LinqDataSource_Selecting does not work - asp.net

Question

I have used a gridview and linqdatasourse , the gridview will be fill by a linq-stored procedure after clicking a search button.
I did it like below but my grid view is empty.
It seems LinqDataSource2_Selecting does not work.
Please help what is the problem?
public void LinqDataSource2_Selecting(object sender, LinqDataSourceSelectEventArgs e)
{
_DataContext = new EDMSDataContext();
var subjectFilter = e.WhereParameters["Subject"];
var query = _DataContext.spQuickSearchDoc(subjectFilter);
e.Result = query;
}
protected void btnSearch_Click(object sender, EventArgs e)
{
_DataContext = new EDMSDataContext();
this.LinqDataSource2.WhereParameters["Subject"].DefaultValue = this.txtSearchKeywords.Text;
GridViewDocuments.Visible = false;
GridViewDocuments_Search.Visible = true;
this.GridViewDocuments_Search.DataBind();
}
Stored procedure:
ALTER PROCEDURE [dbo].[spQuickSearchDoc]
#Searchtext varchar(50)=null
AS
select DocId,DocumentNo,Title,Unit
from tblDocuments
where DocumentNo like '%'+#SearchText + '%'
or Title like '%'+#SearchText + '%'
or Unit like '%'+#SearchText + '%'
Gridview:
<asp:GridView ID="GridViewDocuments_Search" runat="server" AutoGenerateColumns=False Visible="False" onrowcommand="GridViewDocuments_Search_RowCommand"
DataKeyNames="DocID" PageSize="100" >
<Columns>
<asp:TemplateField HeaderText = "Details">
<ItemTemplate>
<asp:Button ID ="btn_Show" Text="Details" runat= "server" CommandName= "Details" CommandArgument='<%#
Container.DataItemIndex%>' />
</ItemTemplate>
</asp:TemplateField>
<asp:BoundField DataField="DocumentNo" HeaderText="DocumentNo" SortExpression="DocumentNo" />
<asp:BoundField DataField="title" HeaderText="Title" SortExpression="title" />
<asp:BoundField DataField="Docid" HeaderText="Docid" Visible="false" />
<asp:CommandField ShowEditButton="True" />
<asp:CommandField ShowDeleteButton="True" />
</Columns>
</asp:GridView>
<asp:LinqDataSource ID="LinqDataSource2" runat="server"
ContextTypeName="EDMSDataContext" OnSelecting="LinqDataSource2_Selecting">
<WhereParameters>
<asp:ControlParameter Name="Subject"
ControlID="txtSearchKeywords"
PropertyName="Text"
Type="String" />
</WhereParameters>
</asp:LinqDataSource>

Answer 1

You need to add DataSourceID to the GridView, as in:
<asp:GridView ID="GridViewDocuments_Search" runat="server"
. . DataSourceID="LinqDataSource2">
Since it's not set, the GridView is being bound to NULL.