当我尝试免费的 TJSONObject 时访问冲突

Question

I have the code below, but when I try to free the variable checkID, I get an access violation error, and if I don't destroy it I will have a memory leak problem.

function TdtmData.CheckID(AID: String): Boolean;
var
  checkID : TJSONObject;
  clientModule : TcmClientModule;
  ok : Boolean;
begin
  Result := False;
  try
    try
      clientModule := TcmClientModule.Create(Self);
      checkID := clientModule.smMethodsServerClient.CheckID(AID);
      ok := checkID.GetValue<Boolean>('Register', False);
      if not(ok) then
        raise Exception.Create('ID ERROR.');
    finally
      clientModule.DisposeOf;
      checkID.Free; // <-- The error is here (Access violation)
    end;
    Result := ok;
  except
    on e : Exception do
      raise Exception.Create(e.Message);
  end;

end;

The smMethodsServerClient.CheckID(AID) method was created automatically through the TDSRestConnection component.

function TsmMethodsServerClient.CheckID(AID: string; const ARequestFilter: string): TJSONObject;
begin
  if FCheckIDCommand = nil then
  begin
    FCheckIDCommand  := FConnection.CreateCommand;
    FCheckIDCommand.RequestType := 'GET';
    FCheckIDCommand.Text := 'TsmMethodsServer.CheckID';
    FCheckIDCommand.Prepare(TsmMethodsServer_CheckID);
  end;
  FCheckIDCommand.Parameters[0].Value.SetWideString(AIDPDV);
  FCheckIDCommand.Execute(ARequestFilter);
  Result := TJSONObject(FCheckIDCommand.Parameters[1].Value.GetJSONValue(FInstanceOwner));
end;

I also used the Datasnap REST Client Module wizard to create my class TcmClientModule.

Answer 1

JSONValue used as a parameter of DataSnap does not need to be Free.

In addition, if you release the memory of the parameter object, an error may occur when you release the DataSnap DataModule or when you call the interface where the parameter was used for the second time.

Even if you create a new JSONValue parameter every time to use the DataSnap interface, there are no problems such as memory leaks.

Moreover, JSONValue objects received as a result of the DataSnap interface should not be freed further.

=================================================

clientModule.DisposeOf;

This frees memory for checkID. However, there is no setting for "checkID:= nil". The conditional statement below will always be executed and an error occurs when executing.

if Assigned(checkId) then 
  checkID.Free;

Answer 2

Maybe when you are doing this thing

clientModule.DisposeOf;

checkID will be destroyed, because checkID is a part of clientModule due to this part of code

clientModule.smMethodsServerClient.CheckID(AID);

You can try clear checkID first and then clear clientModule .

Update: Another way to avoid error is checking checkId before destroying. Maybe this way is suitable:

if Assigned(checkId) then 
  checkID.Free;

Maybe besides this check you need check object for null too.