`File.ReadLines()` vs. `File.ReadAllLines()`: Which is Best for Reading Large Text Files in C#?

Reading large text files in C#: Performance comparison of File.ReadLines() and File.ReadAllLines()

File.ReadLines() and File.ReadAllLines() are both methods in C# for reading text files, but their performance differs significantly.

File.ReadAllLines()

File.ReadAllLines() will read the entire file into memory at once and return a string array. This method is efficient for small files, but has problems with large files. As the file size increases, the time required to read and process the entire file increases significantly. Additionally, File.ReadAllLines() consumes a lot of memory, especially when working with gigabyte-sized files.

File.ReadLines()

File.ReadLines() solves the limitations of File.ReadAllLines() through lazy loading. Instead of reading the entire file into memory at once, it returns a IEnumerable<string> that allows you to iterate through each line of the file as needed. This approach reduces memory footprint and improves performance when processing large files.

Performance comparison

Benchmarks show that File.ReadLines() performs better than File.ReadAllLines() for large files. This difference becomes more noticeable as file size increases. For example, reading a 100 MB file using File.ReadAllLines() might take several seconds, whereas File.ReadLines() can complete the task in less than a second.

Example usage

Use File.ReadLines() to read files:

<code class="language-csharp">using System.IO;

string path = "C:\mytxt.txt";

foreach (var line in File.ReadLines(path))
{
    // 处理每一行
}</code>

Use File.ReadAllLines() to read files:

<code class="language-csharp">using System.IO;

string path = "C:\mytxt.txt";

string[] lines = File.ReadAllLines(path);

// 处理所有行</code>

Conclusion

For large text files, File.ReadLines() has significant performance advantages over File.ReadAllLines(). By lazy loading, File.ReadLines() reduces memory consumption and increases processing speed without sacrificing efficiency.

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