Working with log files or large text datasets? You’ll often need to search for lines containing specific keywords like "ERROR" or "WARNING". Instead of repeating the same logic, a C# Extension Method to Filter Lines offers a clean, reusable solution.

This post walks you through a memory-efficient, scalable approach using this custom method—applicable to both individual files and entire directories.

Why Use an Extension Method?

An extension method in C# lets you “extend” an existing type (like FileInfo) with new functionality without modifying the original class. This improves readability and reuse.

Here’s a clean and efficient implementation with C# Extension Method to Filter Lines :

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

public static class FileExtensions
{
    public static IEnumerable ReadAndFilter(this FileInfo info, Predicate condition)
    {
        if (info == null || condition == null)
            throw new ArgumentNullException();

        using var reader = new StreamReader(info.FullName);
        string? line;
        while ((line = reader.ReadLine()) != null)
        {
            if (condition(line))
            {
                yield return line;
            }
        }
    }
}

Highlights:

• yield return is used to stream results—great for large files.
• Uses a Predicate<string> so you can pass any custom condition.
• Wrapped with basic null checking for safety.

Let’s say you want to find all lines containing the word "ERROR" in a specific file:

string FilePath = "C:\\logs\\app.log";
string SearchString = "ERROR";

var result = new FileInfo(FilePath).ReadAndFilter(s => s.Contains(SearchString));
foreach (var line in result)
{
    Console.WriteLine(line);
}

You can extend this logic to scan all .txt files in a folder:

string path = "C:\\logs";

foreach (string file in Directory.GetFiles(path, "*.txt"))
{
    var result = new FileInfo(file).ReadAndFilter(s => s.Contains(SearchString));
    foreach (string line in result)
    {
        Console.WriteLine($"[{Path.GetFileName(file)}] {line}");
    }
}