CsvReader.cs

List<List<string>> ReadCsv(string fileName) {
    using var reader = new StreamReader(File.Open(fileName, FileMode.Open, FileAccess.Read, FileShare.ReadWrite));
    return ReadCsv(reader);
}

List<List<string>> ReadCsv(TextReader reader) {
    var result = new List<List<string>>();
    var currentRow = new List<string>();
    var currentCell = new StringBuilder();
    int c;

    void ReadNewLine() {
        if (reader.Read() != '\n') throw new FormatException("Found carriage return without following linefeed");
        currentRow.Add(currentCell.ToString());
        result.Add(currentRow);
        currentCell.Clear();
        currentRow = new List<string>();
    }
    
    void AddCell() {
        currentRow.Add(currentCell.ToString());
        currentCell.Clear();
    }
    
    void AddRow() {
        currentRow.Add(currentCell.ToString());
        result.Add(currentRow);
    }
    
    Start: switch (c = reader.Read()) {
        case '\r':
            ReadNewLine();
            goto Start;
        case '"': 
            goto QuotedCell;
        case int _ when c < 0: 
            if (currentRow.Count > 0) result.Add(currentRow);
            return result;
        default: 
            currentCell.Append((char)c);
            goto RawCell;
    }
    
    QuotedCell: switch (c = reader.Read()) {
        case '"': 
            goto EndQuote;
        case int _ when c < 0:
            throw new FormatException($"Unterminated quoted value encountered in record {result.Count + 1}");
        default:
            currentCell.Append((char)c);
            goto QuotedCell;
    }
    
    EndQuote: switch (c = reader.Read()) {
        case '\r':
            ReadNewLine();
            goto Start;
        case ',':
            AddCell();
            goto Start;
        case '"': 
            currentCell.Append('"');
            goto QuotedCell;
        case int _ when c < 0:
            AddCell();
            return result;
        default:
            throw new FormatException($"Unexpected character following trailing quote '{(char)c}'");
    }

    RawCell: switch (c = reader.Read()) {
        case '"':
            throw new FormatException("Encountered quote character (\") in unquoted value");
        case ',':
            AddCell();
            goto Start;
        case '\r':
            ReadNewLine();
            goto Start;
        case int _ when c < 0:
            AddRow();
            return result;
        default:
            currentCell.Append((char)c);
            goto RawCell;
    }
}