In CsvHelper how to catch a conversion error and know what field and what row it happened in?

Currently, there is no way to ignore errors at the field/property level. Your current options are these:

Look at the exception data.

catch( Exception ex )
{
    // This contains useful information about the error.
    ex.Data["CsvHelper"];
}

Ignore reading exceptions. This is on a row level, though, not field. It will allow the whole file to still be read, and just ignore the rows that don’t work. You can get a callback when an exception occurs.

csv.Configuration.IgnoreReadingExceptions = true;
csv.Configuration.ReadingExceptionCallback = ( ex, row ) =>
{
    // Do something with the exception and row data.
    // You can look at the exception data here too.
};