I have implemented the ListDataValidationRule code to add a dropdownlist to the spreadsheet. Based on the code I can find and the searching I have conducted, it seems to be correct, however the dropdown does not appear in the control.

The dropdown list does appear in Excel when I export the file.

 

Dim Context As ListDataValidationRuleContext = New ListDataValidationRuleContext(CurrentWorksheet, CurrentCellIndex)

Context.InputMessageTitle = "Restricted input"
Context.InputMessageContent = "The input is restricted to the week days."
Context.ErrorStyle = ErrorStyle.Stop
Context.ErrorAlertTitle = "Invalid Bedroom Type"
Context.ErrorAlertContent = "The entered value is not valid."
Context.InCellDropdown = True

Context.Argument1 = String.Join(",", _Data.ArchitectCert_UnitBedrooms.AsEnumerable().Select(Function(x) x.Field(Of Integer)("UnitBedroomNumber").ToString()).ToArray())


Dim Rule As ListDataValidationRule = New ListDataValidationRule(Context)

CurrentWorksheet.Cells(CurrentCellIndex).SetDataValidationRule(Rule)