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 the size of the columns is changed using the ColumnSelection.SetWidth(*) method does not update the layout, so subsequent uses of the layout are not calculated correctly. For example BringIntoView method works incorrectly. 

Workaround: call RadSpreadsheet.UpdateLayout() after setting the width.

Cell borders below the cells filled with data are not exported to XLSX.

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.

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.

The COUNTIF function is already available in RadSpreadsheet.

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.

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

Expose WorksheetUIUpdateContextBase.GetDataValidationRuleResult(CellIndex). This method is useful for creating optimized custom Data Validation Circles layers.

Available in LIB Version 2017.2.828.

Provide a way to display function name that is different than the actual names saved in the XML.

Add sheet button is hidden when there are many sheets. This should be checked with all themes.

Provide an easy way to bind or set the ApplicationName in the ribbon

The problem is that the spreadsheet wants to identify the main window. The main window is not known to the plugin and should not be, because it is located in a different app domain due to the specifics of the architecture (https://docs.microsoft.com/en-us/archive/msdn-magazine/2014/january/wpf-build-fault-tolerant-composite-applications).

If you set, for example, two copies, the printed copies will be 4. If you select three copies, the result will be nine copies.

Creating a chart with a title in excel, importing it in RadSpredsheet, and then exporting it to XLSX changes the size of the chart's title and labels.

The SpreadsheetML format has built-in reserved names. When such a name is defined in the scope of a worksheet, a SpreadsheetNameException is thrown during parsing the document.

The style gallery is broken in the fluent theme and crashes the control

Setting up a paper size to A1 and exporting it actually sets the paperSize to 1. Using the same scenario in Excel sets the paperSize to 305.

This leads to inappropriate values when entering the Page Setup options later in Excel.