Why Does File.ReadAllLinesAsync() Sometimes Block the UI Thread in .NET?

Understanding UI Thread Blocking with File.ReadAllLinesAsync() in .NET

While File.ReadAllLinesAsync() in .NET (versions 3.1 and later) aims for non-blocking behavior, it can surprisingly block the UI thread under specific conditions.

The Problem

In WPF applications, using File.ReadAllLinesAsync() might freeze the UI. This stems from inconsistencies in how asynchronous file access APIs are implemented. Microsoft's guidelines suggest asynchronous methods should return a Task after minimal synchronous work. File.ReadAllLinesAsync() deviates from this, leading to prolonged blocking before returning an incomplete Task.

The Solution

The recommended workaround is to utilize the synchronous File.ReadAllLines() method within Task.Run(). This offloads the file reading to a background thread, preventing UI freezes.

Test Results

Tests reading a 6MB file with File.ReadAllLinesAsync() revealed a significant UI blockage of approximately 450 milliseconds – a clear departure from expected asynchronous behavior.

.NET 6 and Beyond

Even with improvements in .NET 6's asynchronous file I/O, File.ReadAllLinesAsync() retains performance limitations. It's considerably slower (roughly double the time) than its synchronous counterpart and isn't fully asynchronous. Therefore, using the synchronous version within Task.Run() remains the best practice until further API optimizations are implemented.

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