Prevent Auto-Deselecting of Items
When you enable the selection functionality of RadGrid, it has a single row selection by default. If you want to allow multi-row selection, you can use the AllowMultiRowSelection property and make avail of the Ctrl and Shift keys to select and deselect items. For convenience, you can also use a GridClientSelectColumn which will generate a CheckBox for every row.
By design, simply clicking on a row will select it, but will also deselect any previously selected rows if you are not holding the required keys mentioned above. This default behavior can be seen in the Grid - Row Selection live demo.
If you prefer to toggle the selected state of the row using single clicks without holding any keys, you can prevent the auto-deselecting using the approach provided below.
<telerik:RadGrid RenderMode="Lightweight" ID="RadGrid1" runat="server" AllowPaging="True" Width="800px"
OnNeedDataSource="RadGrid1_NeedDataSource" AllowMultiRowSelection="true">
<ClientSettings>
<Selecting AllowRowSelect="true" UseClientSelectColumnOnly="true" />
<ClientEvents OnRowClick="toggleSelection" />
</ClientSettings>
<MasterTableView DataKeyNames="OrderID">
<Columns>
<telerik:GridClientSelectColumn UniqueName="SelectColumnName">
</telerik:GridClientSelectColumn>
</Columns>
</MasterTableView>
</telerik:RadGrid>
function toggleSelection(sender, args) {
args.get_gridDataItem().set_selected(!args.get_gridDataItem().get_selected());
}