FileStream.IsAsync Property

public:
 virtual property bool IsAsync { bool get(); };

public virtual bool IsAsync { get; }

member this.IsAsync : bool

Public Overridable ReadOnly Property IsAsync As Boolean

Property Value

true if the handle was opened for overlapped I/O on Windows. On Unix, true if the stream was opened for asynchronous I/O in .NET 10 and earlier or the file descriptor has the O_NONBLOCK flag set in .NET 11 and later; otherwise, false.

Examples

This code example is part of a larger example provided for the FileStream(String, FileMode, FileAccess, FileShare, Int32, Boolean) constructor.

static void Main()
{
    // Create a synchronization object that gets
    // signaled when verification is complete.
    ManualResetEvent manualEvent = new ManualResetEvent(false);

    // Create random data to write to the file.
    byte[] writeArray = new byte[100000];
    new Random().NextBytes(writeArray);

    FileStream fStream =
        new FileStream("Test#@@#.dat", FileMode.Create,
        FileAccess.ReadWrite, FileShare.None, 4096, true);

    // Check that the FileStream was opened asynchronously.
    Console.WriteLine("fStream was {0}opened asynchronously.",
        fStream.IsAsync ? "" : "not ");

    // Asynchronously write to the file.
    IAsyncResult asyncResult = fStream.BeginWrite(
        writeArray, 0, writeArray.Length,
        new AsyncCallback(EndWriteCallback),
        new State(fStream, writeArray, manualEvent));

    // Concurrently do other work and then wait
    // for the data to be written and verified.
    manualEvent.WaitOne(5000, false);
}

// Create a synchronization object that gets
// signaled when verification is complete.
let manualEvent = new ManualResetEvent false

// Create random data to write to the file.
let writeArray = Array.zeroCreate 100000
Random.Shared.NextBytes writeArray

let fStream =
    new FileStream("Test#@@#.dat", FileMode.Create, FileAccess.ReadWrite, FileShare.None, 4096, true)

// Check that the FileStream was opened asynchronously.

if fStream.IsAsync then "" else "not "
|> printfn "fStream was %sopened asynchronously."

// Asynchronously write to the file.
let asyncResult =
    fStream.BeginWrite(
        writeArray,
        0,
        writeArray.Length,
        AsyncCallback endWriteCallback,
        State(fStream, writeArray, manualEvent)
    )

// Concurrently do other work and then wait
// for the data to be written and verified.
manualEvent.WaitOne(5000, false) |> ignore

Shared Sub Main()

    ' Create a synchronization object that gets 
    ' signaled when verification is complete.
    Dim manualEvent As New ManualResetEvent(False)

    ' Create random data to write to the file.
    Dim writeArray(100000) As Byte
    Dim randomGenerator As New Random()
    randomGenerator.NextBytes(writeArray)

    Dim fStream As New FileStream("Test#@@#.dat", _
        FileMode.Create, FileAccess.ReadWrite, _
        FileShare.None, 4096, True)

    ' Check that the FileStream was opened asynchronously.
    If fStream.IsAsync = True
        Console.WriteLine("fStream was opened asynchronously.")
    Else
        Console.WriteLine("fStream was not opened asynchronously.")
    End If

    ' Asynchronously write to the file.
    Dim asyncResult As IAsyncResult = fStream.BeginWrite( _
        writeArray, 0, writeArray.Length, _
        AddressOf EndWriteCallback , _
        New State(fStream, writeArray, manualEvent))

    ' Concurrently do other work and then wait
    ' for the data to be written and verified.
    manualEvent.WaitOne(5000, False)
End Sub

Remarks

The IsAsync property indicates whether the FileStream handle is configured for asynchronous I/O, enabling your code to use the Handle property correctly. In Win32, IsAsync being true means the handle was opened for overlapped I/O, and thus requires different parameters to ReadFile and WriteFile. On Unix, starting in .NET 11, it reflects whether the file descriptor has the O_NONBLOCK flag set.

You specify this value when you create an instance of the FileStream class using a constructor that has an isAsync, useAsync, or options parameter. The property doesn't have to be true to call the ReadAsync, WriteAsync, or CopyToAsync method. When the property is false and you call an asynchronous read or write method, the UI thread isn't blocked, but the I/O is performed synchronously.

See also

• File and Stream I/O
• How to: Read Text from a File
• How to: Write Text to a File