RadGridview 1.BestFitColumns(Telerik.WinControls.UI.BestFitColumnMode.HeaderCells)

CultureInfo.CurrentCulture = CultureInfo.InvariantCulture;
grid.LoadLayout(xml);
CultureInfo.CurrentCulture = CultureInfo.GetCultureInfo("en-US");

DataTable table = new DataTable();
for (int i = 0; i < 20; i++)
{
    table.Columns.Add("Left" + i, typeof(int));
    table.Columns.Add("Right" + i, typeof(int));
}

for (int i = 0; i < 27; i++)
{
    List<object> parameters = new List<object>();
    for (int j = 0; j < table.Columns.Count - 2; j++)
    {
        parameters.Add(100 * i + i);
    }

    table.Rows.Add(parameters.ToArray());
}

this.radGridView1.DataSource = table;

ColumnGroupsViewDefinition def = new ColumnGroupsViewDefinition();
for (int i = 0, j = 0; i < 20; i++)
{
    var group = new GridViewColumnGroup("Group" + 3 + i);
    def.ColumnGroups.Add(group);
    group.Groups.Add(new GridViewColumnGroup("0"));
    group.Groups[0].Rows.Add(new GridViewColumnGroupRow() { MinHeight = 30});
    group.Groups[0].Rows[0].ColumnNames.Add(this.radGridView1.Columns[j].Name);
    this.radGridView1.Columns[j].Width = 58;
    j++;
    group.Groups[0].Rows[0].ColumnNames.Add(this.radGridView1.Columns[j].Name);
    this.radGridView1.Columns[j].Width = 58;
    j++;
}

radGridView1.MasterTemplate.ShowRowHeaderColumn = false;
this.radGridView1.ViewDefinition = def;

// To Export:
GridViewPdfExport pdfExport = new GridViewPdfExport(this.radGridView1);
pdfExport.FitToPageWidth = false;
pdfExport.ExportViewDefinition = true;
pdfExport.ExportVisualSettings = true;
pdfExport.RunExport(@"..\..\pdf", new PdfExportRenderer());

      public RadForm1()
      {
         InitializeComponent();
         PopulateGrid();
      }

      private void radButton1_Click(object sender, EventArgs e)
      {
         GridViewSpreadExport spreadExporter = new GridViewSpreadExport(this.radGridView1);
         SpreadExpo

rtRenderer exportRenderer = new SpreadExportRenderer();
         spreadExporter.ExportHierarchy = true;
         spreadExporter.ExportVisualSettings = true;
         spreadExporter.ChildViewExportMode = Telerik.WinControls.UI.Export.ChildViewExportMode.ExportAllViews;

         string fileName = @"..\..\exportedFile" + DateTime.Now.ToLongTimeString().Replace(":", "_") + ".xlsx";
         spreadExporter.RunExport(fileName, exportRenderer);
         Process.Start(fileName);
      }

      public void PopulateGrid()
      {
         List<GridResults.ChildCollectionFirst> firstCollection = new List<GridResults.ChildCollectionFirst>
         {
            new GridResults.ChildCollectionFirst
            {
               Test1 = "First Child"
            }
         }.ToList();

         List<GridResults.ChildCollectionSecond> secondCollection = new List<GridResults.ChildCollectionSecond>
         {
            new GridResults.ChildCollectionSecond
            {
               Test2 = "Second Child"
            }
         }.ToList();

         List<GridResults.ChildCollectionThird> thirdCollection = new List<GridResults.ChildCollectionThird>
         {
            new GridResults.ChildCollectionThird
            {
               Test3 = "Third Child"
            }
         }.ToList();

         List<GridResults.ChildCollectionFourth> fourthCollection = new List<GridResults.ChildCollectionFourth>
         {
            new GridResults.ChildCollectionFourth
            {
               Test4 = "Fourth Child"
            }
         }.ToList();


         var resultsList = new List<GridResults>
         {



            new GridResults
            {
               Id = "First Parent",
               Value = "Test Value 1",
               ChildCollection1 = new List<GridResults.ChildCollectionFirst>(),

               ChildCollection2 = new List<GridResults.ChildCollectionSecond>(),

               ChildCollection3 = new List<GridResults.ChildCollectionThird>(),

               ChildCollection4 = new List<GridResults.ChildCollectionFourth>()
            },
            new GridResults
            {
               Id = "Second Parent",
               Value = "Test Value 2",
               ChildCollection1 = firstCollection,

               ChildCollection2 = secondCollection,

               ChildCollection3 = thirdCollection,

               ChildCollection4 = fourthCollection
            },

            new GridResults
            {
            Id = "Third Parent",
            Value = "Test Value 3",
            ChildCollection1 = new List<GridResults.ChildCollectionFirst>(),

            ChildCollection2 = new List<GridResults.ChildCollectionSecond>(),

            ChildCollection3 = new List<GridResults.ChildCollectionThird>(),

            ChildCollection4 = new List<GridResults.ChildCollectionFourth>()
         },
            new GridResults
            {
               Id = "Fourth Parent",
               Value = "Test Value 4",
               ChildCollection1 = new List<GridResults.ChildCollectionFirst>(),

               ChildCollection2 = new List<GridResults.ChildCollectionSecond>(),

               ChildCollection3 = new List<GridResults.ChildCollectionThird>(),

               ChildCollection4 = new List<GridResults.ChildCollectionFourth>()
            },
            new GridResults
            {
            Id = "Fifth Parent",
            Value = "Test Value 5",
            ChildCollection1 = new List<GridResults.ChildCollectionFirst>(),

            ChildCollection2 = new List<GridResults.ChildCollectionSecond>(),

            ChildCollection3 = new List<GridResults.ChildCollectionThird>(),

            ChildCollection4 = new List<GridResults.ChildCollectionFourth>()
         },
         new GridResults
         {
            Id = "Sixth Parent",
            Value = "Test Value 6",
            ChildCollection1 = new List<GridResults.ChildCollectionFirst>(),

            ChildCollection2 = new List<GridResults.ChildCollectionSecond>(),

            ChildCollection3 = new List<GridResults.ChildCollectionThird>(),

            ChildCollection4 = new List<GridResults.ChildCollectionFourth>()
         }

         };
      //   radGridView1.BeginUpdate();
         radGridView1.DataSource = resultsList;
      //   radGridView1.EndUpdate();

            foreach (GridViewTemplate t in this.radGridView1.Templates)
            {
                t.AutoSizeColumnsMode= GridViewAutoSizeColumnsMode.Fill;
            }
      }

   public class GridResults
   {
      [DisplayName("ID")]
      public string Id { get; set; }

      [DisplayName("Value")]
      public string Value { get; set; }

      [DisplayName("Child Collection 1")]
      public List<ChildCollectionFirst> ChildCollection1 { get; set; }

      [DisplayName("Child Collection 2")]
      public List<ChildCollectionSecond> ChildCollection2 { get; set; }

      [DisplayName("Child Collection 3")]
      public List<ChildCollectionThird> ChildCollection3 { get; set; }

      [DisplayName("Child Collection 4")]
      public List<ChildCollectionFourth> ChildCollection4 { get; set; }

      [DisplayName("Child Collection 1")]
      public class ChildCollectionFirst
      {
         [DisplayName("Test Header")]
         public string Test1 { get; set; }
      }

      [DisplayName("Child Collection 2")]
      public class ChildCollectionSecond
      {
         [DisplayName("Test Header 2")]
         public string Test2 { get; set; }
      }

      [DisplayName("Child Collection 3")]
      public class ChildCollectionThird
      {
         [DisplayName("Test Header 3")]
         public string Test3 { get; set; }
      }

      [DisplayName("Child Collection 4")]
      public class ChildCollectionFourth
      {
         [DisplayName("Test Header 4")]
         public string Test4 { get; set; }
      }
   }

Message: Object reference not set to an instance of an object.
Telerik.WinControls.UI.RadGridViewDragDropService.GetDragImageHint(ContentAlignment textAlignment, Bitmap hintImage, RectangleF textRectangle, Int32 hintImageWidth)
Telerik.WinControls.UI.ColumnChooserItem.GetDragHintCore()
Telerik.WinControls.RadItem.Telerik.WinControls.ISupportDrag.GetDragHint()
Telerik.WinControls.RadDragDropService.PrepareContext():80
Telerik.WinControls.RadDragDropService.HandleMouseMove(Point mousePos):40
Telerik.WinControls.UI.RadGridViewDragDropService.HandleMouseMove(Point mousePosition):13
Telerik.WinControls.RadDragDropService.Telerik.WinControls.IMessageListener.PreviewMessage(Message& msg):150
Telerik.WinControls.RadMessageFilter.NotifyGetMessageEvent(Message& msg):41
Telerik.WinControls.RadMessageFilter.GetMessageHookProc(Int32 code, IntPtr wParam, IntPtr lParam):36

            DataTable dt = new DataTable();
            dt.Columns.Add("Id", typeof(int));
            dt.Columns.Add("Title", typeof(string));
            dt.Columns.Add("Price", typeof(decimal));

            for (int i = 0; i < 10; i++)
            {
                dt.Rows.Add(i, "Row" + i, 0);
            }

            this.radGridView1.DataSource = dt;
            this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

            GridViewSummaryItem summaryItem = new GridViewSummaryItem();
            summaryItem.Name = "Price";
            summaryItem.Aggregate = GridAggregateFunction.StDev;
            GridViewSummaryRowItem summaryRowItem = new GridViewSummaryRowItem();
            summaryRowItem.Add(summaryItem); 
            this.radGridView1.SummaryRowsBottom.Add(summaryRowItem);