How to modify TList value?

Question

Delphi 2010 How to modify TList < record > value ?

type TTest = record a,b,c:Integer end;
var List:TList;
    A:TTest;
    P:Pointer;
....
..         


        

Answer 1

You've hit upon a snag with using records.

Consider this code:

function Test: TTest;
begin
    ...
end;

Test.a := 1;

What your code looks like to the compiler is actually this:

TTest temp := Test;
temp.a := 1;

The compiler is telling you, with the error message, that the assignment is pointless, since it will only assign a new value to a temporary record value, which will be instantly forgotten.

Also, the @List[10] is invalid because List[10] again returns only a temporary record value, so taking the address of that record is rather pointless.

However, reading and writing the whole record is OK.

So to summarize:

List[10] := A;  <- writing a whole record is OK
List[10].a:=1;  <- List[10] returns a temporary record, pointless assignment
P:=@List[10];   <- List[10] returns a temporary record, its address is pointless