Practice: LINQ and Files

Q: What if property names don't match during JSON deserialization?

Use the [JsonPropertyName] attribute to map names, or customize PropertyNamingPolicy in JsonSerializerOptions .

Student Grade Analyzer

Serialize student data to a JSON file, then deserialize it and use LINQ for multi-dimensional statistical analysis: average score, top three, pass count, and grouping by grade level.

Requirements

Define a Student class with Name and Score properties
Serialize the student list to JSON and save it to a temporary file
Read from the file and deserialize
Use LINQ to calculate: average score, top three, pass count (>=60), and grouping by grade level

Example

CSHARP

using System;
using System.IO;
using System.Linq;
using System.Text.Json;
using System.Collections.Generic;

public class Student
{
    public string Name { get; set; }
    public int Score { get; set; }
}

class Program
{
    static void Main()
    {
        var students = new List<Student>
        {
            new Student { Name = "Zhang San", Score = 92 },
            new Student { Name = "Li Si", Score = 45 },
            new Student { Name = "Wang Wu", Score = 78 },
            new Student { Name = "Zhao Liu", Score = 88 },
            new Student { Name = "Qian Qi", Score = 55 },
            new Student { Name = "Sun Ba", Score = 95 },
            new Student { Name = "Zhou Jiu", Score = 33 },
            new Student { Name = "Wu Shi", Score = 71 }
        };

        string tempDir = Path.GetTempPath();
        string filePath = Path.Combine(tempDir, "students.json");

        var options = new JsonSerializerOptions { WriteIndented = true };
        string json = JsonSerializer.Serialize(students, options);
        File.WriteAllText(filePath, json);
        Console.WriteLine("Written to: " + filePath);

        string readJson = File.ReadAllText(filePath);
        var loaded = JsonSerializer.Deserialize<List<Student>>(readJson);

        double average = loaded.Average(s => s.Score);
        Console.WriteLine($"Average score: {average:F1}");

        var top3 = loaded.OrderByDescending(s => s.Score).Take(3);
        Console.WriteLine("Top three:");
        foreach (var s in top3)
        {
            Console.WriteLine($"  {s.Name} - {s.Score}");
        }

        int passCount = loaded.Count(s => s.Score >= 60);
        Console.WriteLine($"Pass count: {passCount}");

        var grouped = loaded.GroupBy(s => s.Score >= 90 ? "Excellent"
            : s.Score >= 60 ? "Pass" : "Fail");
        Console.WriteLine("Grade groups:");
        foreach (var group in grouped)
        {
            Console.WriteLine($"  {group.Key}: {string.Join(", ", group.Select(s => s.Name))}");
        }

        File.Delete(filePath);
    }
}

▶ Try it Yourself

TEXT

Written to: /tmp/students.json
Average score: 69.6
Top three:
  Sun Ba - 95
  Zhang San - 92
  Zhao Liu - 88
Pass count: 5
Grade groups:
  Excellent: Zhang San, Sun Ba
  Fail: Li Si, Qian Qi, Zhou Jiu
  Pass: Wang Wu, Zhao Liu, Wu Shi

Simple Log Analyzer

Read a text log file, parse each line, and use LINQ to count errors/warnings, find the most frequent error, and filter by date range.

Requirements

Log format: [Date] [Level] Message, e.g. [2025-01-15] [ERROR] Disk full
Count entries by level
Find the most frequently occurring ERROR message
Filter logs by date range

Example

CSHARP

using System;
using System.IO;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        string tempDir = Path.GetTempPath();
        string logPath = Path.Combine(tempDir, "app.log");

        var lines = new List<string>
        {
            "[2025-01-10] [INFO] System started",
            "[2025-01-10] [WARN] Memory usage 80%",
            "[2025-01-11] [ERROR] Database connection failed",
            "[2025-01-11] [ERROR] Disk full",
            "[2025-01-12] [INFO] User login",
            "[2025-01-12] [ERROR] Database connection failed",
            "[2025-01-13] [WARN] CPU usage 90%",
            "[2025-01-13] [ERROR] Network timeout",
            "[2025-01-14] [INFO] Scheduled task completed",
            "[2025-01-14] [ERROR] Database connection failed"
        };
        File.WriteAllLines(logPath, lines);

        var logEntries = File.ReadAllLines(logPath)
            .Select(line =>
            {
                var parts = line.Split(']');
                return new
                {
                    Date = parts[0].TrimStart('[').Trim(),
                    Level = parts[1].TrimStart('[').Trim(),
                    Message = parts[2].Trim()
                };
            })
            .ToList();

        var levelCounts = logEntries
            .GroupBy(e => e.Level)
            .Select(g => new { Level = g.Key, Count = g.Count() })
            .OrderByDescending(x => x.Count);
        Console.WriteLine("Level counts:");
        foreach (var item in levelCounts)
        {
            Console.WriteLine($"  {item.Level}: {item.Count}");
        }

        var topError = logEntries
            .Where(e => e.Level == "ERROR")
            .GroupBy(e => e.Message)
            .OrderByDescending(g => g.Count())
            .First();
        Console.WriteLine($"Most frequent error: {topError.Key} ({topError.Count()} times)");

        var filtered = logEntries
            .Where(e => string.Compare(e.Date, "2025-01-11") >= 0
                     && string.Compare(e.Date, "2025-01-13") <= 0)
            .ToList();
        Console.WriteLine("Logs from Jan 11-13:");
        foreach (var entry in filtered)
        {
            Console.WriteLine($"  [{entry.Date}] [{entry.Level}] {entry.Message}");
        }

        File.Delete(logPath);
    }
}

▶ Try it Yourself

TEXT

Level counts:
  ERROR: 5
  INFO: 3
  WARN: 2
Most frequent error: Database connection failed (3 times)
Logs from Jan 11-13:
  [2025-01-11] [ERROR] Database connection failed
  [2025-01-11] [ERROR] Disk full
  [2025-01-12] [INFO] User login
  [2025-01-12] [ERROR] Database connection failed
  [2025-01-13] [WARN] CPU usage 90%
  [2025-01-13] [ERROR] Network timeout

Batch File Rename Tool

List files in a directory, filter by extension using LINQ, then batch-add a prefix or suffix.

Requirements

Create sample files in a temporary directory
Filter files by extension
Add a prefix and suffix to file names (preserving the extension)
Display rename logic (output old vs. new name comparison)

Example

CSHARP

using System;
using System.IO;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        string workDir = Path.Combine(Path.GetTempPath(), "rename_demo");
        Directory.CreateDirectory(workDir);

        var demoFiles = new[] { "photo1.jpg", "photo2.jpg", "doc.txt", "notes.txt", "data.csv" };
        foreach (var f in demoFiles)
        {
            File.WriteAllText(Path.Combine(workDir, f), "demo");
        }
        Console.WriteLine("Original files:");
        foreach (var f in Directory.GetFiles(workDir))
        {
            Console.WriteLine("  " + Path.GetFileName(f));
        }

        string targetExt = ".txt";
        string prefix = "backup_";

        var filesToRename = Directory.GetFiles(workDir)
            .Where(f => Path.GetExtension(f).Equals(targetExt, StringComparison.OrdinalIgnoreCase))
            .ToList();

        Console.WriteLine($"\nFiltering extension {targetExt}:");
        foreach (var f in filesToRename)
        {
            string dir = Path.GetDirectoryName(f);
            string nameNoExt = Path.GetFileNameWithoutExtension(f);
            string newName = prefix + nameNoExt + targetExt;
            string newPath = Path.Combine(dir, newName);
            Console.WriteLine($"  {Path.GetFileName(f)} -> {newName}");
            File.Move(f, newPath);
        }

        Console.WriteLine("\nAfter renaming:");
        foreach (var f in Directory.GetFiles(workDir))
        {
            Console.WriteLine("  " + Path.GetFileName(f));
        }

        Directory.Delete(workDir, true);
    }
}

▶ Try it Yourself

TEXT

Original files:
  photo1.jpg
  photo2.jpg
  doc.txt
  notes.txt
  data.csv

Filtering extension .txt:
  doc.txt -> backup_doc.txt
  notes.txt -> backup_notes.txt

After renaming:
  photo1.jpg
  photo2.jpg
  backup_doc.txt
  backup_notes.txt
  data.csv

❓ FAQ

Q What if property names don't match during JSON deserialization?

A Use the [JsonPropertyName] attribute to map names, or customize PropertyNamingPolicy in JsonSerializerOptions.

Q Is the order of GroupBy results guaranteed?

A No, GroupBy does not guarantee group order. Use OrderBy to sort before iterating.

Q What happens if the target file already exists when using File.Move to rename?

A It throws an IOException. You should check whether the target path exists first, or use File.Replace.

Q Is ReadAllLines appropriate for reading large log files?

A No, ReadAllLines loads all content into memory at once. For large files, use File.ReadLines for lazy, line-by-line reading.

📖 Summary

Comprehensively apply LINQ and JSON serialization for data persistence and statistical analysis
Master log parsing, LINQ grouping and aggregation, and date filtering techniques
Use Directory and Path classes combined with LINQ for file filtering and batch operations
All file operation examples use Path.GetTempPath() for cross-platform compatibility
Temporary files and directories are cleaned up after use to avoid leftovers

📝 Exercises

Extend the student grade analyzer to count students by score range (0-59/60-79/80-89/90-100) and output a bar chart (using * characters)
Add a feature to the log analyzer: count ERROR entries per day and output the date range with the most consecutive ERROR occurrences
Refactor the file rename tool to accept command-line arguments for directory path, extension filter, and prefix, and add a --dry-run mode that only shows a preview without executing

Web-Tutorial Tech Team

A team of developers maintaining programming tutorials. Each tutorial is written and reviewed by developers with expertise in that field. We work to keep our content accurate and reliable — if you spot an issue, please let us know.

About Us

404 Not Found

Practice: LINQ and Files

Student Grade Analyzer

Requirements

Example

Simple Log Analyzer

Requirements

Example

Batch File Rename Tool

Requirements

Example

❓ FAQ

📖 Summary

📝 Exercises

🙏 帮我们做得更好