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

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

Efficient file handling is crucial in programming. In C#, both File.ReadLines() and File.ReadAllLines() methods can read file contents into a string array, but their performance differs significantly when processing large files.

File.ReadLines()

File.ReadLines() returns a IEnumerable<string>, which is a lazy-loaded collection. This means it reads the file line by line, making it more efficient when working with large files. You can use foreach to loop through each line without loading the entire file into memory.

File.ReadAllLines()

File.ReadAllLines() reads the entire file at once and returns an array of string[]. For large files, this approach is less efficient because it requires loading the entire file into memory, which can lead to performance bottlenecks or even memory overflows.

Performance comparison

The following table summarizes the main performance differences between File.ReadLines() and File.ReadAllLines():

方法	读取方式	大型文件性能
`File.ReadLines()`	逐行读取	更高效
`File.ReadAllLines()`	一次性读取	效率较低

Example

The following code examples demonstrate how to use File.ReadLines() to process large text files, and how to use File.ReadAllLines() to process small text files:

<code class="language-csharp">// 处理大型文件
string[] lines = File.ReadLines("large.txt").Where(x => x.Contains("keyword")).ToArray();

// 处理小型文件
string[] lines = File.ReadAllLines("small.txt");</code>

Conclusion

For large files, File.ReadLines() is a better choice as it provides better performance by reading the file incrementally. File.ReadAllLines() Still works for small files, where the performance difference is less critical.

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