File.ReadLines() vs. File.ReadAllLines(): Which Method Should You Choose for Optimal File Reading Performance?

C# file reading performance: Comparison of File.ReadLines() and File.ReadAllLines()

In C#’s System.IO namespace, both File.ReadLines() and File.ReadAllLines() are common methods for reading text files. Both return arrays of strings, but there are significant differences in terms of performance, especially when working with large files.

File.ReadLines()

File.ReadLines() returns a IEnumerable<string>, supporting lazy loading. This means that it doesn't read the entire file at once, but rather iteratively, line by line. For large files, the advantage of this approach is that it reduces memory overhead and avoids performance bottlenecks that may result from loading the entire file into memory.

File.ReadAllLines()

File.ReadAllLines() will first read the entire file content into memory and then return the string array. This method is more efficient for small files or when you need to get the entire contents of the file immediately. However, for large files, it can significantly impact performance and may even throw exceptions due to insufficient memory.

Performance comparison

The following code example demonstrates how to use both methods:

<code class="language-csharp">string[] lines1 = File.ReadAllLines("C:\mytxt.txt");
foreach (var line in File.ReadLines("C:\mytxt.txt"))
{
    // 处理每一行
}</code>

Summary

Generally speaking, File.ReadLines() is more suitable for reading large files or only processing part of the file. And File.ReadAllLines() is more suitable for reading small files or when you need to get the entire contents of the file immediately. Which method to choose depends on the size of the file and the specific application scenario. When processing large files, File.ReadLines()'s lazy loading mechanism can effectively improve performance and avoid memory overflow errors.

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