TDataSet Descendant

Question

I have a very good DirectMySQL unit, which is ready to be used and i want it to be a TDataset descendant so i can use it with QuickReport, i just want MySQL Query with DirectMySQL which descendant from TDataset.

Everything was ok until i tried to access a big table with 10.000 rows and more. It was unstable, the error was unpredictable and not always shown but it likely happened after you played with other tables.

It happened in GetFieldData(Field: TField; Buffer: Pointer): boolean; which used to get the field value from MySQL rows.

Here's the code,

function TMySQLQuery.GetFieldData(Field: TField; Buffer: Pointer): Boolean;
var
  I, CT: Integer;
  Row: TMySQL_Row;
  TBuf: PChar;
  FD: PMySQL_FieldDef;
begin
  UpdateCursorPos; ------------> This code is after i got the error but no result
  Resync([]);      ------------> This code is after i got the error but no result
  Result := false;

  Row := oRecordset.CurrentRow;
  I := Field.FieldNo-1;
  FD := oRecordset.FieldDef(I);
  if Not Assigned(FD) then
    FD := oRecordset.FieldDef(I);
  TBuf := PP(Row)[i];

  Try
    CT := MySQLWriteFieldData(fd.field_type, fd.length, fd.decimals, TBuf, PChar(Buffer));
    Result := Buffer <> nil;

  Finally
    Row := nil; ------------> This code is after i got the error but no result
    FD := nil; ------------> This code is after i got the error but no result
    TBuf := nil; ------------> This code is after i got the error but no result
    Buffer := nil; ------------> This code is after i got the error but no result
  End;
end;

{
These codes below are to translate the data type 
from MySQL Data type to a TDataset data type 
and move mysql row (TBuf) to TDataset buffer to display. 
And error always comes up from this function 
when moving mysql row to buffer.
}
function TMySQLQuery.MySQLWriteFieldData(AType: byte;
  ASize: Integer; ADec: cardinal; Source, Dest: PChar): Integer;
var
  VI: Integer;
  VF: Double;
  VD: TDateTime;  
begin
     Result := MySQLDataSize(AType, ASize, ADec);

     case AType of
       FIELD_TYPE_TINY, FIELD_TYPE_SHORT, FIELD_TYPE_LONG, FIELD_TYPE_LONGLONG,
       FIELD_TYPE_INT24:
         begin
              if Source <> '' then
                 VI := StrToInt(Source)
              else
                  VI := 0;
              Move(VI, Dest^, Result);
         end;
       FIELD_TYPE_DECIMAL, FIELD_TYPE_NEWDECIMAL:
         begin
              if source <> '' then
                VF := internalStrToCurr(Source)
              else
                VF := 0;
              Move(VF, Dest^, Result);
         end;
       FIELD_TYPE_FLOAT, FIELD_TYPE_DOUBLE:
         begin
              if Source <> '' then
                 VF := InternalStrToFloat(Source)
              else
                  VF := 0;
              Move(VF, Dest^, Result);
         end;
       FIELD_TYPE_TIMESTAMP:
         begin
              if Source <> '' then
                 VD := InternalStrToTimeStamp(Source)
              else
                  VD := 0;
              Move(VD, Dest^, Result);
         end;
       FIELD_TYPE_DATETIME:
         begin
              if Source <> '' then
                 VD := InternalStrToDateTime(Source)
              else
                  VD := 0;
              Move(VD, Dest^, Result);
         end;
       FIELD_TYPE_DATE:
         begin
              if Source <> '' then
                 VD := InternalStrToDate(Source)
              else
                  VD := 0;
              Move(VD, Dest^, Result);
         end;
       FIELD_TYPE_TIME:
         begin
              if Source <> '' then
                 VD := InternalStrToTime(Source)
              else
                  VD := 0;
              Move(VD, Dest^, Result);
         end;
       FIELD_TYPE_STRING, FIELD_TYPE_VAR_STRING,
       FIELD_TYPE_ENUM, FIELD_TYPE_SET:
         begin
              if Source = nil then
                 Dest^ := #0
              else
                Move(Source^, Dest^, Result);
         end;

        Else
          Result := 0;
          Raise EMySQLError.Create( 'Write field data  -  Unknown type field' );
     end;
end;

My guess for now is it's memory related problem.

I am stacked. Anyone could help? I also need TDataset documentation which list availlable descendant function and how to use it, or how to descendant from TDataset. anyone have them? I am lack of this kind of doumentation.

Answer 1

1. GetFieldData cannot have UpdateCursorPos and Resync calls. Otherwise you may get unpredicatable errors.
2. FD := oRecordset.FieldDef(I) ... FD := oRecordset.FieldDef(I); - looks strange. Second assigment is not needed.
3. finally ... end with local variables reset is not needed.
4. I have no idea what returns MySQLDataSize. For example, MySQLDataSize may return size in Delphi data type representation units, or may return length of data returned by MySQL. But depending on that MySQLWriteFieldData may be correct or may be not.
5. I dont know how DirectMySQL works. If it uses raw TCP/IP to talk to MySQL, then the problem may be there. For example, it incorrectly handles a sequence of packets.
6. And finally - what are the errors you are getting ? What is your Delphi version ? What is your MySQL client and server versions ?
7. And so on ....

IOW, that will be really hard to say, what is wrong. To do so, I for example, will need to get all sources, sit at Delphi IDE debugger and analyze many details of what is going on - sorry, no time :)

Answer 2

现在通过在行尾添加＃0解决了...非常感谢所有回答我问题的人。