FileStream.BeginRead Method (Byte(), Int32, Int32, AsyncCallback, Object)
Begins an asynchronous read operation. (Consider using ReadAsync instead; see the Remarks section.)
Assembly: mscorlib (in mscorlib.dll)
<HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading := True)> Public Overrides Function BeginRead ( array As Byte(), offset As Integer, numBytes As Integer, userCallback As AsyncCallback, stateObject As Object ) As IAsyncResult
Parameters
- array
-
Type:
System.Byte()
The buffer to read data into.
- offset
-
Type:
System.Int32
The byte offset in array at which to begin reading.
- numBytes
-
Type:
System.Int32
The maximum number of bytes to read.
- userCallback
-
Type:
System.AsyncCallback
The method to be called when the asynchronous read operation is completed.
- stateObject
-
Type:
System.Object
A user-provided object that distinguishes this particular asynchronous read request from other requests.
| Exception | Condition |
|---|---|
| ArgumentException | The array length minus offset is less than numBytes. |
| ArgumentNullException | array is null. |
| ArgumentOutOfRangeException | offset or numBytes is negative. |
| IOException | An asynchronous read was attempted past the end of the file. |
In the .NET Framework 4 and earlier versions, you have to use methods such as BeginRead and EndRead to implement asynchronous file operations. These methods are still available in the .NET Framework 4.5 to support legacy code; however, the new async methods, such as ReadAsync, WriteAsync, CopyToAsync, and FlushAsync, help you implement asynchronous file operations more easily.
EndRead must be called exactly once for every call to BeginRead. Failing to end a read process before beginning another read can cause undesirable behavior such as deadlock.
FileStream provides two different modes of operation: synchronous I/O and asynchronous I/O. While either can be used, the underlying operating system resources might allow access in only one of these modes. By default, FileStream opens the operating system handle synchronously. In Windows, this slows down asynchronous methods. If asynchronous methods are used, use the FileStream(String, FileMode, FileAccess, FileShare, Int32, Boolean) constructor.
Note |
|---|
Use the CanRead property to determine whether the current instance supports reading. For additional information, see CanRead. |
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 IO request, occur on the thread pool thread and become visible upon a call to EndRead.
EndRead must be called with this IAsyncResult to find out how many bytes were read.
Multiple simultaneous asynchronous requests render the request completion order uncertain.
For a list of common file and directory operations, see Common I-O Tasks.
This code example is part of a larger example provided for the FileStream(String, FileMode, FileAccess, FileShare, Int32, Boolean) constructor.
Private Shared Sub EndWriteCallback(asyncResult As IAsyncResult) Dim tempState As State = _ DirectCast(asyncResult.AsyncState, State) Dim fStream As FileStream = tempState.FStream fStream.EndWrite(asyncResult) ' Asynchronously read back the written data. fStream.Position = 0 asyncResult = fStream.BeginRead( _ tempState.ReadArray, 0 , tempState.ReadArray.Length, _ AddressOf EndReadCallback, tempState) ' Concurrently do other work, such as ' logging the write operation. End Sub
Available since 1.1
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
