Using Async for File Access (Visual Basic)

You can use the Async feature to access files. By using the Async feature, you can call into asynchronous methods without using callbacks or splitting your code across multiple methods or lambda expressions. To make synchronous code asynchronous, you just call an asynchronous method instead of a synchronous method and add a few keywords to the code.

You might consider the following reasons for adding asynchrony to file access calls:

  • Asynchrony makes UI applications more responsive because the UI thread that launches the operation can perform other work. If the UI thread must execute code that takes a long time (for example, more than 50 milliseconds), the UI may freeze until the I/O is complete and the UI thread can again process keyboard and mouse input and other events.

  • Asynchrony improves the scalability of ASP.NET and other server-based applications by reducing the need for threads. If the application uses a dedicated thread per response and a thousand requests are being handled simultaneously, a thousand threads are needed. Asynchronous operations often don’t need to use a thread during the wait. They use the existing I/O completion thread briefly at the end.

  • The latency of a file access operation might be very low under current conditions, but the latency may greatly increase in the future. For example, a file may be moved to a server that's across the world.

  • The added overhead of using the Async feature is small.

  • Asynchronous tasks can easily be run in parallel.

Running the Examples

To run the examples in this topic, you can create a WPF Application or a Windows Forms Application and then add a Button. In the button's Click event, add a call to the first method in each example.

In the following examples, include the following Imports statements.

Imports System  
Imports System.Collections.Generic  
Imports System.Diagnostics  
Imports System.IO  
Imports System.Text  
Imports System.Threading.Tasks  

Use of the FileStream Class

The examples in this topic use the FileStream class, which has an option that causes asynchronous I/O to occur at the operating system level. By using this option, you can avoid blocking a ThreadPool thread in many cases. To enable this option, you specify the useAsync=true or options=FileOptions.Asynchronous argument in the constructor call.

You can’t use this option with StreamReader and StreamWriter if you open them directly by specifying a file path. However, you can use this option if you provide them a Stream that the FileStream class opened. Note that asynchronous calls are faster in UI apps even if a ThreadPool thread is blocked, because the UI thread isn’t blocked during the wait.

Writing Text

The following example writes text to a file. At each await statement, the method immediately exits. When the file I/O is complete, the method resumes at the statement that follows the await statement. Note that the async modifier is in the definition of methods that use the await statement.

Public Async Sub ProcessWrite()  
    Dim filePath = "temp2.txt"  
    Dim text = "Hello World" & ControlChars.CrLf  
    Await WriteTextAsync(filePath, text)  
End Sub  
Private Async Function WriteTextAsync(filePath As String, text As String) As Task  
    Dim encodedText As Byte() = Encoding.Unicode.GetBytes(text)  
    Using sourceStream As New FileStream(filePath,  
        FileMode.Append, FileAccess.Write, FileShare.None,  
        bufferSize:=4096, useAsync:=True)  
        Await sourceStream.WriteAsync(encodedText, 0, encodedText.Length)  
    End Using  
End Function  

The original example has the statement Await sourceStream.WriteAsync(encodedText, 0, encodedText.Length), which is a contraction of the following two statements:

Dim theTask As Task = sourceStream.WriteAsync(encodedText, 0, encodedText.Length)  
Await theTask  

The first statement returns a task and causes file processing to start. The second statement with the await causes the method to immediately exit and return a different task. When the file processing later completes, execution returns to the statement that follows the await. For more information, see Control Flow in Async Programs (Visual Basic).

Reading Text

The following example reads text from a file. The text is buffered and, in this case, placed into a StringBuilder. Unlike in the previous example, the evaluation of the await produces a value. The ReadAsync method returns a Task<Int32>, so the evaluation of the await produces an Int32 value (numRead) after the operation completes. For more information, see Async Return Types (Visual Basic).

Public Async Sub ProcessRead()  
    Dim filePath = "temp2.txt"  
    If File.Exists(filePath) = False Then  
        Debug.WriteLine("file not found: " & filePath)  
            Dim text As String = Await ReadTextAsync(filePath)  
        Catch ex As Exception  
        End Try  
    End If  
End Sub  
Private Async Function ReadTextAsync(filePath As String) As Task(Of String)  
    Using sourceStream As New FileStream(filePath,  
        FileMode.Open, FileAccess.Read, FileShare.Read,  
        bufferSize:=4096, useAsync:=True)  
        Dim sb As New StringBuilder  
        Dim buffer As Byte() = New Byte(&H1000) {}  
        Dim numRead As Integer  
        numRead = Await sourceStream.ReadAsync(buffer, 0, buffer.Length)  
        While numRead <> 0  
            Dim text As String = Encoding.Unicode.GetString(buffer, 0, numRead)  
            numRead = Await sourceStream.ReadAsync(buffer, 0, buffer.Length)  
        End While  
        Return sb.ToString  
    End Using  
End Function  

Parallel Asynchronous I/O

The following example demonstrates parallel processing by writing 10 text files. For each file, the WriteAsync method returns a task that is then added to a list of tasks. The Await Task.WhenAll(tasks) statement exits the method and resumes within the method when file processing is complete for all of the tasks.

The example closes all FileStream instances in a Finally block after the tasks are complete. If each FileStream was instead created in an Imports statement, the FileStream might be disposed of before the task was complete.

Note that any performance boost is almost entirely from the parallel processing and not the asynchronous processing. The advantages of asynchrony are that it doesn’t tie up multiple threads, and that it doesn’t tie up the user interface thread.

Public Async Sub ProcessWriteMult()  
    Dim folder = "tempfolder\"  
    Dim tasks As New List(Of Task)  
    Dim sourceStreams As New List(Of FileStream)  
        For index = 1 To 10  
            Dim text = "In file " & index.ToString & ControlChars.CrLf  
            Dim fileName = "thefile" & index.ToString("00") & ".txt"  
            Dim filePath = folder & fileName  
            Dim encodedText As Byte() = Encoding.Unicode.GetBytes(text)  
            Dim sourceStream As New FileStream(filePath,  
                FileMode.Append, FileAccess.Write, FileShare.None,  
                bufferSize:=4096, useAsync:=True)  
            Dim theTask As Task = sourceStream.WriteAsync(encodedText, 0, encodedText.Length)  
        Await Task.WhenAll(tasks)  
        For Each sourceStream As FileStream In sourceStreams  
    End Try  
End Sub  

When using the WriteAsync and ReadAsync methods, you can specify a CancellationToken, which you can use to cancel the operation mid-stream. For more information, see Fine-Tuning Your Async Application (Visual Basic) and Cancellation in Managed Threads.

See also