Title: Reading not required dataset fields
Question: how to avoid the "invalid Variant conversion" Exception
when reading a NULL Field
Answer:
function GetFieldValueDefault(ds:TDataSet; const FieldName:string; default:Variant):Variant;
var
f : TField;
begin
f := ds.FieldByName(FieldName);
if not f.IsNull then
Result := f.Value
else
Result := default;
end;
procedure Example;
var
s : string;
i : integer;
begin
// not safe, because a Variant conversion Exception could be trown
s := table1.FieldValues['Field1'];
i := table1.FieldValues['Field2'];
// safe
s := GetFieldValueDefault(table1, 'Field1', '');
i := GetFieldValueDefault(table1, 'Field2', 0);
// you could provide a default value if the field is NULL
s := GetFieldValueDefault(table1, 'Field1', '(unassigned)');
i := GetFieldValueDefault(table1, 'Field2', -1);
end;
// If you are not shure, if the field exists at all
// use this function
function GetFieldValueDefault2(ds:TDataSet; const FieldName:string; default:Variant):Variant;
var
f : TField;
begin
f := ds.FindField(FieldName);
if Assigned(f) and not f.IsNull then
Result := f.Value
else
Result := default
end;