An ArgumentNullException is thrown when you try to open a document containing relationship namespace declared in the tag.

<sheet name="Management Position View" sheetId="1" d3p1:id="rId1" xmlns:d3p1="http://schemas.openxmlformats.org/officeDocument/2006/relationships" />

The exception is thrown in the QuarterAutoFillGenerator TryParse method due to the unexpected length of the value.

Workaround: Do not use SetValueAsText method by passing an empty string to it. Use SetValue or ClearValue instead.

At the moment the theme of the RadSpreadsheetRibbon can be set only by setting the theme of the entire application. The individual setting (using telerik:StyleManager.Theme="Windows8") should also be supported.

When a sheet that has a new line in a text value is exported and the resulting file is opened in Excel, the value is displayed without a new line. If you enter edit mode and then exit (without confirming the value by pressing enter) the new line will appear. The reason for this behavior is that when there is a new line in a text value Excel automatically applies wrap text on the cell while RadSpreadsheet does not do that. A possible workaround is to apply wrap text on new values with new line in them.

Implement mechanism for disabling, enabling or changing of the commands of the CellEditor classes, like Copy and Paste.

When a row is removed through code and an image is created immediately after that in the same method, the UI does not update. The change is made in the model, but not visualized. If you scroll up and down the change is reflected in the UI.

When trying to set the VisibleSize property of RadSpreadsheet on the ActiveSheetChanged event a null exception is thrown in the UpdateSheet method of RadSheetEditorBase. 

Possible workaround is to record the visible size at the SheetChanged event and set it at a later time, e.g. using a timer.

if you use Excel, you use a lot of formulas. 
But In Telerik WPF there isn't enough formulas.(even like SUMIFS , SUBTOTAL) 

But I search for other library like Kendo-UI, there is more fomulas than WPF. 
Why are these different? 

Please make this formulas in your library.

Selecting a column with two or more merged cells, selects not only the selected column but the rest of columns that the merged cells are from. The behavior when working with rows is analogical.

If you begin entering e.g. "=" in FormulaBar, the characters are not visible in dark themes. If the focus is in the cell it works as expected, if the focus is in FormulaBar, the Foreground is dark and cannot be seen on dark background. 

Workaround:
-  set the following style (add BaseOn if using implicit styles)

<Style  TargetType="spreadsheetControls:RadSpreadsheetFormulaBar" BasedOn="{StaticResource RadSpreadsheetFormulaBarStyle}">
            <Setter Property="Foreground" Value="White"/>
</Style>

Available in LIB Version 2017.2.828.

The class would be used for customization of the printing behavior of the control.

Implement automatic completion of cell value based on the values typed above, in the same column.

#.##0,00;-#.##0,00;;@ does not work in RadSpreadsheet. It should suppress visualizing of ) values.

Workaround: #.##0,00;-#.##0,00; ;@ works fine (there is ' ' [space] set for the zero values).

Setting the visible size property to a small number, like 10,10, will result in missing the last column right border and last row bottom border.

NullReferenceException is thrown when the VisibleSize property of RadSpreadsheet is set more than once on two (or more) adjacent lines. For example:

spreadsheet.VisibleSize = new SizeI(1, 1);
spreadsheet.VisibleSize = new SizeI(5, 5);

To work this around you can increase the time between the settings of the VisibleSize property. For example you can use the ActiveSheetEditorChanged event of RadSpreadsheet and the ActivePresenterChanged event of the ActiveWorksheetEditor, and the UIUpdated event of the ActivePresenter. Each time the VisibleSize is changed, the ActiveWorksheetEditor will be recreated along with its ActivePresenter. You should make sure the second, third, etc. setting of the VisibleSize is executed after the new ActivePresenter is loaded. This can be done in the UIUpdated event.

When a de-DE culture is applied the 1,1 should be parsed as a number. On the other hand the 1.1 (with a dot) should be parsed as 01.Jan. Currently, both values (1,1 and 1.1) are parsed as datetime.

Decline reason: This is reproducible only if the cell was previously formatted as DateTime. In this case the separator is not taken into account which is expected. The same behavior can be observed also in Excel.

When the VisibleSize property of the spreadsheet control is set (for example new SizeI(10, 10)) and column is selected via its header (click on it), the IsColumnSelection property of the ActiveWorksheetEditor's Selection returns False. This applies also for the IsRowSelection property.