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Gets a value that represents the format code of the associated range object as displayed in the current user interface.
Use this property to retrieve the effective number format currently shown for the range. This displayed format can differ from the range's own format settings when display-affecting features such as conditional formatting or table styles are applied.
string NumberFormat { get; }
ReadOnly Property NumberFormat As String
IRange range = worksheet.Range["A1"];
range.NumberFormat = "$#,##0.00";
range.Value = 1234.5;
string numberFormat = range.DisplayFormat.NumberFormat;