Stream.BeginRead(Byte[], Int32, Int32, AsyncCallback, Object) Stream.BeginRead(Byte[], Int32, Int32, AsyncCallback, Object) Stream.BeginRead(Byte[], Int32, Int32, AsyncCallback, Object) Stream.BeginRead(Byte[], Int32, Int32, AsyncCallback, Object) Method

定义

开始异步读操作。Begins an asynchronous read operation. (请考虑改用 ReadAsync(Byte[], Int32, Int32)。)(Consider using ReadAsync(Byte[], Int32, Int32) instead.)

public:
 virtual IAsyncResult ^ BeginRead(cli::array <System::Byte> ^ buffer, int offset, int count, AsyncCallback ^ callback, System::Object ^ state);
public virtual IAsyncResult BeginRead (byte[] buffer, int offset, int count, AsyncCallback callback, object state);
abstract member BeginRead : byte[] * int * int * AsyncCallback * obj -> IAsyncResult
override this.BeginRead : byte[] * int * int * AsyncCallback * obj -> IAsyncResult
Public Overridable Function BeginRead (buffer As Byte(), offset As Integer, count As Integer, callback As AsyncCallback, state As Object) As IAsyncResult

参数

buffer
Byte[]

数据读入的缓冲区。The buffer to read the data into.

offset
Int32 Int32 Int32 Int32

buffer 中的字节偏移量,从该偏移量开始写入从流中读取的数据。The byte offset in buffer at which to begin writing data read from the stream.

count
Int32 Int32 Int32 Int32

最多读取的字节数。The maximum number of bytes to read.

callback
AsyncCallback AsyncCallback AsyncCallback AsyncCallback

可选的异步回调,在完成读取时调用。An optional asynchronous callback, to be called when the read is complete.

state
Object Object Object Object

一个用户提供的对象,它将该特定的异步读取请求与其他请求区别开来。A user-provided object that distinguishes this particular asynchronous read request from other requests.

返回

表示异步读取的 IAsyncResult(可能仍处于挂起状态)。An IAsyncResult that represents the asynchronous read, which could still be pending.

异常

尝试的异步读取超过了流的结尾,或者发生了磁盘错误。Attempted an asynchronous read past the end of the stream, or a disk error occurs.

一个或多个自变量无效。One or more of the arguments is invalid.

在流关闭后调用方法。Methods were called after the stream was closed.

当前 Stream 实现不支持读取操作。The current Stream implementation does not support the read operation.

注解

在 .NET Framework 4 和更早的版本中,您必须使用 BeginReadEndRead 之类的方法来实现异步 I/O 操作。In the .NET Framework 4 and earlier versions, you have to use methods such as BeginRead and EndRead to implement asynchronous I/O operations. 这些方法是在中仍然可用.NET Framework 4.5.NET Framework 4.5以支持旧版代码; 但是,新的异步方法,如ReadAsyncWriteAsyncCopyToAsync,和FlushAsync,帮助您更轻松地实现异步 I/O 操作。These methods are still available in the .NET Framework 4.5.NET Framework 4.5 to support legacy code; however, the new async methods, such as ReadAsync, WriteAsync, CopyToAsync, and FlushAsync, help you implement asynchronous I/O operations more easily.

默认实现BeginRead流调用Read方法以同步方式,这意味着,Read可能会阻止某些流。The default implementation of BeginRead on a stream calls the Read method synchronously, which means that Read might block on some streams. 但是,实例的类,如FileStreamNetworkStream完全支持异步操作,如果实例已异步打开。However, instances of classes such as FileStream and NetworkStream fully support asynchronous operations if the instances have been opened asynchronously. 因此,调用BeginRead不会阻止对这些流。Therefore, calls to BeginRead will not block on those streams. 您可以重写BeginRead(通过使用异步委托,例如) 提供异步行为。You can override BeginRead (by using async delegates, for example) to provide asynchronous behavior.

传递IAsyncResult返回值为EndRead方法以确定已读取的字节数并释放操作系统资源用于进行读取的流。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 必须为每个调用一次调用BeginReadEndRead must be called once for every call to BeginRead. 您可以执行此操作是通过调用的相同代码BeginRead或在传递给回调BeginReadYou can do this either by using the same code that called BeginRead or in a callback passed to BeginRead.

当异步读取或写入发出时,I/O 操作完成时不会更新中流的当前位置。The current position in the stream is updated when the asynchronous read or write is issued, not when the I/O operation completes.

多个同时进行的异步请求呈现请求完成顺序不确定。Multiple simultaneous asynchronous requests render the request completion order uncertain.

使用CanRead属性来确定当前实例是否支持读取。Use the CanRead property to determine whether the current instance supports reading.

如果流已关闭或传递了无效的参数,将立即引发异常从BeginReadIf a stream is closed or you pass an invalid argument, exceptions are thrown immediately from BeginRead. 异步读取请求,例如,I/O 请求过程的磁盘故障过程中发生的错误发生在线程池线程和调用时引发异常EndReadErrors 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.

适用于

另请参阅