CSV and Binary Files — Processing Structured File Formats

Beyond JSON, many real-world applications process CSV files (data exports, bulk imports), binary files (images, PDFs, compiled data), and need to inspect file metadata. CSV is ubiquitous in enterprise data exchange — every ERP, CRM, and analytics tool exports CSV. Binary formats appear in file uploads, report generation, and image processing. FileInfo and DirectoryInfo provide rich metadata access. Knowing how to handle these formats reliably, including edge cases like different encodings and locked files, completes the file I/O skill set.

CSV Processing #

// ── Manual CSV parsing — handles simple cases ─────────────────────────────
public async Task<List<UserImport>> ParseCsvAsync(string filePath)
{
    var users = new List<UserImport>();
    using var reader = new StreamReader(filePath, Encoding.UTF8);

    // Skip header
    await reader.ReadLineAsync();

    string? line;
    int lineNum = 1;
    while ((line = await reader.ReadLineAsync()) is not null)
    {
        lineNum++;
        if (string.IsNullOrWhiteSpace(line)) continue;

        var parts = line.Split(',');
        if (parts.Length < 3)
        {
            Console.WriteLine($"Warning: line {lineNum} has too few columns, skipping.");
            continue;
        }

        users.Add(new UserImport
        {
            Name  = parts[0].Trim(),
            Email = parts[1].Trim(),
            Role  = parts[2].Trim(),
        });
    }

    return users;
}

// ── Writing CSV ───────────────────────────────────────────────────────────
public async Task ExportToCsvAsync(IEnumerable<User> users, string filePath)
{
    await using var writer = new StreamWriter(filePath, append: false, Encoding.UTF8);
    await writer.WriteLineAsync("Id,Name,Email,CreatedAt");

    foreach (var user in users)
    {
        // Escape commas in field values
        string safeName = user.Name.Contains(',') ? $"\"{user.Name}\"" : user.Name;
        await writer.WriteLineAsync($"{user.Id},{safeName},{user.Email},{user.CreatedAt:O}");
    }
}

BinaryReader and BinaryWriter #

// ── Write binary data ─────────────────────────────────────────────────────
using var writeStream = File.Create("data.bin");
using var writer = new BinaryWriter(writeStream);

writer.Write(42);                      // int (4 bytes)
writer.Write(3.14);                    // double (8 bytes)
writer.Write("Hello");                 // length-prefixed string
writer.Write(new byte[] { 1, 2, 3 }); // raw bytes

// ── Read binary data ──────────────────────────────────────────────────────
using var readStream = File.OpenRead("data.bin");
using var reader = new BinaryReader(readStream);

int    i = reader.ReadInt32();      // 42
double d = reader.ReadDouble();     // 3.14
string s = reader.ReadString();     // "Hello"
byte[] b = reader.ReadBytes(3);     // { 1, 2, 3 }

FileInfo — Rich Metadata #

// FileInfo provides metadata without opening the file
var info = new FileInfo("uploads/report.pdf");

Console.WriteLine($"Name:      {info.Name}");
Console.WriteLine($"Directory: {info.DirectoryName}");
Console.WriteLine($"Extension: {info.Extension}");
Console.WriteLine($"Size:      {info.Length / 1024.0 / 1024.0:F2} MB");
Console.WriteLine($"Created:   {info.CreationTimeUtc:O}");
Console.WriteLine($"Modified:  {info.LastWriteTimeUtc:O}");
Console.WriteLine($"ReadOnly:  {info.IsReadOnly}");
Console.WriteLine($"Exists:    {info.Exists}");

// Common validation in file upload handlers
public bool IsValidUpload(FileInfo file) =>
    file.Exists
    && file.Length <= 10 * 1024 * 1024   // max 10MB
    && AllowedExtensions.Contains(file.Extension.ToLowerInvariant());

private static readonly HashSet<string> AllowedExtensions =
    new(StringComparer.OrdinalIgnoreCase) { ".jpg", ".jpeg", ".png", ".webp", ".pdf" };

Common Mistakes #

Mistake 1 — Splitting CSV on comma without handling quoted fields #

❌ Wrong — breaks on "Smith, John",john@example.com,Admin:

var parts = line.Split(',');   // splits "Smith, John" into two parts!

✅ Correct — use CsvHelper or implement RFC 4180 compliant parsing for production CSV.

Mistake 2 — Not validating file size and type before processing uploads #

❌ Wrong — no size check allows multi-GB uploads that exhaust server memory.

✅ Correct — validate file size and extension immediately on upload before any processing begins.