To reproduce:

public Form1()
{
    InitializeComponent();

    GridViewDecimalColumn idColumn = new GridViewDecimalColumn("ID");         
    radGridView1.MasterTemplate.Columns.Add(idColumn);

    GridViewDecimalColumn parentIdColumn = new GridViewDecimalColumn("ParentID");         
    radGridView1.MasterTemplate.Columns.Add(parentIdColumn);

    GridViewTextBoxColumn nameColumn = new GridViewTextBoxColumn("Name");
    radGridView1.MasterTemplate.Columns.Add(nameColumn);

    GridViewTextBoxColumn addressColumn = new GridViewTextBoxColumn("Address");
    radGridView1.MasterTemplate.Columns.Add(addressColumn);

    GridViewTextBoxColumn typeColumn = new GridViewTextBoxColumn("Type");
    radGridView1.MasterTemplate.Columns.Add(typeColumn);

    radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
    this.radGridView1.Relations.AddSelfReference(this.radGridView1.MasterTemplate, "ID", "ParentID");

    AddNewRow("1", null, "Project", "USA", "Project");
    AddNewRow("2", "1", "Site 1", "New York", "Site");
    AddNewRow("3", "2", "Bldg 1", "Road 1", "Building");
    AddNewRow("4", "1", "Site 2", "New York", "Site");
    AddNewRow("5", "4", "Bldg 2", "Road 2", "Building");
    AddNewRow("20", "3", "Floor 1", "Road 20", "Floor");
    AddNewRow("30", "3", "Floor 2", "Road 30", "Floor");
    AddNewRow("40", "3", "Floor 3", "Road 40", "Floor");
}

public void AddNewRow(
    params object[] values)
{
    if (values.Length != radGridView1.Columns.Count)
        return;

    GridViewRowInfo newRow = radGridView1.Rows.AddNew();
    newRow.Cells[0].Value = values[0];
    newRow.Cells[1].Value = values[1];
    newRow.Cells[2].Value = values[2];
    newRow.Cells[3].Value = values[3];
    newRow.Cells[4].Value = values[4];
}

When you run the project you will notice that the last row is missing (AddNewRow("40", "3", "Floor 3", "Road 40", "Floor");). Please refer to the attached screenshots.

Workaround: use the Rows.Add(params) method instead:

To reproduce:

List<Coordinate> coordinates_ = new List<Coordinate>();

public Form1()
{
    InitializeComponent();
   
    for (int i = 0; i < 10; i++)
    {
        coordinates_.Add(new Coordinate(i * 0.25, i * 0.33, i * 0.46));
    }

    this.radGridView1.AutoGenerateColumns = false;

    string mask = "F2";          

    this.radGridView1.Columns.Add(CreateDecimalColumn("X", "X", mask));
    this.radGridView1.Columns.Add(CreateDecimalColumn("Y", "Y", mask));
    this.radGridView1.Columns.Add(CreateDecimalColumn("Z", "Z", mask));
  
    this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

    SetRows();
}

GridViewDataColumn CreateDecimalColumn(string name, string headertext, string mask)
{
    var format = "{0:" + mask + "}";
    return new GridViewMaskBoxColumn()
    {
        Name = name,
        HeaderText = headertext,
        MinWidth = 50,
        MaskType = MaskType.Numeric,
        Mask = mask,
        FormatString = format,
        DataType = typeof(double),
        TextAlignment = ContentAlignment.MiddleRight
    };
}

void SetRows()
{
    foreach (var c in coordinates_)
    {
        var ri = radGridView1.Rows.AddNew();
        ri.Cells["X"].Value = c.X;
        ri.Cells["Y"].Value = c.Y;
        ri.Cells["Z"].Value = c.Z;
    }
}

public class Coordinate
{
    public double X { get; set; }

    public double Y { get; set; }

    public double Z { get; set; }

    public Coordinate(double x, double y, double z)
    {
        this.X = x;
        this.Y = y;
        this.Z = z;
    }
}

Workaround:
private void radGridView1_EditorRequired(object sender, EditorRequiredEventArgs e)
{
    if (e.EditorType==typeof(RadMaskedEditBoxEditor))
    {
        e.Editor = new RadMaskedEditBoxEditor();
    }
}  

Version 2014.3.1202.40 scrolling worsened

How to reproduce:
 public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            DataTable dataTable = new DataTable();
            dataTable.Columns.Add("Id", typeof(int));
            dataTable.Columns.Add("Id2", typeof(int));
            dataTable.Columns.Add("Id3", typeof(int));
            dataTable.Columns.Add("Id4", typeof(int));

            for (int i = 0; i < 10000; i++)
            {
                dataTable.Rows.Add(i, i + 10, i +20, i+30);
            }

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

            this.Load += Form1_Load;
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            ExpressionFormattingObject idObj = new ExpressionFormattingObject("1", "Id % 2 = 0", false);
            idObj.CellBackColor = Color.SkyBlue;
            idObj.CellForeColor = Color.Red;
            this.radGridView1.Columns["Id"].ConditionalFormattingObjectList.Add(idObj);

            ExpressionFormattingObject idObj2 = new ExpressionFormattingObject("1", "Id2 % 2 = 0", false);
            idObj2.CellBackColor = Color.LightGray;
            idObj2.CellForeColor = Color.Red;
            this.radGridView1.Columns["Id2"].ConditionalFormattingObjectList.Add(idObj2);

            ExpressionFormattingObject idObj3 = new ExpressionFormattingObject("1", "Id3 % 2 = 0", false);
            idObj3.CellBackColor = Color.LightGreen;
            idObj3.CellForeColor = Color.Red;
            this.radGridView1.Columns["Id3"].ConditionalFormattingObjectList.Add(idObj3);

            ExpressionFormattingObject idObj4 = new ExpressionFormattingObject("1", "Id4 % 2 = 0", false);
            idObj4.CellBackColor = Color.LightYellow;
            idObj4.CellForeColor = Color.Red;
            this.radGridView1.Columns["Id4"].ConditionalFormattingObjectList.Add(idObj4);
        }
    }

Workaround: 
Apply formatting on CellFormatting event

Workaround - hide the expander items if space is not enough:

 private void radGridView1_CellFormatting(object sender, CellFormattingEventArgs e)
 {
     GridDataCellElement cell = e.CellElement as GridDataCellElement;
     if (cell != null && cell.SelfReferenceLayout != null)
     {
         foreach (RadElement element in cell.SelfReferenceLayout.StackLayoutElement.Children)
         {
             GridExpanderItem expanderItem = element as GridExpanderItem;
             if (expanderItem != null)
             {
                 if (cell.ColumnInfo.Width < cell.SelfReferenceLayout.StackLayoutElement.Size.Width)
                 {
                     expanderItem.Opacity = 0;
                 }
                 else
                 {
                     expanderItem.Opacity = 1;
                 }
             }
         }
     }
 }

To reproduce:

             this.radGridView1.MultiSelect = true;

            GroupDescriptor descriptor1 = new GroupDescriptor();
            descriptor1.GroupNames.Add("ProductId", ListSortDirection.Ascending );
            this.radGridView1.GroupDescriptors.Add(descriptor1);

Please refer to the attached gif file.

The conditional formatting collection get deleted after closing the property builder.

To reproduce :

Add a gridview with a datasource.
Open property builder
Select a column
Advance -> ConditionalFormattingObjectList
Add a expression, set a name
Press Ok

If you click the ConditionalFormattingObjectList, you can see the expression

If you click close the Property Builder, the list is not saved when you open it again.

To reproduce:
public RadForm1()
{
    InitializeComponent();
    DataTable master = new DataTable();
    master.Columns.Add("ID", typeof(int));
    master.Columns.Add("F_ID", typeof(int));
    master.Columns.Add("test", typeof(string));

    DataTable child = new DataTable();
    child.Columns.Add("F_ID", typeof(int));
    child.Columns.Add("test", typeof(string));
    child.Columns.Add("CheckBox", typeof(bool));

    for (int i = 0; i < 10; i++)
    {
        master.Rows.Add(i, i , "Row " + i);
        child.Rows.Add(i , "Child " + i, true);
    }

    radGridView1.DataSource = master;
    GridViewTemplate template = new GridViewTemplate();
 
    template.DataSource = child;
    radGridView1.MasterTemplate.Templates.Add(template);

    GridViewRelation relation = new GridViewRelation(radGridView1.MasterTemplate);
    relation.ChildTemplate = template;
    relation.RelationName = "Test";
    relation.ParentColumnNames.Add("F_ID");
    relation.ChildColumnNames.Add("F_ID");
    radGridView1.Relations.Add(relation);
    this.Load += RadForm1_Load;

}

void RadForm1_Load(object sender, EventArgs e)
{
    GridViewCheckBoxColumn col = radGridView1.MasterTemplate.Templates[0].Columns[2] as GridViewCheckBoxColumn;
    col.EnableHeaderCheckBox = true;
   
}

- Expand some rows and click on header check box.

Use the following code snippet and follow the illustrated steps on the attached gif file:

 private void Form1_Load(object sender, EventArgs e)
 {
     this.customersTableAdapter.Fill(this.nwindDataSet.Customers);
     this.radGridView1.DataSource = this.customersBindingSource;
     this.radGridView1.BestFitColumns(BestFitColumnMode.AllCells);
     ColumnGroupsViewDefinition view = new ColumnGroupsViewDefinition();
     view.ColumnGroups.Add(new GridViewColumnGroup("Customer Contact"));
     view.ColumnGroups.Add(new GridViewColumnGroup("Details"));
     view.ColumnGroups[1].Groups.Add(new GridViewColumnGroup("Address"));
     view.ColumnGroups[1].Groups.Add(new GridViewColumnGroup("Contact"));
     view.ColumnGroups[0].Rows.Add(new GridViewColumnGroupRow());
     view.ColumnGroups[0].Rows[0].Columns.Add(this.radGridView1.Columns["CompanyName"]);
     view.ColumnGroups[0].Rows[0].Columns.Add(this.radGridView1.Columns["ContactName"]);
     view.ColumnGroups[0].Rows[0].Columns.Add(this.radGridView1.Columns["ContactTitle"]);

     view.ColumnGroups[1].Groups[0].Rows.Add(new GridViewColumnGroupRow());
     view.ColumnGroups[1].Groups[0].Rows[0].Columns.Add(this.radGridView1.Columns["Address"]);
     view.ColumnGroups[1].Groups[0].Rows[0].Columns.Add(this.radGridView1.Columns["City"]);
     view.ColumnGroups[1].Groups[0].Rows[0].Columns.Add(this.radGridView1.Columns["Country"]);

     view.ColumnGroups[1].Groups[1].Rows.Add(new GridViewColumnGroupRow());
     view.ColumnGroups[1].Groups[1].Rows[0].Columns.Add(this.radGridView1.Columns["Phone"]);
     view.ColumnGroups[1].Groups[1].Rows[0].Columns.Add(this.radGridView1.Columns["Fax"]);
     radGridView1.ViewDefinition = view;
 }

When you use a custom GroupComparer for the MasterTemplate, grouping is performed successfully for the first time. However, if you remove all groups and perform grouping again for the same columns, the grid behavior is not as expected: you will notice groups mismatching or duplication. Here is a sample code snippet which incorrect behavior is illustrated on the attached gif file:

public Form1()
{
    InitializeComponent();

    DataTable dt = new DataTable();
    dt.Columns.Add("Reference");
    dt.Columns.Add("Test Case");
    dt.Columns.Add("ChBW");
    dt.Columns.Add("Voltage");
    dt.Columns.Add("Environmental Conditions");
    dt.Columns.Add("RadioAccessTechnology");

    dt.Rows.Add("4.2.1", "E-Utran FDD...", "10", "C Nominal-2", "T Nominal - V Nominal", "E-UTRAN Only");
    dt.Rows.Add("5.2.1", "E-Utran FDD...", "3", "C Nominal-1", "T Nominal - V Nominal", "E-UTRAN Only");
    dt.Rows.Add("14.2.1", "E-Utran FDD...", "1", "C Nominal-3", "T Nominal - V Nominal", "E-UTRAN Only");
    dt.Rows.Add("2.2.1", "E-Utran FDD...", "4", "C Nominal-2", "T Nominal - V Nominal", "E-UTRAN Only");
    dt.Rows.Add("7.2.1", "E-Utran FDD...", "12", "C Nominal-1", "T Nominal - V Nominal", "E-UTRAN Only");
    dt.Rows.Add("2.2.1", "E-Utran FDD...", "2", "C Nominal-3", "T Nominal - V Nominal", "E-UTRAN Only");

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

    this.radGridControlSelection.MasterTemplate.GroupComparer = new GroupComparer();
}

public class GroupComparer : IComparer<Group<GridViewRowInfo>>
{
    public int Compare(Group<GridViewRowInfo> x, Group<GridViewRowInfo> y)
    {
        if (x.Level != y.Level)
        {
        }
        DataGroup group = x as DataGroup;

        var a = x.Header;
        var b = y.Header;

        int valueA;
        int valueB;

        if (group == null)
        {
            group = y as DataGroup;
        }
        if (group != null && group.GroupDescriptor != null && group.GroupDescriptor.GroupNames.Count > 0)
        {
            string propertyName = group.GroupDescriptor.GroupNames.First().PropertyName;
            if ((propertyName.ToUpper() == "VOLTAGE"))
            {
                int indexA = GetIndexContain(a.ToString().Split(new char[] { '-' })[1]);
                int indexB = GetIndexContain(b.ToString().Split(new char[] { '-' })[1]);

                if (indexA == indexB)
                {
                    return 0;
                }
                else if (indexA < indexB)
                {
                    return -1;
                }
                else
                {
                    return 1;
                }
            }
            else if (propertyName.ToUpper() == "RADIOACCESSTECHNOLOGY")
            {
                return x.Key.ToString().CompareTo(y.Key.ToString());
            }
            // BAND
            else if (propertyName.ToUpper() == "CHBW")
            {
                Int32.TryParse(a, out valueA);
                Int32.TryParse(b, out valueB);

                //ASCENDING SELECTED
                if (group.GroupDescriptor.GroupNames.First().Direction == ListSortDirection.Ascending)
                {
                    if (valueA > valueB)
                    {
                        return 1;
                    }
                    else if (valueA < valueB)
                    {
                        return -1;
                    }
                    else
                    {
                        return 0;
                    }
                }
                //DESCENDING
                else
                {
                    if (valueA > valueB)
                    {
                        return -1;
                    }
                    else if (valueA < valueB)
                    {
                        return 1;
                    }
                    else
                    {
                        return 0;
                    }
                }
            }
        }
        return x.Key.ToString().CompareTo(y.Key.ToString());
    }

    private int GetIndexContain(string a)
    {
        int parsedValue;
        if (int.TryParse(a, out parsedValue))
        {
            return 10 - parsedValue;
        }
        return -1;
    }
}


Workaround:

public class Grid:RadGridView
{
    public override string ThemeClassName  
    { 
        get 
        { 
            return typeof(RadGridView).FullName;  
        }
    }

    protected override RadGridViewElement CreateGridViewElement()
    {
        return new CustomGridViewElement();
    }
}

public class CustomGridViewElement : RadGridViewElement
{
    protected override Type ThemeEffectiveType     
    { 
        get    
        { 
            return typeof(RadGridViewElement);     
        }
    }

    protected override MasterGridViewTemplate CreateTemplate()
    {
        return new CustomMasterGridViewTemplate();
    }
}

public class CustomMasterGridViewTemplate : MasterGridViewTemplate
{
    protected override GridViewListSource CreateListSource()
    {
        return new CustomGridViewListSource(this);
    }
}

public class CustomGridViewListSource : GridViewListSource
{
    public CustomGridViewListSource(GridViewTemplate template) : base(template)
    {
    }

    protected override RadCollectionView<GridViewRowInfo> CreateDefaultCollectionView()
    {
        return new CustomGridDataView(this);
    }
}

public class CustomGridDataView : GridDataView
{
    public CustomGridDataView(GridViewListSource listSource) : base(listSource)
    {
    }

    protected override GroupBuilder<GridViewRowInfo> CreateGroupBuilder()
    {
        return new CustomGroupBuilder(this.Indexer);
    }
}

public class CustomGroupBuilder : GridGroupBuilder
{
    public CustomGroupBuilder(Index<GridViewRowInfo> index) : base(index)
    {
    }

    protected override Group<GridViewRowInfo> GetGroup(GroupCollection<GridViewRowInfo> cache,
        Group<GridViewRowInfo> newGroup, Group<GridViewRowInfo> parent, object key, int level)
    {
        GroupDescriptor currentDescriptor = this.CollectionView.GroupDescriptors[level];
        DataGroup group = (DataGroup)base.GetGroup(null, newGroup, parent, key, level);
        if (group.GroupDescriptor != null && group.GroupDescriptor != currentDescriptor)
        {
            SetGroupDescriptor(group, null);
            IGroupFactory<GridViewRowInfo> groupFactory = this.CollectionView.GroupFactory;
            group = (DataGroup)groupFactory.CreateGroup(key, parent);
            group.GroupBuilder = this;
        }
        SetGroupDescriptor(group, currentDescriptor);
        return group;
    }

    private void SetGroupDescriptor(DataGroup dataGroup, GroupDescriptor currentDescriptor)
    {
        FieldInfo fi = typeof(DataGroup).GetField("groupDescriptor", BindingFlags.NonPublic | BindingFlags.Instance);
        fi.SetValue(dataGroup, currentDescriptor);
    }
}

To reproduce:
            string fileName = @"..\..\..\exported" + DateTime.Now.Hour + DateTime.Now.Minute + DateTime.Now.Second + ".xlsx";
            SpreadExport spreadExporter = new SpreadExport(radGridView1);
            spreadExporter.ExportVisualSettings = false;      
            spreadExporter.RunExport(fileName);


Workaround:
spreadExporter.WorkbookCreated += spreadExporter_WorkbookCreated;
private void spreadExporter_WorkbookCreated(object sender, WorkbookCreatedEventArgs e)
{

    Telerik.Windows.Documents.Spreadsheet.PropertySystem.CellStyle defaultStyle = e.Workbook.Styles.Add("DefaultStyle");
    defaultStyle.FontSize = Telerik.Windows.Documents.Spreadsheet.Utilities.UnitHelper.PointToDip(11);
    Telerik.Windows.Documents.Spreadsheet.Model.Worksheet sheet = e.Workbook.Worksheets.First();
    sheet.Cells[0, 0, sheet.UsedCellRange.RowCount, sheet.UsedCellRange.ColumnCount].SetStyleName("DefaultStyle");
    sheet.Columns[sheet.UsedCellRange].AutoFitWidth();
}

To reproduce:
 this.radGridView1.MultiSelect = true;

Please refer to the attached gif file illustrating better the behavior.

1. Select a row and filter the grid in a way to keep the selected row visible.
2. The first row in the ChildRows collection is selected.
3. Clear the filter. The selection is stored and only one row is selected.
4. Repeat the above steps, but perform such filtering that hides the selected cell. When you clear the filter, two rows are selected instead of one.

Workaround:
private void radGridView1_FilterChanged(object sender, Telerik.WinControls.UI.GridViewCollectionChangedEventArgs e)
        {
            this.radGridView1.ClearSelection();
            if (this.radGridView1.CurrentCell!=null)
            {
                this.radGridView1.CurrentCell.IsSelected = true;
                this.radGridView1.CurrentRow.IsSelected = true;
                this.radGridView1.GridNavigator.Select(this.radGridView1.CurrentRow, this.radGridView1.CurrentColumn);
            }
        }

Currently, when you have an object which has more than one parent, the grid falls in an invalid state and shows the child under all of the parents, however, it does not behave correctly. As this is not supported scenario, the user should be notified via exception.

Workaround:

Subscribe  to CellFormatting event:

void radGridView1_CellFormatting(object sender, CellFormattingEventArgs e)
{
    if (e.CellElement is GridCheckBoxCellElement)
    {
        e.CellElement.ToolTipText = "ErrorMessage for CheckBoxColumn";
        e.CellElement.Children[0].ToolTipText = "ErrorMessage for CheckBoxColumn";
        e.CellElement.Children[0].Children[0].ToolTipText = "ErrorMessage for CheckBoxColumn";
    }
}

To reproduce: use the following code snippet and follow the steps illustrated on the attached gif file:

private void Form1_Load(object sender, EventArgs e)
{
    this.customersTableAdapter.Fill(this.nwindDataSet.Customers);
    this.radGridView1.DataSource = this.customersBindingSource;
    this.radGridView1.BestFitColumns(BestFitColumnMode.AllCells);
    ColumnGroupsViewDefinition view = new ColumnGroupsViewDefinition();
    view.ColumnGroups.Add(new GridViewColumnGroup("Customer Contact"));
    view.ColumnGroups.Add(new GridViewColumnGroup("Details"));
    view.ColumnGroups[1].Groups.Add(new GridViewColumnGroup("Address"));
    view.ColumnGroups[1].Groups.Add(new GridViewColumnGroup("Contact"));
    view.ColumnGroups[0].Rows.Add(new GridViewColumnGroupRow());
    view.ColumnGroups[0].Rows[0].Columns.Add(this.radGridView1.Columns["CompanyName"]);
    view.ColumnGroups[0].Rows[0].Columns.Add(this.radGridView1.Columns["ContactName"]);
    view.ColumnGroups[0].Rows[0].Columns.Add(this.radGridView1.Columns["ContactTitle"]);

    view.ColumnGroups[1].Groups[0].Rows.Add(new GridViewColumnGroupRow());
    view.ColumnGroups[1].Groups[0].Rows[0].Columns.Add(this.radGridView1.Columns["Address"]);
    view.ColumnGroups[1].Groups[0].Rows[0].Columns.Add(this.radGridView1.Columns["City"]);
    view.ColumnGroups[1].Groups[0].Rows[0].Columns.Add(this.radGridView1.Columns["Country"]);

    view.ColumnGroups[1].Groups[1].Rows.Add(new GridViewColumnGroupRow());
    view.ColumnGroups[1].Groups[1].Rows[0].Columns.Add(this.radGridView1.Columns["Phone"]);
    view.ColumnGroups[1].Groups[1].Rows[0].Columns.Add(this.radGridView1.Columns["Fax"]);
    radGridView1.ViewDefinition = view;
}

Sometimes the incorrect behavior is obtained immediately after you drop the column, but you need to scroll the horizontal scroll-bar to replicate it.

Workaround:
RadGridViewDragDropService svc = this.radGridView1.GridViewElement.GetService<RadGridViewDragDropService>();
svc.Stopped += svc_Stopped;

private void svc_Stopped(object sender, EventArgs e)
{
    int horizontalScrollvalue = this.radGridView1.TableElement.HScrollBar.Value;
    this.radGridView1.MasterTemplate.Refresh();
    this.radGridView1.TableElement.HScrollBar.Value = horizontalScrollvalue;
}

To reproduce: use the following code snippet and refer to the attached gif file:

public Form1()
{
    InitializeComponent();
    for (int i = 0; i < 20; i++)
    {
        this.radGridView1.Columns.Add("Col" + i);
    }

    for (int i = 0; i < 10; i++)
    {
        GridViewDataRowInfo row = this.radGridView1.Rows.AddNew() as GridViewDataRowInfo;
        foreach (GridViewColumn col in this.radGridView1.Columns)
        {
            row.Cells[col.Name].Value = "Data" + row.Index + "." + col.Index;
        }
    }

    this.radGridView1.MultiSelect = true;
    this.radGridView1.SelectionMode = GridViewSelectionMode.CellSelect;
}


Workaround:
int startColumn = int.MaxValue;
int endColumn = 0;
int startRow = int.MaxValue;
int endRow = 0;

private void radGridView1_MouseDown(object sender, MouseEventArgs e)
{
    GridDataCellElement cellElement = this.radGridView1.ElementTree.GetElementAtPoint(e.Location) as GridDataCellElement;
    if (cellElement != null)
    {
        startColumn = cellElement.ColumnIndex;
        startRow = cellElement.RowIndex;
    }
}

private void radGridView1_MouseUp(object sender, MouseEventArgs e)
{
    GridDataCellElement cellElement = this.radGridView1.ElementTree.GetElementAtPoint(e.Location) as GridDataCellElement;
    if (cellElement != null)
    {
        endColumn = cellElement.ColumnIndex;
        endRow = cellElement.RowIndex;
    }

    if (endColumn < startColumn)
    {
        int swap = endColumn;
        endColumn = startColumn;
        startColumn = swap;
    }
    if (endRow < startRow)
    {
        int swap = endRow;
        endRow = startRow;
        startRow = swap;
    }

    this.radGridView1.ClearSelection();
    int scrollBarValue = this.radGridView1.TableElement.HScrollBar.Value;
    this.radGridView1.BeginUpdate();
    for (int i = startRow; i < endRow + 1; i++)
    {
        for (int j = startColumn; j < endColumn + 1; j++)
        {
            if (!this.radGridView1.Rows[i].Cells[j].IsSelected)
            {
                this.radGridView1.Rows[i].Cells[j].IsSelected = true;
            }
        }
    }
    this.radGridView1.EndUpdate();
    this.radGridView1.TableElement.HScrollBar.Value = scrollBarValue;
}

Hello,

I succeed to reproduce the GridViewComboboxColumn exception in this forum post:
http://www.telerik.com/forums/nullreferenceexception-4a6181b2453b#cwDrbIqzp0CPxcgh90b4rQ

I attach a sample project, the database (SQL Server 2012 Express) and a video from the exception.

To reproduce:
- Run the project,
- Sort the column "Állapot" descending.
- Click on column and drop down the list.
- Choose an another value, and click very fast twice. On a slow PC is much easier to reproduce the issue. The important thing, that you need select a value from the combobox and select another row very fast. (See the attached video)

I use the latest Trial version of Winforms.

If you have any question, please contact me.

Best Regards,
László

Workaround:

Private Sub gridMunkak_CreateCell(sender As Object, e As GridViewCreateCellEventArgs) Handles gridMunkak.CreateCell
    If e.CellType = GetType(GridComboBoxCellElement) Then
        e.CellElement = New MyGridComboBoxCellElement(e.Column, e.Row)
    End If
End Sub

Public Class MyGridComboBoxCellElement
    Inherits GridComboBoxCellElement
    Public Sub New(column As GridViewColumn, row As GridRowElement)
        MyBase.New(column, row)
    End Sub

    Public Overrides Sub SetContent()
        If Me.ColumnInfo IsNot Nothing Then
            MyBase.SetContent()
        End If
    End Sub
    Protected Overrides ReadOnly Property ThemeEffectiveType() As Type
        Get
            Return GetType(GridComboBoxCellElement)
        End Get
    End Property
End Class