How to debug exceptions in C#?

Question

in my Windows Forms app I keep getting an unhandled ArgumentException ("input array is longer than number of columns in table") in line 56 (within catch). For debugging purposes I'm trying to show a message box, but I always get the exception and the message box never shows.

1. Why is the message box not shown?
2. How can I get the current values of row.Length, errMsg etc. and print them somewhere where I can read them?

In the watch window it says "value not available at current pointer".

How can I print simple debugging information in Windows Forms apps, just like I would do in console apps?

P.S. The method below is run after a button is clicked in the forms app.

using System;
using System.Data;
using System.IO;
using System.Text;
using CsvHelper;
using System.Windows.Forms;

namespace ARautomat_visual {

    class Model {

        // read CSV-file to DataTable
        // hasHeader is passed as FALSE
        public static DataTable GetDataTableFromCsv (string file, bool hasHeader = true) {

            DataTable dataTable = new DataTable();

            using (TextReader infile = new StreamReader(file, Encoding.Default))
            using (CsvParser csv = new CsvParser(infile)) {
                csv.Configuration.Delimiter = ";";

                bool datevFormat = false;
                string[] row;

                for (int i = 0; (row = csv.Read()) != null; i++) {

                    if (i == 0) {

                        if (datevFormat == false && (row[0] == "EXTF" || row[0] == "DTVF")) {
                            datevFormat = true;
                            i--;
                        }
                        else if (hasHeader == true) {
                            try {
                                string errMsg = $"row.Length: {row.Length}\n";

                                foreach (DataColumn item in dataTable.Columns) {
                                    errMsg += $"{item.ColumnName}, ";
                                }

                                Program.TriggerError(errMsg);

                                for (int j = 0; j < row.Length; j++) {
                                    dataTable.Columns.Add(row[j]);
                                }
                            }
                            catch (Exception e) {
                                string errMsg = $"Fehler: {e.Message}; row.Length:     {row.Length.ToString()}; Columns:     {dataTable.Columns.ToString()}";    

                                MessageBox.Show("Mist!");
                                Program.TriggerError(errMsg);
                                throw; // this is line 56
                            }
                        }
                        else {
                            dataTable.Rows.Add(row);
                        }
                    }
                    else if (i > 0) {
                        dataTable.Rows.Add(row);
                    }
                }
            }

            return dataTable;                
        }
    }
}

Program.cs

static class Program {

    // print all warnings
    public static void PrintWarnings() {
      if (!String.IsNullOrEmpty(warningsTriggered)) {
        TriggerStatus(warningsTriggered, "Warnung");
      }
    }

    // print warnings, print errors, exit
    public static void TriggerError (string msg) {
      PrintWarnings();
      TriggerStatus("Fehler: " + msg + @"Programm wird nach Klick auf 'OK' beendet.", "Fehler");
      //Application.Exit();
      Form.ActiveForm.Close();
    }

}

Answer 1

Turns out the problem was within the else-clause: I tried to add a new row to the empty dataTable without creating columns in beforehand.

I added the for-loop below and now the code runs fine.

else {

  for (int j = 1; j <= row.Length; j++) {
    dataTable.Columns.Add("Column " + j);
  }

  dataTable.Rows.Add(row);
}

What did confuse me a lot was the fact that Visual Studio was actually placing the exception symbol at the end of the catch-block, despite the fact that the else if-block wasn't even entered and the exception was actually thrown in the else-block.

Thanks to Chetan and PaulF for guiding me with debugging using break points!!

Answer 2

catch (Exception e)
{
    string errMsg = $"Fehler: {e.Message}; row.Length: {row.Length.ToString()}; Columns: {dataTable.Columns.ToString()}";    

    MessageBox.Show("Mist!");
    Program.TriggerError(errMsg);
    throw; // this is line 56
}

Your problem is that you are using the throw keyword. The throw keyword is used to re-throw an exception. See this: What does "throw;" by itself do?