ConfigureAwait(true) - Explicitly Resuming on the Context

This snippet demonstrates how to explicitly configure an await to resume on the original context. While `ConfigureAwait(false)` is generally preferred for libraries, there might be specific scenarios where you need to ensure the continuation runs on the originating context.

Understanding ConfigureAwait(true)

ConfigureAwait(true) explicitly instructs the awaitable to resume on the original SynchronizationContext or TaskScheduler. This effectively enforces the default behavior, but it makes your intention clear in the code. While rarely necessary, it can be useful in specific situations or for code clarity.

The Code Snippet

This code snippet is a simplified example within a Windows Forms application. The Button1_Click event handler is triggered when a button is clicked. It calls an asynchronous method DoSomethingAsync, which simulates some work with Task.Delay. Crucially, ConfigureAwait(true) is used after the await in both Button1_Click and DoSomethingAsync. This ensures that after the delay, the label1.Text is updated on the UI thread, preventing cross-thread exceptions. The Console.WriteLine statements will show the thread ID to illustrate the context preservation.

using System;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace ConfigureAwaitExample
{
    public partial class MyForm : Form
    {
        public MyForm()
        {
            InitializeComponent();
        }

        private async void Button1_Click(object sender, EventArgs e)
        {
            label1.Text = "Starting...";
            await DoSomethingAsync().ConfigureAwait(true);
            label1.Text = "Completed on UI thread.";
        }

        private async Task DoSomethingAsync()
        {
            Console.WriteLine($"DoSomethingAsync started on thread: {Environment.CurrentManagedThreadId}");
            await Task.Delay(1000).ConfigureAwait(true); // Simulate some work
            Console.WriteLine($"DoSomethingAsync continued on thread: {Environment.CurrentManagedThreadId}");
        }
    }
}

When to use ConfigureAwait(true)

• Ensuring UI Thread Execution: When you absolutely need to guarantee that the code after the await runs on the UI thread. This is often when you're directly updating UI elements.
• Code Clarity: In rare cases, you might use ConfigureAwait(true) to explicitly document the intended behavior, even though it's the default. This can improve code readability for other developers.

Real-Life Use Case

Consider a data binding scenario in a WPF application. You're fetching data asynchronously, and you need to update a UI control that is bound to that data. If you don't resume on the UI thread after the await, the data binding might not work correctly, leading to UI inconsistencies. Using ConfigureAwait(true) in this case would ensure the data is updated on the UI thread, triggering the binding mechanism.

Contrast with ConfigureAwait(false)

The key difference is the resumption context. ConfigureAwait(false) avoids resuming on the original context, potentially improving performance and preventing deadlocks in libraries. ConfigureAwait(true) forces resumption on the original context, which is necessary for direct UI updates in application code.

Benefits of Explicit Context Control

While ConfigureAwait(true) is rarely necessary, explicitly controlling the context can make your code more predictable and easier to reason about, especially in complex asynchronous scenarios. It can also serve as a form of documentation, indicating that the continuation relies on the original context.

Interview Tip

Be able to explain the trade-offs between ConfigureAwait(true) and ConfigureAwait(false). Understand that ConfigureAwait(true) enforces the default behavior, and discuss scenarios where it might be useful to make this explicit.