Change a record property with a variable/dynamic name - delphi

Question

I have a simple Delphi record:
type
TCustomer = record
name : string[30];
age : byte;
end;
And I know I can set this record's field by hard coding the field name in code:
var
customer : TCustomer;
begin
// Set up our customer record
customer.name := 'Fred Bloggs';
customer.age := 23;
end;
But I have a single TEdit, a single TComboBox next to it, and a single TButton. The combobox is fixed and has two items, "Name" and "Age". It will first be set to "Name". User types their name value into the edit box. The is a Save type of button that would have an OnClick event like:
procedure TMainForm.SaveButtonClick(Sender: TObject);
begin
if(MyComboBox.Text = 'Name') then
begin
customer.name := MyEditBox.Text;
end
else
begin
customer.age := MyEditBox.Text;
end;
end;
The record was initialized someplace else. What I'm getting at here is in my cases there are 101 possible combobox items. Should I make a massive case statement to handle this or can I consolidate the code by matching the reorder's field name to a dynamic piece of information being set by another control (in this case a combobox)?

Answer 1

If you have a Delphi Version that has enhanced RTTI (Delphi 2010 and higher) you can do it.
However there are a few traps:
1.Short strings have to defined as type for the compiler to create typeinfo for these fields (as I did with the String30)
2.TValue which is the type to carry things around in the enhanced RTTI does not automatic type conversion (like the string from the edit into the Integer for the Age field). That is why I took the way into a Variant and converted that into the correct type for the field (just for ShortString and Integer, the rest is left as an exercise to the reader).
3.TValue does not like conversions from different ShortString types (String30 is not the same as ShortString) that is why I used TValue.Make. What is missing there is a check if the provided value matches the type (like it it exceeds 30 chars). Also it of course is not unicode compatible.
uses
Rtti,
TypInfo;
type
String30 = string[30];
TCustomer = record
name: String30;
age: byte;
end;
var
c: TCustomer;
function CastFromVariant(ATypeInfo: PTypeInfo; const AValue: Variant): TValue;
var
asShort: ShortString;
begin
case ATypeInfo.Kind of
tkInteger: Result := TValue.From<Integer>(AValue);
tkString:
begin
asShort := AValue;
TValue.Make(#asShort, ATypeInfo, Result);
end;
end;
end;
procedure TForm5.Button1Click(Sender: TObject);
var
ctx: TRttiContext;
t: TRttiType;
f: TRttiField;
v: TValue;
begin
t := ctx.GetType(TypeInfo(TCustomer));
f := t.GetField(ComboBox1.Text);
v := CastFromVariant(f.FieldType.Handle, Edit1.Text);
f.SetValue(#c, v);
end;
procedure TForm5.ComboBox1Change(Sender: TObject);
var
ctx: TRttiContext;
t: TRttiType;
f: TRttiField;
v: TValue;
begin
t := ctx.GetType(TypeInfo(TCustomer));
f := t.GetField(ComboBox1.Text);
v := f.GetValue(#c);
Edit1.Text := v.ToString;
end;

Answer 2

The on-screen components are their own variables. They exist independently of your records.
You need to copy from one to the other as needed.
(Use my code as a guide -- it may not be exactly correct syntactically.)
// UI component declarations within the form
TForm1 = class(TForm)
. . .
cbo : TComboBox;
edt : TEdit;
. . .
end;
. . .
var
Form1 : TForm1;
. . .
// to copy values from customer to UI components:
cbo.ItemIndex := customer.age; // assuming this is what the combobox is used for,
// and it starts at zero
edt := customer.name;
// to copy from UI components into customer, you'll need to do it inside of one
// of the event handlers.
TForm1.cboCloseUp( sender : TObject ); // the onCloseUp handler, when the combo drop-down closes
begin
customer.age := cbo.ItemIndex; // or StrToInt(cbo.Text)
end;
TForm1.edtChange( sender : TObject ); // the onChange handler whenever the edit changes
begin
customer.name := edt.Text;
end;
That's the gist of it. If the ages in the combobox don't start at zero, then you'll want to adjust the offset accordingly when you copy bewteen the customer record and the combobox. You can either set ItemIndex+offset if the values are linear, or simply write a value into cbo.Text.