Exception message shown when ExcelDataReader closed is misleading

[khensunny]

Hi there. I have been debugging an issue with some excel sheet throwing an error when enumerating its rows. The error message says: Sheet "Sheet1" has no heading. However, the sheet in question does have a heading. Fast forward, I found the root cause of the issue and as you can see with the code below:

 private static void Main(string[] args)
 {
    // NOTE: sheet.ReadRows uses yields to return the rows and at this point the ExcelDataReader has already been closed
    // because of the use of `using` to open the ExcelImporter
     var rows = ReadSheet(); 
     Console.WriteLine(rows.Count());
 }

 private static IEnumerable<ExcelPolicyDetailsImportRow> ReadSheet()
 {
     using (var stream = File.OpenRead("testing_sheet.xlsx"))
     {
         using (var importer = new ExcelImporter(stream))
         {
             importer.Configuration.RegisterClassMap<PolicyDetailsImportMap>();
             var sheet = importer.ReadSheet();
             return sheet.ReadRows<ExcelPolicyDetailsImportRow>();
         }
     }
 }

The rows are being read after the ExcelImporter has already been closed/disposed. My suggestion is that since ExcelDataReader has property IsClosed to tell whether the reader has been disposed of, that can be used to determine which message to show in situations where ExcelDataReader.Read() returns false.