"An item with the same key has already been added" Exception when creating ExcelTable.

[Dude3030]

If we have two equal cell values with illegal xml characters, <test> for example, and try to add
a table containing this cells:
var table = sheet.Tables.Add(sheet.Cells[sheet.Dimension.Address], "dataTable");
then we get the following exception:

ArgumentException: An item with the same key has already been added
System.ThrowHelper.ThrowArgumentException(ExceptionResource resource) in System.Collections.Generic.Dictionary'2.Insert(TKey key, TValue value, Boolean add) in OfficeOpenXml.Table.ExcelTable.GetStartXml(String name, Int32 tblId) in OfficeOpenXml.Table.ExcelTable..ctor(ExcelWorksheet sheet, ExcelAddressBase address, String name, Int32 tblId) в OfficeOpenXml.Table.ExcelTableCollection.Add(ExcelAddressBase Range, String Name) в Mfc.Web.Results.ExcelTableResult'1.CreatePackage() in D:\Agent_work\6\s\Mfc\Mfc.Web.Controllers\Results\ExcelTableResult{T}.cs:line 110

Looking at the source,

EPPlus/EPPlus/Table/ExcelTable.cs

Line 172 in 4dacf27

if (cell.Value == null || names.ContainsKey(cell.Value.ToString()))

the second part of the condition looks for cell value before xml encoding.

EPPlus/EPPlus/Table/ExcelTable.cs

Line 184 in 4dacf27

colName = SecurityElement.Escape(cell.Value.ToString());

Code above does not check encoded column value for presence in the dictionary.
So, for our example:

1. String <test> does not exist in the names dictionary.
2. After xml encoding we get the string <test> and add this value to the dictionary.
3. In the next iteration we try to find the second string <test>, but this value do not exist in the dictionary.
4. Therefore we try to add an item with the key <test> twice.