<telerik:RadGrid ID="RadGrid1" runat="server" AutoGenerateColumns="false" AllowFilteringByColumn="true"
    OnNeedDataSource="RadGrid1_NeedDataSource" OnInsertCommand="RadGrid1_InsertCommand">
    <MasterTableView CommandItemDisplay="Top" DataKeyNames="ID">
        <Columns>
            <telerik:GridEditCommandColumn></telerik:GridEditCommandColumn>
            <telerik:GridButtonColumn CommandName="Delete"></telerik:GridButtonColumn>

            <telerik:GridBoundColumn DataField="ID" HeaderText="ID" UniqueName="ID" ReadOnly="true"
                CurrentFilterFunction="Contains" AutoPostBackOnFilter="true" />

            <telerik:GridBoundColumn DataField="Name" HeaderText="Name" UniqueName="Name"
                AutoPostBackOnFilter="false" />

            <telerik:GridBoundColumn DataField="Category" HeaderText="Category" UniqueName="Category" DataType="System.String"
                CurrentFilterFunction="Contains" AutoPostBackOnFilter="true" />

            <telerik:GridBoundColumn DataField="Price" HeaderText="Price" UniqueName="Price"
                CurrentFilterFunction="Contains" AutoPostBackOnFilter="true" DataFormatString="{0:C}" />
        </Columns>
        <EditFormSettings InsertCaption="Add New Item" />
    </MasterTableView>
</telerik:RadGrid>

private const string DataTableSessionKey = "RadGridDataTable";
private DataTable SessionDataSource
{
    get
    {
        DataTable dt = Session[DataTableSessionKey] as DataTable;

        if (dt == null || !IsPostBack)
        {
            // Create an empty DataTable with schema defined
            dt = new DataTable();

            dt.Columns.Add("ID", typeof(int));
            dt.Columns.Add("Name", typeof(string));
            dt.Columns.Add("Category", typeof(string));
            dt.Columns.Add("Price", typeof(decimal));

            dt.PrimaryKey = new DataColumn[] { dt.Columns["ID"] };

            Session[DataTableSessionKey] = dt;
        }

        return dt;
    }
}
protected void RadGrid1_NeedDataSource(object sender, GridNeedDataSourceEventArgs e)
{
    (sender as RadGrid).DataSource = SessionDataSource;
}

protected void RadGrid1_InsertCommand(object sender, GridCommandEventArgs e)
{
    GridEditableItem insertItem = (GridEditableItem) e.Item;

    Hashtable newValues = new Hashtable();

    insertItem.ExtractValues(newValues);

    DataRow findLastItem = SessionDataSource.Select("ID=MAX(ID)").FirstOrDefault();

    newValues["ID"] = findLastItem != null ? (int) findLastItem["ID"] + 1 : 0;

    DataRow rowToInsert = SessionDataSource.NewRow();

    foreach (DictionaryEntry entry in newValues)
    {
        rowToInsert[entry.Key.ToString()] = entry.Value ?? DBNull.Value;
    }

    SessionDataSource.Rows.Add(rowToInsert);
}