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Delphi: How to get the address of an event variable?

How do i get the address of the variable holding an event handler?

e.g.

TExample = class(TObject)
private
    FOnChange: TNotifyEvent;
end;

i want the address of the FOnChange private member, event handler, variable.

Why?

i'm trying to figure out who is overwriting my FOnChange handler variable with junk.

i am stepping through code:

if Assigned(FOnChange) then
    FOnChange(Self);

No event handler is ever assigned, and for a while the FOnChange variable is nil in the Watch Window:

@FOnChange: nil
Addr(FOnChange): nil

But later the FOnChange variable is getting turned into junk:

@FOnChange: $2C
Addr(FOnChange): $2C

So i want to watch the FOnChange variable in data pane of the CPU window, so that i can watch it to from:

00410018 00000000

to

00410018 0000002C

Except i don't know the address of FOnChange; i just made up the $410018.

How can i find the address of an event variable?

Things i've tried

Watch List 

OnChange: nil
@OnChange: nil
@@OnChange: Variable required
@FOnChange: nil
Assigned(OnChange): False
Assigned(FOnChange): False
@@FOnChange: $253B588
addr(addr(FOnChange)): $253B588

Alt+F5

  • OnChange: OnChange: TNotifyEvent $253B588
  • FOnChange: Error inspecting 'FOnChange': expression error
  • Self.FOnChange: Error inspecting 'Self.FOnChange': expression error
  • @OnChange: @OnChange: Pointer $253B588
  • @@OnChange: Error inspecting '@@OnChange': expression error
  • @FOnChange: @FOnChange: Pointer $253B588
  • @@FOnChange: @@FOnChange: ^Untyped (no address) Data: @@FOnChange $253B588`

The concensus seems to be at address 0x253B588.

Yet when i run some sample code:

MyControl1.OnChange := TheOnChangeHandler;

That turns into:

mov edx,[ebp+$08]         ;move stack variable $08 into edx
mov [eax+$00000208],edx   ;and then into offset $208 of my control

mov edx,[ebp+$0c]         ;move stack variable $0c into edx
mov [eax+$0000020c],edx   ;and then into offset $20c of my control

No wonder i can't find an address of FOnChange, it's two addresses!

Solution

  • You can get the address through the Debug Inspector. To get the address of a field, put a breakpoint in your code at some point before the change has happened, for example right after you call the constructor. Then open your object in the Debug Inspector. Not sure how you get it in the old IDE style, but in D2010 you can get this from the Run->Inspect... menu command, from a button in Evaluate/Modify, or by hitting ALT-F5 on the keyboard. (Be careful you don't hit ALT-F4!)

    The Debug Inspector will show you your object with all its fields. Double-click on one of the fields and it will open in a new Debug Inspector window. In the edit box-like bar at the top will be the address of your field. You can use this to set a memory breakpoint to find where the value changes.