I use TBlobfield and TBlobStream for this purpose, but to get to TBlobField (as far as I can work out with FireDAC) I have to access change the dataset's RecNo to , which I don't always wish to do. That being so, I don't seem to have any other choice than the GetData/SetData. I would be very pleased to be wrong!


I have the Professional version of Delphi. It gives you only limited access to the FireDAC source code and does not include any of the source code for these functions. Also, there does not appear to be any documentation relating to TFDDATSROW.

As I mention in the response to Peter, to use TBlobstream I have to be able to access TBlobField, to do which I have to move the cursor to the current record in the dataset. On some occasions I want to avoid doing this whilst reading the blob data from another record. The only way I can see of doing this is via TFDATSROW type which only allows retrieval and setting of data via GetData and SetData functions as mentioned in my original post and with which I am having problems.

For the record, I have discovered that the TBlobField type also has LoadFromStream and SaveToStream functions which are simpler to use than TBlobStream.

I'm unfamiliar with CloneCursor, but it sounds like it might be useful. I'll take a look. Thanks.

Out of interest, do you have any idea what is wrong with the code I posted in terms of extracting a dynamic array from the pointer returned by GetData? The code is in another branch of this thread.

I've just done a little research on CloneCursor in Cary Jensen's "Delphi in Depth: FireDAC". He says: " ... which means it can be called by any FDDataSet, not just FDMemTables. For the most part, however, you will not call CloneCursor with any FDDataSet other than FDMemTables. Trying to do so often leads to access violations and other issues. When I asked the author of FireDAC about these issues, he explained that CloneCursor was intended for use only from FDMemTable".

The documentation on CloneCursor does not mention that it is designed for use only with FDMemTable, bit I note the example provided with the documentation is in relation to FDMemTable.

Edited by: Mark Williams on Apr 5, 2018 3:30 AM

I'm kind of a long way down the road with FireDAC to abandon it. How do you open a second cursor without using cloneCursor?

I misunderstood. I thought you were advising against the user of FireDAC!

The dataset involved can be rather large. Opening it twice may be a bit overkill. But thanks for the suggestion. The CloneCursor procedure gives you a second cursor without the overhead, but it seems not to be advisable for my specific requirements.

It's a TFDQuery. There is nothing in the documentation to suggest that CloneCursor can't be used with a TFDQuery component, but a recent book I have on the subject suggests otherwise (see above).

Thanks for the suggestion. That's certainly one way to go. There is a way of getting to it behind the scenes in FireDAC, however, I am having problems working with the raw data returned (see the sub-thread in this thread). I have reposted the problem on the FireDAC group and hopefully I can get an answer that way. If not, I will probably implement your suggestion. Thanks again.

Many thanks for the help.

I didn't really have any code at the time of posting. My knowledge of pointers is limited (very) and I was getting nowhere fast. Your suggested code has helped me to take things further.

I have array data stored in a test table. I can save to and retrieve successfully from this using TBlobField and streams.

I have tried to retrieve the array using GetData:

  with FDQuery1 do begin
          setLength(MyIntArray, Integer(Table.Rows[table.Rows.Count-1].GetData(2)));  //this is the database column which stores the number of items in the array         
          Len2 := Length(MyIntArray) * sizeOf(Integer);
          if Table.Rows[table.Rows.Count-1].GetData(1, rvDefault, Pointer(MyIntArray), Len, Len2, true) then begin
             for i := 0 to  High(MyIntArray) do
                s := s + MyIntArray[i].ToString + ', ';
          Showmessage(s);          

It seems you are correct that AByVal needs to be set to true. As for the ABuffLen and ADataLen params I am still unsure what they do. However, I have tried setting ABuffLen to the value of Len2. I have also tried setting it to SizeOf(Integer) with ADataLen set to the length of the array and so on with every possible combination I can think of! Neither really seem to affect the outcome, but that may be because of the way I am trying to handle the array after. The result of my showmessage is always something like "122060280, 0, 0, 0, 0,..." (with 0s for the remaining items).

Can you see anything obviously wrong in the above code?

Many thanks