Access Violation when using MemoryStream.CopyFrom - delphi

Question

Well this is going to be long!
I made a retransmission scheme for managing dropped UDP packets, for a LAN simulation protocol I am working on.
Packet Storage
TDataBuffer = record
PacketID : WORD;
Packet : TMemoryStream;
end;
PDataBuffer = ^TDataBuffer;
Related DataModule Class Members
fRxDataPacketList : TThreadList20;
fTxDataPacketList : TThreadList20;
procedure CreateDataBuffer
(PacketID : WORD; Packet : TMemoryStream;
var DataBuffer : PDataBuffer);
procedure DestroyDataBuffer
(var DataBuffer : PDataBuffer);
procedure AddPacketToPacketList
(PacketID : WORD; Packet : TMemoryStream;
RecievedPacket : Boolean);
function GetPacketFromTxDataPacketList
(PacketID : WORD; var Packet : TMemoryStream): Boolean;
TThreadList20: It is my own thread friendly, encryption and compression supporting wrapper class for TList.
There is another procedure for processing the Rx side which doesn't concern my question so I am skipping it.
Creation
procedure TDataModuleClient.CreateDataBuffer
(PacketID : WORD; Packet : TMemoryStream;
var DataBuffer : PDataBuffer);
begin
New (DataBuffer);
DataBuffer.PacketID := PacketID;
DataBuffer.Packet := TMemoryStream.Create;
if Assigned (Packet) then
begin
DataBuffer.Packet.CopyFrom (Packet,Packet.Size); // NO AV HERE
DataBuffer.Packet.Position := 0;
end;
end;
Destruction
procedure TDataModuleClient.DestroyDataBuffer
(var DataBuffer : PDataBuffer);
begin
DataBuffer.Packet.Free;
Dispose (DataBuffer);
end;
Addition To List
procedure TDataModuleClient.AddPacketToDataPacketList
(PacketID : WORD; Packet : TMemoryStream; RecievedPacket : Boolean);
var
DataBuffer : PDataBuffer;
begin
CreateDataBuffer (PacketID,Packet,DataBuffer);
if RecievedPacket then
fRxDataPacketList.Add (TObject (DataBuffer))
else
begin
fTxDataPacketList.Lock;
try
fTxDataPacketList.Add (TObject (DataBuffer));
if fRxDataPacketList.Count = 21 then
begin
DataBuffer := PDataBuffer (fTxDataPacketList [0]);
DestroyDataBuffer (DataBuffer);
fTxDataPacketList.Delete (0);
end;
finally fTxDataPacketList.Unlock;
end;
end;
end;
Extraction From List
function TDataModuleClient.GetPacketFromTxDataPacketList
(PacketID : WORD; var Packet : TMemoryStream): Boolean;
var
DataBuffer : PDataBuffer;
I : Integer;
begin
Result := False;
fTxDataPacketList.Lock;
try
for I := fTxDataPacketList.Count - 1 downto 0 do
begin
DataBuffer := PDataBuffer (fTxDataPacketList [I]);
if DataBuffer.PacketID < PacketID then
begin
DestroyDataBuffer (DataBuffer);
fTxDataPacketList.Delete (I);
end
else if DataBuffer.PacketID = PacketID then
begin
Result := True;
Packet := TMemoryStream.Create;
Packet.CopyFrom
(DataBuffer.Packet,DataBuffer.Packet.Size); // AV HERE
Packet.Position := 0;
DestroyDataBuffer (DataBuffer);
fTxDataPacketList.Delete (I);
break;
end;
end;
finally fTxDataPacketList.Unlock;
end;
end;
Packet: Output Variable.
Please help me out, I know it looks like a tall order to deduce something thanks to the amount of code.

Answer 1

Based on this limited code, I am making some guesses:
1) the input Packet that is being passed to GetPacketFromTxDataPacketList() has probably not actually been instantiated yet before you call CopyFrom(). That would account for the AV.
2) since the Packet parameter is declared as a var, that suggests to me that it is an output parameter that GetPacketFromTxDataPacketList() is supposed to create and return, instead of having the caller create the TMemoryStream and pass it into GetPacketFromTxDataPacketList() to be filled with data. That would also play into #1, since GetPacketFromTxDataPacketList() is not creating a new TMemoryStream object, it is assuming the object has already been created beforehand.