Stream.BeginRead Method (System.IO)

Switch View :

Classic

Lightweight

ScriptFree

Site Feedback

.NET Framework Class Library for Silverlight

Stream.BeginRead Method

Begins an asynchronous read operation.

Namespace: System.IO
Assembly: mscorlib (in mscorlib.dll)

Syntax

Visual Basic (Declaration)

Public Overridable Function BeginRead ( _
	buffer As Byte(), _
	offset As Integer, _
	count As Integer, _
	callback As AsyncCallback, _
	state As Object _
) As IAsyncResult

public virtual IAsyncResult BeginRead(
	byte[] buffer,
	int offset,
	int count,
	AsyncCallback callback,
	Object state
)

Parameters

buffer: Type: System.Byte[]
The buffer to read the data into.

offset: Type: System.Int32
The byte offset in buffer at which to begin writing data read from the stream.

count: Type: System.Int32
The maximum number of bytes to read.

callback: Type: System.AsyncCallback
An optional asynchronous callback, to be called when the read is complete.

state: Type: System.Object
A user-provided object that distinguishes this particular asynchronous read request from other requests.

Return Value

Type: System.IAsyncResult
An IAsyncResult that represents the asynchronous read, which could still be pending.

Exceptions

Exception	Condition
IOException	Attempted an asynchronous read past the end of the stream, or a disk error occurs.
ArgumentException	One or more of the arguments is invalid.
ObjectDisposedException	Methods were called after the stream was closed.
NotSupportedException	The current Stream implementation does not support the read operation.

Remarks

The default implementation of BeginRead on a stream calls the Read method synchronously, which means that Read might block on some streams. However, instances of classes such as FileStream fully support asynchronous operations if the instances have been opened asynchronously. Therefore, calls to BeginRead will not block on those streams. You can override BeginRead (by using async delegates, for example) to provide asynchronous behavior.

Pass the IAsyncResult return value to the EndRead method of the stream to determine how many bytes were read and to release operating system resources used for reading. EndRead must be called once for every call to BeginRead. You can do this either by using the same code that called BeginRead or in a callback passed to BeginRead.

The current position in the stream is updated when the asynchronous read or write is issued, not when the I/O operation completes.

Multiple simultaneous asynchronous requests render the request completion order uncertain.

Use the CanRead property to determine whether the current instance supports reading.

If a stream is closed or you pass an invalid argument, exceptions are thrown immediately from BeginRead. Errors that occur during an asynchronous read request, such as a disk failure during the I/O request, occur on the thread pool thread and throw exceptions when calling EndRead.

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

Reference

Stream Class

System.IO Namespace

Community Content

Alex Heitzmann

Utility class for asynchronously reading a stream into a string.

Here's an example of reading a byte stream into a string using the async stream operations.


public class AsyncStreamReader
{
    private const int DEFAULT_CHUNK_SIZE = 16384; // 16kb
    private byte[] chunk;
    private int chunkSize;
    private bool isOperationInProgress;

    public Encoding StreamEncoding { get; set; }
    public string StreamContentsAsString { get; private set; }
    public event EventHandler ReadStreamCompleted;

    public AsyncStreamReader(int chunkSize = DEFAULT_CHUNK_SIZE)
    {
        this.chunkSize = chunkSize;
        this.isOperationInProgress = false;
    }

    /// <summary>
    /// Asynchronously reads the contents of the given stream into a string.
    /// The ReadStreamCompleted event will be fired when the end of
    /// the stream is reached, and the destination string cann be accessed
    /// through the StreamContentsAsString property.
    /// </summary>
    public void ReadStreamAsync(Stream stream)
    {
        if (isOperationInProgress) {
            throw new InvalidOperationException("Async operation already in progress.");
        }

        StreamEncoding = Encoding.UTF8;
        StreamContentsAsString = "";
        isOperationInProgress = true;
        ReadStreamAsyncImpl(stream);
    }

    private void ReadStreamAsyncImpl(Stream stream)
    {
        chunk = new byte[chunkSize];
        stream.BeginRead(chunk, 0, chunkSize, new AsyncCallback(BeginReadCallback), stream);
    }

    private void BeginReadCallback(IAsyncResult ar)
    {
        Stream stream = ar.AsyncState as Stream;
        int bytesRead = stream.EndRead(ar);
        StreamContentsAsString += StreamEncoding.GetString(chunk, 0, bytesRead);

        if (bytesRead < chunkSize) {
            // Finished
            isOperationInProgress = false;
            stream.Close();
            if (null != ReadStreamCompleted) {
                ReadStreamCompleted(this, new EventArgs());
            }
        } else {
            ReadStreamAsyncImpl(stream);
        }
    }
}