How can you tell the source of the data when using the Stream.BeginRead Method?

When using the Stream.BeginRead Method, and you are reading from a stream into a memory, how is it determined where you are reading the data from?

See: http://msdn.microsoft.com/en-us/library/system.io.stream.beginread.aspx

In the list of parameters, I do not see one that tells where the data is being read from:

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
• c开发者_Python百科ount 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.

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It reads the data from wherever the stream is up to, as determined by the Position property.

Note that not all streams support seeking, so you cannot necessarily set Position.