For counting CSV rows. Helpful when querying with EF Core .AsAsyncEnumerable().

CsvWriterCounter.cs

using System.Globalization;
using CsvHelper;

//namespace AppFoo;

// Counts rows.  Helps when streaming to CSV from query that uses EF Core `.AsAsyncEnumerable()`.
public class CsvWriterCounter : CsvWriter
{
    int _counter;

    public CsvWriterCounter(TextWriter textWriter, CultureInfo culture = null) : base(textWriter, culture ?? CultureInfo.InvariantCulture)
    {
    }

    public int Count => _counter - 1;
    public Action<int> OnNext { get; set; }

    public override void NextRecord()
    {
        ++_counter;
        OnNext?.Invoke(_counter);
        base.NextRecord();
    }

    public override Task NextRecordAsync()
    {
        ++_counter;
        OnNext?.Invoke(_counter);
        return base.NextRecordAsync();
    }

}