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BeginRead  
Method  
Starts an asynchronous stream read operation
FileStream Class
System.IO NameSpace
CF1.  Function BeginRead ( Bytes : Array of Bytes; StartIndex : Integer; Count : Integer; CompletionMethod : AsyncCallBack; ReadControl : Object ) : AsyncResult:AsyncResult;
CF : Methods with this mark are Compact Framework Compatible
Description
Asynchronous operations take place independent of the current process. After BeginRead is called, control immediately returns to the current process thread of execution.
 
You specify the Count of bytes that will be read into the Bytes array from StartIndex.
 
You specify the procedure that is called when the read is completed. It must have the following form :
 
Procedure Name (AsyncResult : IAsyncResult);
 
You pass one of your objects ReadControl to the read to allow control of multiple threads. This gets returned in the IAsyncResult callback method parameter.
 
You need not rely on the return method - you can instead wait for the read to finish by calling EndRead. But this tends to negate the meaning of the asynchronous process in the first place.
Microsoft MSDN Links
System.IO
System.IO.FileStream
System.IO.FileStream.BeginRead
 
 
A simple example
program Project1;
{$APPTYPE CONSOLE}

uses
  System.IO;

var
  Stream    : System.IO.FileStream;
  ByteArray : Array of Byte;

procedure ReadDone(AsyncResult : IAsyncResult);
var
  Index : Integer;
begin
  Console.WriteLine('Read is finished :');
  Console.WriteLine;

  // Display our array contents
  for Index := 0 to Length(ByteArray)-1 do
    Console.WriteLine(ByteArray[Index].ToString);
end;

begin
  // Create our File Stream
  Stream := System.IO.FileStream.Create('C:\DelphiBasics.txt', FileMode.Create);

  // Write to the stream
  Stream.WriteByte(1);
  Stream.WriteByte(2);
  Stream.WriteByte(3);
  Stream.WriteByte(4);
  Stream.WriteByte(5);
  Stream.WriteByte(6);
  Stream.WriteByte(7);
  Stream.WriteByte(8);
  Stream.WriteByte(9);

  // Move to the start of the stream
  Stream.Seek(0, SeekOrigin.Begin);

  // Declare our array to hold the returned data
  SetLength(ByteArray, 12);

  // Read from the stream asynchronously
  Stream.BeginRead(ByteArray, 0, 10, ReadDone, TObject.Create);

  // Close the file stream
  Stream.Close;

  Console.Readline;
end.
   Read is finished :
  
   1
   2
   3
   4
   5
   6
   7
   8
   9
   0
   0
   0
 
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