Why Does File.ReadAllLinesAsync() Block the UI Thread in WPF Applications?

Unravel the mystery: File.ReadAllLinesAsync() Why the WPF UI thread is blocked

Introduction

When using asynchronous programming in WPF, it is critical to understand why certain methods block the UI thread. File.ReadAllLinesAsync() is one such method whose unexpected behavior raised questions among developers. This article will delve into the reasons behind this issue and explore possible solutions.

Question

File.ReadAllLinesAsync() is an asynchronous method for retrieving the contents of a text file asynchronously. However, in a WPF application, using this method may cause the UI thread to block, as shown in the following code snippet:

private async void Button_OnClick(object sender, RoutedEventArgs e)
{
    Button.Content = "Loading...";
    var lines = await File.ReadAllLinesAsync("D:\temp.txt"); // 阻塞 UI 线程
    Button.Content = "Show";
}

Explanation

File.ReadAllLinesAsync() The reason for blocking the UI thread is due to the internal implementation of asynchronous file operations in .NET Core 3.1. Contrary to the recommended design principles for asynchronous methods, this method performs a lot of synchronous work before returning the task. This synchronization includes initializing file access, allocating memory, and loading the file contents into memory. Therefore, the UI thread is blocked while performing these tasks.

Performance impact

To illustrate the performance impact of this issue, we can run a simple test that involves reading a large text file. The results (in milliseconds) are as follows:

Task<string> task = File.ReadAllLinesAsync("LargeFile.txt");
long duration1 = stopwatch.ElapsedMilliseconds;
bool isCompleted = task.IsCompleted;
stopwatch.Restart();
string[] lines = await task;
long duration2 = stopwatch.ElapsedMilliseconds;

Output

<code>创建: 450 毫秒, Task.IsCompleted: False
等待: 5 毫秒, 行数: 204,000</code>

As you can see from the output, File.ReadAllLinesAsync() blocks the UI thread for almost half a second (450 milliseconds) before returning the incomplete task. The subsequent await operation completes very quickly, taking only 5 milliseconds.

Possible solutions

To resolve this issue, consider the following alternatives:

• Use the synchronous File.ReadAllLines() method. Although synchronous, this method avoids blocking the UI thread. You can wrap its call in Task.Run to ensure it runs asynchronously.

    var lines = await Task.Run(() => File.ReadAllLines("LargeFile.txt"));

• Consider using a third-party library with more efficient asynchronous file access capabilities. Libraries such as System.IO.Pipelines and DotNetReactor.IO are designed to improve asynchronous performance in file operations.

Conclusion

Due to an implementation inefficiency in .NET Core 3.1, File.ReadAllLinesAsync() blocks the UI thread in WPF applications. This issue can negatively impact the responsiveness of the UI. By leveraging a synchronized or optimized third-party solution, you can avoid this potential bottleneck and ensure a smooth user experience.

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