`HttpClient`

A network access takes some time to finish. This causes a pause in our program's execution. With Async and Await we use HttpClient to download pages in a better way.

With asynchronous programming, we download files without interrupting other parts of our program. Many details are required to use HttpClient.

An example

Let us examine this simple program. We first add an "Imports System.Net.Http" statement at the top. In Main we create a Task.

Here We specify the Sub DownloadPageAsync with the AddressOf operator. This Sub is run when the Task starts.

Note We create an HttpClient instance in a Using-statement. This ensures disposal of resources.

Also We use GetAsync, Content and ReadAsStringAsync to download the web file with HttpClient.

Result The program will display the first 50 characters in a text version of a Wikipedia page.

Imports System.Net.Http

Module Module1

    Sub Main()
        ' Create new Task.
        ' ... Use AddressOf to reference a method.
        Dim t As Task = New Task(AddressOf DownloadPageAsync)
        ' Start the task.
        t.Start()
        ' Print a message as the page downloads.
        Console.WriteLine("Downloading page...")
        Console.ReadLine()
    End Sub

    Async Sub DownloadPageAsync()
        Dim page As String = "http://en.wikipedia.org/"

        ' Use HttpClient in Using-statement.
        ' ... Use GetAsync to get the page data.
        Using client As HttpClient = New HttpClient()
            Using response As HttpResponseMessage = Await client.GetAsync(page)
                Using content As HttpContent = response.Content
                    ' Get contents of page as a String.
                    Dim result As String = Await content.ReadAsStringAsync()
                    ' If data exists, print a substring.
                    If result IsNot Nothing And result.Length > 50 Then
                        Console.WriteLine(result.Substring(0, 50) + "...")
                    End If
                End Using
            End Using
        End Using
    End Sub

End ModuleDownloading page...
<!DOCTYPE html>
<html lang="en" dir="ltr" class="c...

Some notes

HttpClient is a more advanced class, and was added later, than other classes like WebClient. This class is built for asynchronous use.

For downloading web pages, it is better to enable and support GZIP compression. Another header can be added to HttpClient. We can use VB.NET to decompress these files.

HttpClient is a powerful class. And it is an effective way to download web pages and other files through HTTP (a protocol).