Why Do Async File I/O Operations Still Block the UI Thread in .NET (and How to Fix It)?

Addressing UI Thread Blocking in Asynchronous File I/O Operations

The goal of asynchronous programming in GUI applications is to prevent UI thread blockage, ensuring a responsive user experience. However, in .NET Core 3.1 and earlier versions, asynchronous file system APIs didn't always adhere to best practices.

Specifically, File.ReadAllLinesAsync() could block the UI thread, despite its asynchronous design. This is because the method didn't fully adhere to the recommended asynchronous pattern: minimal synchronous work before returning the task.

To circumvent this issue in older .NET versions, avoid using asynchronous file system APIs directly within GUI applications. Instead, encapsulate synchronous APIs within Task.Run(). For instance:

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

Performance Observations:

Tests reveal the blocking behavior of File.ReadAllLinesAsync(). Processing a 6MB file on an SSD resulted in a 450-millisecond UI thread block before an incomplete task was returned in older .NET versions.

.NET 6 Improvements:

.NET 6 brought improvements to asynchronous file system APIs, significantly reducing the blocking time to approximately 19 milliseconds. However, these asynchronous APIs remain slower (roughly double the speed) than their synchronous counterparts and aren't fully asynchronous. Therefore, using synchronous APIs wrapped in Task.Run() remains a recommended approach for optimal performance and UI responsiveness.

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