Author: Tomas Rutkauskas
How can I create a generic procedure that will load data from a table into a
TListBox?
Answer:
Actually, this tip pertains to any control that has a property such as an Items
property of type TStrings or is a descendant of TStrings. For example, the
following controls have properties of type TStrings: TComboBox, TMemo, TQuery (SQL
Property), TDBRadioGroup, TDirectoryListBox, TDriveComboBox, TFileListBox,
TFilterComboBox, TListBox, TRadioGroup.
I should say that TStrings is actually an abstract class, and though you will see
the help file list properties such as Items as TStrings, they're actually
descendants of TStrings. More commonly, you will see something like the TStringList
being used almost interchangeably (at least in concept) with TStrings. Essentially,
a TStrings object is a list of strings, with array-like properties. The advantage
of a TStrings object over an array is that you don't have to worry about allocating
and deallocating memory to add and subtract items (which is why they're ideal when
working with a loose collection of strings).
To load data from a column in a table into something like a TListBox is easy. All
it takes is stepping through the table, picking out the values you want and adding
them to the TStrings object - all in one line of code.
1
2 {Loads a list using values from a specific field from an open table}
3
4 procedure DBLoadList(srcTbl: TTable; srcFld: string; const lst: TStrings);
5 {srcTbl: Source Table, srcFld: Source Field, lst: Target List}
6 begin
7 with srcTbl do
8 if (RecordCount > 0) then
9 begin
10 {Don't bother if there are no records}
11 lst.Clear;
12 while not EOF do
13 begin
14 lst.Add(FieldByName(srcFld).AsString);
15 next;
16 end;
17 end;
18 end;
The code above assumes you have an open TTable whose values you can access. The
real workhorse of the procedure is the while loop which steps through the table and
grabs the value from the specified field and assigns it to a new entry into the
TStrings object.
Operations such as the one above can be done any of those objects because the properties descend from a common ancestor.
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