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

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

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

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

callback
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

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. .NET Framework 4.5.NET Framework 4.5这些方法在中仍可用以支持旧版代码; 但是,新的异步方法( ReadAsync如、 WriteAsync CopyToAsync、和FlushAsync)可帮助你更轻松地实现异步 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.

流上的的BeginRead默认实现以Read同步Read方式调用方法,这意味着可能会阻止某些流。The default implementation of BeginRead on a stream calls the Read method synchronously, which means that Read might block on some streams. 但是,如果实例已异步打开FileStreamNetworkStream则类的实例(例如和)完全支持异步操作。However, instances of classes such as FileStream and NetworkStream fully support asynchronous operations if the instances have been opened asynchronously. 因此,对的BeginRead调用不会在这些流上阻塞。Therefore, calls to BeginRead will not block on those streams. 可以重写BeginRead (例如,使用异步委托)来提供异步行为。You can override BeginRead (by using async delegates, for example) to provide asynchronous behavior.

将返回值传递到流EndRead的方法以确定读取的字节数,以及释放用于读取的操作系统资源。 IAsyncResultPass 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对于每次调用BeginRead, 都必须调用一次。EndRead must be called once for every call to BeginRead. 可以通过在传递给BeginRead BeginRead的回调中使用调用或的相同代码来执行此操作。You can do this either by using the same code that called BeginRead or in a callback passed to BeginRead.

发出异步读取或写入操作时,将更新流中的当前位置,而不是在 i/o 操作完成时进行更新。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会导致不确定请求完成顺序。Multiple simultaneous asynchronous requests render the request completion order uncertain.

CanRead使用属性可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的参数无效,则会立即从中BeginRead引发异常。If a stream is closed or you pass an invalid argument, exceptions are thrown immediately from BeginRead. 在异步读取请求期间发生的错误(例如,i/o 请求过程中的磁盘故障)将在线程池线程上发生,并在调用EndRead时引发异常。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.

适用于

另请参阅