How to get selected rows data in code behind in .NET WebForms Grid?

<asp:Button ID="button" runat="server" Text="SelectedRow" OnClick="button_Click" />
 
      <ej:Grid ID="FlatGrid" runat="server" AllowPaging="True">
            <Columns>
                <ej:Column Field="OrderID" HeaderText="Order ID" IsPrimaryKey="True" TextAlign="Right" Width="75" />
                <ej:Column Field="CustomerID" HeaderText="Customer ID" Width="80" />
                <ej:Column Field="EmployeeID" HeaderText="Employee ID" TextAlign="Right" Width="75"/>
                <ej:Column Field="Freight" HeaderText="Freight" TextAlign="Right" Width="75" Format="{0:C}"/>
                <ej:Column Field="OrderDate" HeaderText="Order Date" TextAlign="Right" Width="80" Format="{0:MM/dd/yyyy}"/>
                <ej:Column Field="ShipCity" HeaderText="Ship City" Width="110" />
            </Columns>
        </ej:Grid>
    

  protected void button_Click(object sender, EventArgs e)
        {
            var index = this.FlatGrid.SelectedRowIndex;     //get the index of the selected row in current
            List<Orders> data = ViewState["DataSource"] as List<Orders>;
            if(this.FlatGrid.PageSettings.CurrentPage > 1)
               index = this.FlatGrid.PageSettings.PageSize * (this.FlatGrid.PageSettings.CurrentPage - 1) + index;        //get index to retrieve the data from the dataSource based on the pagesize and currenpage
            var selectedData = data[index];    //selectedData contains the current selected Row data
        } 
     }
  }