Download & Export System

public Task<byte[]> GenerateExcelReportAsync(AuditExportData data, CancellationToken ct = default)
{
    using var workbook = new XLWorkbook();

    BuildSummarySheet(workbook, data);   // Sheet 1: Executive Summary & KPIs
    BuildDataSheet(workbook, data, ct);  // Sheet 2: Audit Logs Table (failures in light-red)
    BuildSecuritySheet(workbook, data);  // Sheet 3: Security Alerts & Suspicious IPs

    using var ms = new MemoryStream();
    workbook.SaveAs(ms);
    return Task.FromResult(ms.ToArray());
}

public Task<byte[]> GenerateCsvAsync(AuditExportData data, CancellationToken ct = default)
{
    using var ms = new MemoryStream();
    
    // Write UTF-8 BOM first for automatic Excel double-click encoding detection
    ms.Write(Encoding.UTF8.GetPreamble());

    using var writer = new StreamWriter(ms, new UTF8Encoding(false), leaveOpen: true);
    using var csv = new CsvWriter(writer, new CsvConfiguration(CultureInfo.InvariantCulture)
    {
        HasHeaderRecord = true,
        ShouldQuote = args => true // Security: always quote fields to prevent CSV Injection
    });

    // Write header and rows...
    csv.NextRecord();
    writer.Flush();
    return Task.FromResult(ms.ToArray());
}

// Parse Range header: "bytes=1024-2048"
private (long? Start, long? End) ParseRangeHeader(long fileSize)
{
    var range = rangeHeader.ToString(); // "bytes=1024-2048"
    var parts = range["bytes=".Length..].Split('-');
    
    long? start = string.IsNullOrEmpty(parts[0]) ? null : long.Parse(parts[0]);
    long? end = string.IsNullOrEmpty(parts[1]) ? null : long.Parse(parts[1]);

    // Handle suffix range (last N bytes): "bytes=-500" -> last 500 bytes
    if (!start.HasValue && end.HasValue)
    {
        start = fileSize - end.Value;
        end = fileSize - 1;
    }

    return (start, end);
}

HTTP/1.1 206 Partial Content
Content-Range: bytes 1024-2048/10240
Accept-Ranges: bytes

// Generate deterministic ETag from file metadata
private static string GenerateETag(FileInfo fileInfo)
{
    var data = `${fileInfo.FullName}|${fileInfo.Length}|${fileInfo.LastWriteTimeUtc:O}`;
    var hash = MD5.HashData(System.Text.Encoding.UTF8.GetBytes(data));
    return $"\"{Convert.ToHexString(hash)}\"";
}

public async Task<string> CreateDownloadSessionAsync(string filePath, TimeSpan? expiration, ...)
{
    var sessionId = Guid.NewGuid().ToString("N"); // 32-char hex string
    var exp = expiration ?? TimeSpan.FromHours(1);

    await _cacheService.SetAsync(
        $"download:session:{sessionId}",
        new DownloadSession(filePath, DateTime.UtcNow.Add(exp)),
        exp);

    return sessionId;
}

private string GetFullPath(string filePath)
{
    var normalizedPath = filePath.Replace("..", "").TrimStart('/', '\\');
    return Path.Combine(_settings.StoragePath ?? "uploads", normalizedPath);
}

var stream = new FileStream(
    fullPath,
    FileMode.Open,
    FileAccess.Read,
    FileShare.Read,      // Allow concurrent reads
    bufferSize: 64 * 1024, // 64KB buffer (4x default)
    useAsync: true         // Async I/O for non-blocking reads
);