[Lazarus] TDBEdit, TStringField Size, DataSize, DisplayWidth and MaxLength
tony.whyman at mccallumwhyman.com
Tue Oct 11 14:46:06 CEST 2016
An IBX user came to me with a problem and the problem seems to be a deep
seated disconnect between multi-byte character sets, TStringField.Size
and TDBEdit.MaxLength. Something needs to give - but I am not sure what
If you go back to Delphi, TField.DataSize is the memory needed to hold
the Field's value. The DisplayWidth is the number of characters to be
displayed, and Size is, for datatype ftstring, "the maximum number of
characters in the string". How literally this last definition should be
taken, I'm not sure, as it may well have been written assuming a single
byte character set.
On the other hand, the FPC documentation is consistent with Delphi for
DisplayWidth and DataSize, but more opaque for TField.Size where it is
the "logical size" - whatever that means, although TStringField is more
definitive by saying it is the maximum size (in characters) - their
brackets not mine.
That seems to be consistent with TDBEdit.Maxlength which should be the
maximum number of characters that can appear in the control and, if you
look at the code, TDBEdit will source the default value from
FDatalink.Size (And also seems to ignore DisplayWidth).
The problem comes when you look at the code for TStringField.GetValue,
where it starts off as:
function TStringField.GetValue(var AValue: string): Boolean;
var Buf, TBuf : TStringFieldBuffer;
DynBuf, TDynBuf : Array of char;
if DataSize <= dsMaxStringSize then
Buf[Size]:=#0; //limit string to Size
If Result then
If nothing else, this is a "bug in waiting". TStringField.GetDataSize
always returns "Size+1", so "Buff[Size]:=#0; should work - but only as
long as the virtual method "GetDataSize" is not overridden (GetValue is
non-virtual) and Size is the byte length of the string!
There is a built-in assumption here that "Size" is the byte length of
the string and not the character length. If you have a multi-byte
character set and set size to the number of characters and DataSize to
e.g. for UTF8 4*(no of characters)+1, then you will get string
corruption as a result of the above.
IBX handles multi-byte character sets and does so by defining
TIBStringField as a subclass of TStringFIeld and setting size to the
byte length and the Default DisplayWidth to the character width. This is
compatible with TStringField as it works today. It also seems to be
compatible with TDBGrid, which uses Field.DisplayWidth. However, it does
result in TDBEdit accepting too many characters.
What should be done?
It's a problem. Ideally, the TStringField code should be aligned with
the documentation. However, that could break existing code and would
need to handled carefully. TStringFIeld also needs fixing i.e. to
Buf[DataSize-1]:=#0 in order to make this a reality.
Alternatively, the documentation could be amended to reflect the
implementation. This means that TDBEdit (and maybe more) have to be
updated - but why doesn't TDBEdit respect the DisplayWidth property anyway?
Perhaps, it is also about time that TStringField got a characterWidth
property to hold the maximum number of bytes for each character. That
would at least allow the DataSize to be automatically computed from the
If I had to write a bug report today, I would write it to avoid changes
to IBX - but then is that the right answer?
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