Delphi, FileStream and MemoryStream

Question

I want to copy a part of a FileStream to a memory stream.

FileStream.Write(Pointer(MemoryStream)^, MemoryStream.Size);
FileStream.Read(Pointer(MemoryStream)^, count);

Is that right? it isn't working for me.

Answer 1

You have to Read() from the FileStream into a separate buffer and then Write() that to the MemoryStream, ie:

var
  Buffer: PByte;

GetMem(Buffer, NumberOfBytes);
try
  FileStream.ReadBuffer(Buffer^, NumberOfBytes);
  MemoryStream.WriteBuffer(Buffer^, NumberOfBytes);
finally
  FreeMem(Buffer);
end;

Since you are dealing with two TStream objects, it would be easier to use the TStream.CopyFrom() method instead, ie:

MemoryStream.CopyFrom(FileStream, NumberOfBytes);