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

StackOverflow https://stackoverflow.com/questions/3086623

Question

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!

Was it helpful?

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.

OTHER TIPS

Not sure in Delphi 5 but you should be able to put a Data Breakpoint (or an Address Breakpoint) on your AExample.FOnChange.
It would break whenever the value changes.

Since you can't use the smart solution suggested by François, it's time to get hacking! Somewhere, anywhere you can put a brakepoint, write code like this:

var X:TExample;
X.OnChange := nil;

Put a brakepoint on the X.OnChange := nil line; When the debugger stops there, take a look at the disassembly pane, you'll see something like this:

; assembler blah blah to get the address of X
xor EDX ; or whatever the compiler finds appropriate this time of day
mov [eax + $00000288], edx
mov [eax + $0000028c], edx

You don't care about the registers used by the compiler, you care about the $288, the offset used for the first MOV instruction. That's the OFFSET from the "X" address to the FOnChange field. Note it down. Now go back to your buggy program, set a brakepoint somewhere, hit Alt+F5 to invoke the Debug Inspector (followed by Ctrl+N if you're not presented with an empty edit box to type your query in) and write "Integer(MyExampleVariable)"; Whatever you get, add up the number you noted down at the previous step and you've got the address of the FOnChange filed for the MyExampleVariable instance, you can now set a address brakepoint.

I don't know if they exists in Delphi 5, but the type TMethod and the function MethodAddress should be helpful.

Why?

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

I can answer that bit - you are! Or you are looking at the wrong instance that is not created correctly? not referenced correctly? It will be something like that. The Delphi debugger is good at making some thing that does not exist look like it does, until...

Step back a bit, try and see the wood not the trees. I have been where you are many times, you are probably barking up the wrong tree (sorry pun intended) and shortly will call yourself all sorts of names :)

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