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Home Interviews How do you implement a file-based cache system? – MindStick
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1 Answers

ICSM Computer
ICSM Computer 13 May 2025
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Implementing a file-based cache system in C# allows you to store and retrieve data on disk efficiently, which is useful for scenarios like:

  1. Caching API responses
  2. Storing expensive computation results
  3. Avoiding redundant downloads or processing

Basic Structure

  1. A simple file-based cache can:
  2. Use the filename as a key.
  3. Store serialized data (e.g., JSON or binary).
  4. Check timestamps or a TTL (time-to-live) to determine staleness.
ICSM Computer

Step-by-Step Implementation

using System;
using System.IO;
using System.Text.Json;

public class FileCache
{
    private readonly string _cacheDir;
    private readonly TimeSpan _defaultTtl;

    public FileCache(string cacheDir, TimeSpan? ttl = null)
    {
        _cacheDir = cacheDir;
        _defaultTtl = ttl ?? TimeSpan.FromMinutes(30);
        Directory.CreateDirectory(_cacheDir);
    }

    private string GetCachePath(string key)
    {
        string safeName = Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(key));
        return Path.Combine(_cacheDir, safeName + ".json");
    }

    public void Set<T>(string key, T value)
    {
        var path = GetCachePath(key);
        var cacheItem = new CacheItem<T>
        {
            Timestamp = DateTime.UtcNow,
            Value = value
        };

        File.WriteAllText(path, JsonSerializer.Serialize(cacheItem));
    }

    public bool TryGet<T>(string key, out T value)
    {
        var path = GetCachePath(key);
        value = default;

        if (!File.Exists(path))
            return false;

        try
        {
            var json = File.ReadAllText(path);
            var cacheItem = JsonSerializer.Deserialize<CacheItem<T>>(json);

            if (DateTime.UtcNow - cacheItem.Timestamp < _defaultTtl)
            {
                value = cacheItem.Value;
                return true;
            }
            else
            {
                File.Delete(path); // Cache expired
            }
        }
        catch
        {
            // corrupted cache file or deserialization failure
            File.Delete(path);
        }

        return false;
    }

    private class CacheItem<T>
    {
        public DateTime Timestamp { get; set; }
        public T Value { get; set; }
    }
}

Usage Example

var cache = new FileCache("cache");

string key = "user_profile_123";

if (cache.TryGet(key, out string profileJson))
{
    Console.WriteLine("Cache hit: " + profileJson);
}
else
{
    // Simulate slow data fetch
    profileJson = "{ \"name\": \"John\" }";
    cache.Set(key, profileJson);
    Console.WriteLine("Cache miss. Data fetched and cached.");
}

Features You Can Add:

  1. TTL per entry
  2. Custom serialization (binary or XML)
  3. Compression (e.g., GZip)
  4. Max file count or max total size limit
  5. Cleanup policy (oldest files deleted when disk use is high)
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