Delphi XE2中查询中的访问冲突

Question

I have a query say qryDoABC in function DoABC . When I call this function first time, all works fine but 2nd time when I call it, at line with qryDoABC do, it throws access violation exception.

procedure TMyForm.DoABC;
begin
  with qryDoABC do
  --
  -- 
end;

I googled and found out that I should check whether query is assigned or not. So now I am also checking whether query is assigned or not like this:

procedure TMyForm.DoABC;
begin
  if assigned qryDoABC then
  with qryDoABC do
  --
  -- 
end;

Now no exception is there because 2nd time query is not assigned. But I have to fire this qry 2nd time also. How should I assign this query 2nd time.

Full Code:

function TMyForm.DoABC:boolean;
begin
  try
    if assigned(qryDoABC)then 
    with qryDoABCdo
    begin
      Close  ;
      SQL.Clear;
      SQL.Text :=
        'Some query';

      Parameters.ParamByName('ABC').Value := ABC;
      Parameters.ParamByName('XYZ').Value := XYZ;
      Open;
      if (Recordcount = 0) then
        result := false
      else
      begin
        result := true;
      end;
      Close;
    end;
  except
    on E : Exception do
    begin
      result := false;
      exit;
    end;
  end;
end;

Answer 1

Use Query Variable as a Local. Declare it in your function. And use 'TRY...FINALLY' to FREE query.

function TMyForm.DoABC:boolean;
var
  qryDoABC: TADOQuery;
begin
  try
    qryDoABC := TADOQuery.Create(nil);
    //Set Appropriate Connection String (below for MS-Access)
    qryDoABC.ConnectionString := 'Provider=Microsoft.Jet.OLEDB.4.0; Data Source=MDB_file';
    try
      with qryDoABC do
      begin
        SQL.Clear;
        SQL.Text := 'Some query';
        Parameters.ParamByName('ABC').Value := 'ABC';
        Parameters.ParamByName('XYZ').Value := 'XYZ';
        Open;
        if (Recordcount = 0) then
          result := false
        else
        begin
          result := true;
        end;
        Close;
      end;
    except
      on E : Exception do
      begin
        result := false;
      end;
    end;
  finally
    FreeAndNil(qryDoABC);
  end;
end;

Answer 2

Is qryDoABC a component on a form? If so, it is probably called the second time, if the form is removed. In that case, the form is either released too early or the query is called to late.

If you free the form manually, and the second query call is the result of an event handler, you can free the form using Release, this frees the form only when the message queue is handled so there are no pending messages for the form left.

Answer 3

I was using TADOQuery Syntax like this

      with qryDoABC do
      begin
        SQL.Clear;
        SQL.Text := 'Some query';
        Parameters.ParamByName('ABC').Value := 'ABC';
        Parameters.ParamByName('XYZ').Value := 'XYZ';
        Open;
        if (Recordcount = 0) then
          result := false
        else
        begin
          result := true;
        end;
        Close;
      end;

But I discarded using With statement and wrote my code like this

qryDoABC.SQL.Clear;
    qryDoABC.SQL.Text := 'Some query';
    qryDoABC.Parameters.ParamByName('ABC').Value := 'ABC';
    qryDoABC.Parameters.ParamByName('XYZ').Value := 'XYZ';
    qryDoABC.Open;
    if (qryDoABC.Recordcount = 0) then
      result := false
    else
    begin
      result := true;
    end;
qryDoABC.Close;