`File.ReadLines() vs. File.ReadAllLines(): When Should I Use Which for Reading Text Files in C#?`

Comparison of File.ReadLines() and File.ReadAllLines() methods in C#: When to use which to read text files?

When processing text files in C#, File.ReadLines() and File.ReadAllLines() are two commonly used methods to read the file content. Both return an array of strings, with each element in the array representing a line of the file, but they have significant performance differences.

File.ReadAllLines()

File.ReadAllLines() method will read the entire file into memory at once and then return the string array. This method is very efficient for small or medium-sized files, but becomes significantly less efficient for large files.

File.ReadLines()

In contrast, the File.ReadLines() method returns a IEnumerable<string> object that supports lazy loading of files. This means it doesn't load the entire file into memory at once, making it a better choice when working with large files, especially in scenarios where performance is critical.

Performance Difference

As mentioned before, File.ReadLines() performs far better than File.ReadAllLines() for large files. This is because File.ReadLines() reads the file in chunks and returns rows incrementally, allowing the data to be processed immediately. File.ReadAllLines(), on the other hand, needs to wait for the entire file to be loaded into memory before returning the array, which can lead to performance bottlenecks and high memory usage, especially when dealing with large files.

Examples of usage

Example of using File.ReadAllLines():

<code class="language-csharp">string[] lines = File.ReadAllLines("C:\mytxt.txt");

foreach (string line in lines)
{
    // 处理每一行
}</code>

Example of using File.ReadLines():

<code class="language-csharp">foreach (string line in File.ReadLines("C:\mytxt.txt"))
{
    // 处理每一行（按读取顺序）
}</code>

Summary

For small files, the performance difference between using File.ReadAllLines() or File.ReadLines() is negligible. However, when dealing with large files, File.ReadLines() becomes a better choice due to its lazy loading mechanism, ensuring optimal performance and memory usage efficiency.

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