course.Course_Name" ReadOnly="True">
</telerik:GridBoundColumn>
<telerik:GridBoundColumn DataField="Start_Time" DataType="System.DateTime"
FilterControlAltText="Filter Start_Time column" HeaderText="Start Time"
ReadOnly="True" SortExpression="Start_Time" UniqueName="Start_Time" DataFormatString="{0:dd-MMM-yyyy hh:mm tt}">
</telerik:GridBoundColumn>
<telerik:GridBoundColumn DataField="End_Time" DataType="System.DateTime"
FilterControlAltText="Filter End_Time column" HeaderText="End Time"
ReadOnly="True" SortExpression="End_Time" UniqueName="End_Time" DataFormatString="{0:dd-MMM-yyyy hh:mm tt}">
</telerik:GridBoundColumn>
<telerik:GridHyperLinkColumn Text="View" HeaderText="Online Teaching Material" DataNavigateUrlFields="db_training_session.MaterialURL" Target="_blank" UniqueName="HyperLinkColumn" AllowFiltering="false">
</telerik:GridHyperLinkColumn>
</Columns>
<EditFormSettings>
<EditColumn FilterControlAltText="Filter EditCommandColumn column"></EditColumn>
</EditFormSettings>
<PagerStyle PageSizeControlType="RadComboBox"></PagerStyle>
</MasterTableView>
<PagerStyle PageSizeControlType="RadComboBox"></PagerStyle>
<FilterMenu EnableImageSprites="False"></FilterMenu>
</telerik:RadGrid>
<asp:LinqDataSource ID="LDS_UpcomingTraining" runat="server"
ContextTypeName="dcLRDBDataContext" EntityTypeName=""
Select="new (db_Course, Start_Time, End_Time, db_Training_Session)"
TableName="db_Employee_Enrollments"
Where="UserID == #UserID && db_Training_Session.Status == #db_Training_Session and Start_Time >= DateTime.Now" OrderBy="Start_Time">
<WhereParameters>
<asp:SessionParameter Name="UserID" SessionField="UserID" Type="Int32" />
<asp:Parameter DefaultValue="Confirmed" Name="db_Training_Session"
Type="String" />
</WhereParameters>
</asp:LinqDataSource>
Code behind
Protected Sub rdl_filter_date_SelectedIndexChanged(ByVal sender As Object, ByVal e As Telerik.Web.UI.DropDownListEventArgs) Handles rdl_filter_date.SelectedIndexChanged
If rdl_filter_date.SelectedValue = "2mths" Then
ElseIf rdl_filter_date.SelectedValue = "all" Then
End If
End Sub

Answer 1

You can set that even in .aspx:
(you'll have to modify Where condition based on param)
<asp:LinqDataSource runat="server" ...>
...
<WhereParameters>
<asp:ControlParameter ControlID="rdl_filter_date" Name="Param" PropertyName="SelectedValue" />
</WhereParameters>
</asp:LinqDataSource>
Second option - use codebehind to set it all. Markup:
<asp:LinqDataSource ID="lds" runat="server" OnSelecting="lds_Selecting" />
Codebehind:
protected void lds_Selecting(object sender, LinqDataSourceSelectEventArgs e) {
var query = ...; // make your query here
e.Result = query;
}