Asynchronous Programming in Visual Basic (.NET)

Asynchronous programming allows your application to perform long-running operations without blocking the user interface or other threads. This leads to a more responsive and efficient application.

Understanding Asynchronous Operations

Traditionally, when you call a method that takes a long time to complete (e.g., reading a large file, making a network request), your application pauses until that method returns. This is known as synchronous execution. In asynchronous programming, these operations are started, and control is returned to the caller immediately. The operation then runs in the background, and you are notified when it's complete, allowing your application to continue with other tasks.

Key Concepts: `Async` and `Await`

Visual Basic (.NET) provides the Async and Await keywords to simplify asynchronous programming.

The Async Keyword

You mark a method as asynchronous by adding the Async keyword to its declaration. An asynchronous method can execute asynchronously with respect to its caller. Asynchronous methods are implicitly asynchronous. If an asynchronous method returns Void, it's an async void method. If it returns a value, it must return a Task or a Task(Of TResult). A method that returns a Task or a Task(Of TResult) is an asynchronous method, even if it doesn't use the Await keyword.

The Await Keyword

The Await keyword is used within an asynchronous method. When the execution reaches an Await expression, control is returned to the caller of the asynchronous method if the awaited task has not yet completed. When the awaited task completes, execution resumes in the asynchronous method.

Important: The Await keyword can only be used inside a method marked with the Async keyword.

Example: Performing a Network Request Asynchronously

Consider a scenario where you need to download content from a URL. Doing this synchronously would freeze your application. Here's how you can do it asynchronously:


Imports System.Net.Http
Imports System.Threading.Tasks

Public Class AsyncDownloader
    
    Public Async Function DownloadContentAsync(url As String) As Task(Of String)
        Using client As New HttpClient()
            Console.WriteLine($"Starting download from {url}...")
            Dim content As String = Await client.GetStringAsync(url)
            Console.WriteLine($"Download complete. Received {content.Length} characters.")
            Return content
        End Using
    End Function

    Public Async Function ProcessWebPageAsync() As Task
        Dim url As String = "https://www.example.com"
        Try
            Dim pageContent As String = Await DownloadContentAsync(url)
            ' Process the downloaded content here
            Console.WriteLine("Web page content processed.")
        Catch ex As Exception
            Console.WriteLine($"An error occurred: {ex.Message}")
        End Try
    End Function
End Class

Explanation:

Benefits of Asynchronous Programming

Tip: Always aim to return Task or Task(Of TResult) from your asynchronous methods. Avoid Async Void methods unless they are event handlers, as unhandled exceptions in Async Void methods can terminate the application.

Common Scenarios for Asynchronous Programming

Further Reading