telrik RadGrid deletes ViewState values After Selecting Records From RadGrid

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C#

ViewState

DataTable

Telerik

I have very simple code in which I just want to check user have selected the record in grid or not for this what I have done is

public void chkSelect_CheckedChanged(object sender, EventArgs e)
{

CheckBox cbChkAllocate = (CheckBox)sender;
GridDataItem dataItem = (GridDataItem)cbChkAllocate.NamingContainer;

if (cbChkAllocate.Checked == true)
{
dtRoom = getDataTable();
dtRoom.Rows[dataItem.ItemIndex]["Transtatus"] = cbChkAllocate.Checked;

gwRoom.DataSource = dtRoom;
gwRoom.DataBind();
ViewState["dtRoom"] = dtRoom;

}
}

and In my aspx file I have placed this for check box

<telerik:GridTemplateColumn  DataField="AllocationFlag" UniqueName="AllocationFlag" SortExpression="trans_status" >


                                            <ItemTemplate>
                                                <asp:CheckBox class="checkbox" ID="chkAllocate" runat="server"   OnCheckedChanged="chkSelect_CheckedChanged" AutoPostBack="true" />
                                            </ItemTemplate>
                                        </telerik:GridTemplateColumn>

this event of checkbox selecteing gets fired every time which is ok and my datatable gets its value but after selecting more than many records and pressing submit button I am unable to get value from ViewState["dtRoom"] which gets null I have noting in my page load event