Why Does `File.ReadAllLinesAsync()` Block the UI Thread Despite Being Asynchronous?

Understanding Why File.ReadAllLinesAsync() Can Block the UI Thread

Asynchronous methods ideally perform minimal synchronous work before returning a pending task. However, File.ReadAllLinesAsync() doesn't always follow this best practice.

The Issue:

The method can unexpectedly block the UI thread for a considerable duration before the asynchronous operation actually begins.

Initial Misconception:

It was initially assumed that the blocking stemmed from the inherently synchronous nature of file system operations. Testing, however, showed that the thread blocks before any file access occurs.

The Real Problem:

The implementation of File.ReadAllLinesAsync() doesn't fully embrace asynchronous principles. It performs a significant amount of synchronous pre-processing before handing off the task.

Recommended Solution:

To prevent UI thread blockage, use the synchronous File.ReadAllLines() method wrapped in Task.Run() for asynchronous execution:

var lines = await Task.Run(() => File.ReadAllLines(@"D:\temp.txt"));

Performance Comparison:

Tests using a 6MB file showed File.ReadAllLinesAsync() blocking for approximately 450 milliseconds before returning an incomplete task (which then completed in 5 milliseconds). Conversely, the synchronous method, when run asynchronously via Task.Run(), exhibited negligible delay.

.NET 6 and Beyond:

While .NET 6 brought performance improvements to File.ReadAllLinesAsync(), it remains slower than the synchronous approach when used asynchronously and doesn't fully exhibit true asynchronous behavior. Therefore, using Task.Run() with the synchronous method remains the suggested approach for optimal UI responsiveness.

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