MonoTouch NullReferenceException on tableview scroll - xamarin.ios

Question

In my app i simply trying to add a checkmark to the selected row. I have around 20 items in my row and want to show a checkmark for the selected row. When i scroll to the bottom of the page and select a row it is throwing NullReferenceException. I have checkmark code reference from here
using System;
using System.Drawing;
using MonoTouch.Foundation;
using MonoTouch.UIKit;
using MonoTouch.ObjCRuntime;
using System.Collections.Generic;
namespace SampleApp
{
public partial class FirstViewController : UITableViewController
{
DataSource dataSource;
public FirstViewController () : base ("FirstViewController", null)
{
Title = NSBundle.MainBundle.LocalizedString ("First", "First");
TabBarItem.Image = UIImage.FromBundle ("first");
}
public override void DidReceiveMemoryWarning ()
{
// Releases the view if it doesn't have a superview.
base.DidReceiveMemoryWarning ();
// Release any cached data, images, etc that aren't in use.
}
public override void ViewDidLoad ()
{
base.ViewDidLoad ();
// Perform any additional setup after loading the view, typically from a nib.
// Add Table
TableView.Source = dataSource = new DataSource (this);
}
public override bool ShouldAutorotateToInterfaceOrientation (UIInterfaceOrientation toInterfaceOrientation)
{
// Return true for supported orientations
return (toInterfaceOrientation != UIInterfaceOrientation.PortraitUpsideDown);
}
class DataSource : UITableViewSource
{
static readonly NSString CellIdentifier = new NSString ("DataSourceCell");
FirstViewController controller;
private List<String> _listData = new List<String> ();
private NSIndexPath _previousRow;
public DataSource (FirstViewController controller)
{
this.controller = controller;
// POPULATE DISPOSITION LIST
PopulateDatabase();
_previousRow = NSIndexPath.FromRowSection(Settings.SelectedIndex,0);
}
void PopulateDatabase()
{
_listData.Add("val1");
_listData.Add("val2");
_listData.Add("val3");
_listData.Add("val4");
_listData.Add("val5");
_listData.Add("val6");
_listData.Add("val7");
_listData.Add("val8");
_listData.Add("val9");
_listData.Add("val10");
_listData.Add("val11");
_listData.Add("val12");
_listData.Add("val13");
_listData.Add("val14");
_listData.Add("val15");
}
// Customize the number of sections in the table view.
public override int NumberOfSections (UITableView tableView)
{
return 1;
}
public override int RowsInSection (UITableView tableview, int section)
{
return _listData.Count;
}
// Customize the appearance of table view cells.
public override UITableViewCell GetCell (UITableView tableView, MonoTouch.Foundation.NSIndexPath indexPath)
{
var cell = tableView.DequeueReusableCell (CellIdentifier);
if (cell == null) {
cell = new UITableViewCell (UITableViewCellStyle.Default, CellIdentifier);
//cell.Accessory = UITableViewCellAccessory.DisclosureIndicator;
}
cell.TextLabel.Text = _listData[indexPath.Row];
cell.TextLabel.Font = UIFont.SystemFontOfSize(18);
return cell;
}
// Row Select
public override void RowSelected (UITableView tableView, NSIndexPath indexPath)
{
// Uncheck the previous row
if (_previousRow != null)
tableView.CellAt(_previousRow).Accessory = UITableViewCellAccessory.None;
// Do something with the row
var row = indexPath.Row;
Settings.SelectedIndex = row;
tableView.CellAt(indexPath).Accessory = UITableViewCellAccessory.Checkmark;
//Console.WriteLine("{0} selected",_controller.Items[row]);
_previousRow = indexPath;
// This is what the Settings does under Settings>Mail>Show on an iPhone
tableView.DeselectRow(indexPath,false);
}
// Set row height
public override float GetHeightForRow(UITableView tableView, NSIndexPath indexPath)
{
return 50f;
}
}
public class Settings
{
public static int SelectedIndex = 0;
}
}
}
I get the exception at line
tableView.CellAt(_previousRow).Accessory =UITableViewCellAccessory.None;
When i debug i found that Accessory
tableView.CellAt(_previousRow).Accessory Unknown member: Accessory
Any idea what is going wrong???
Got the anser
// Row Select
public override void RowSelected (UITableView tableView, NSIndexPath indexPath)
{
// Uncheck the previous row
if (_previousRow != null)
{
NSIndexPath [] temp = tableView.IndexPathsForVisibleRows;
if (temp.Contains(preIndex))
{
tableView.CellAt(_previousRow).Accessory = UITableViewCellAccessory.None;
}
}
// Do something with the row
var row = indexPath.Row;
Settings.SelectedIndex = row;
tableView.CellAt(indexPath).Accessory = UITableViewCellAccessory.Checkmark;
//Console.WriteLine("{0} selected",_controller.Items[row]);
_previousRow = indexPath;
// This is what the Settings does under Settings>Mail>Show on an iPhone
tableView.DeselectRow(indexPath,false);
}