How to Read a File in C#: Quick and Efficient Methods

Table of Contents

1. ReadAllText – The Quickest Way
2. ReadAllLines – Easy Line-by-Line
3. ReadLines – Best for Large Files
4. StreamReader – Full Control
5. Asynchronous Reading
6. Benchmark Example
7. Encoding Considerations
8. File Existence Check
9. Comparison Summary
10. GitHub Example
11. Frequently Asked Questions (FAQ)

1. ReadAllText – The Quickest Way

string content = File.ReadAllText("example.txt");
Console.WriteLine(content);

Best for small files when you need the entire content at once.

2. ReadAllLines – Easy Line-by-Line

string[] lines = File.ReadAllLines("example.txt");
foreach (var line in lines)
{
    Console.WriteLine(line);
}

Ideal when processing individual lines but the file size is moderate.

3. ReadLines – Best for Large Files

foreach (var line in File.ReadLines("example.txt"))
{
    Console.WriteLine(line);
}

This method uses lazy evaluation, making it memory-efficient for large files.

4. StreamReader – Full Control

using (var reader = new StreamReader("example.txt", Encoding.UTF8))
{
    string? line;
    while ((line = reader.ReadLine()) != null)
    {
        Console.WriteLine(line);
    }
}

Use when you need to specify encoding or implement custom logic for each line.

5. Asynchronous Reading

Reading an entire file asynchronously:

string content = await File.ReadAllTextAsync("example.txt");
Console.WriteLine(content);

For asynchronous line-by-line reading:

await foreach (var line in ReadLinesAsync("example.txt"))
{
    Console.WriteLine(line);
}

async IAsyncEnumerable<string> ReadLinesAsync(string path)
{
    using var reader = new StreamReader(path);
    while (await reader.ReadLineAsync() is { } line)
    {
        yield return line;
    }
}

Recommended for GUI applications or web servers where blocking is undesirable.

6. Benchmark Example

var sw = Stopwatch.StartNew();
string content = File.ReadAllText("example.txt");
sw.Stop();
Console.WriteLine($"Elapsed time: {sw.ElapsedMilliseconds} ms");

Use similar code to compare different reading methods in your environment.

7. Encoding Considerations

To specify a particular encoding:

string content = File.ReadAllText("example.txt", Encoding.GetEncoding("Windows-1252"));

Useful encodings include:

• Encoding.UTF8 (default)
• Encoding.Unicode (UTF-16)
• Encoding.ASCII
• Encoding.GetEncoding("ISO-8859-1")

8. File Existence Check

Always check if the file exists to avoid exceptions:

if (!File.Exists("example.txt"))
{
    Console.WriteLine("File not found.");
    return;
}

9. Comparison Summary

10. GitHub Example

The full example project will be available at:

https://github.com/AdrianBailador/FileReadingDemo

11. Frequently Asked Questions (FAQ)

What is considered a large file to read into memory?

As a general rule:

• Under 10 MB: Safe for full read
• 10–100 MB: Use with caution
• Over 100 MB: Use streaming (ReadLines or StreamReader)

How to read a CSV file in C#?

For simple CSV parsing:

foreach (var line in File.ReadLines("data.csv"))
{
    var fields = line.Split(',');
    Console.WriteLine($"Name: {fields[0]}, Age: {fields[1]}");
}

For complex CSV files, use a library like CsvHelper.

How to check if a file exists before reading it in C#?

Use File.Exists:

if (!File.Exists("example.txt"))
{
    Console.WriteLine("The file does not exist.");
    return;
}