I was wondering if anyone has had a similar experience. I am trying to trace the source of a problem but am coming up with nil. I have a project in Delphi 5 which has Report Builder reports on it. I needed an upgraded version of reportbuilder so I tried running the project in Delphi 7. When my project runs and I click a button to view a report, it views fine. However, if I use a paramstr to run the report (showmainform is set false) and show report procedure runs, I get get a stack overflow error.
The original code was :
if lowercase(ParamStr(1)) = 'termsexceeded' then begin
reportsdata.termsexceeded.close;
reportsdata.termsexceeded.open;
reports.ppTermsExceeded.print;
reportsdata.termsexceeded.close;
application.terminate;
end;
And it gave me the stack overflow error on the .print function.
The code that works in Delphi 7 is :
if lowercase(ParamStr(1)) = 'termsexceeded' then begin
reportsdata.termsexceeded.close;
reportsdata.termsexceeded.open;
reports.left := -10000;
reports.show;
reports.ppTermsExceeded.print;
reportsdata.termsexceeded.close;
application.terminate;
end;
Has anybody got a suggestion on how I could debug this to see if the problem lies with my Delphi 7 or with Reportbuilder ?
There are no events on the .show event of the reports form.
Any advice on how to get to the bottom of this would be appreciated.
Regards
When you get a stack overflow, use the debugger. It will interrupt your program when the OS throws the exception, and at that time, you can use the debugger's call stack window to see the path a function calls that lead there. You'll probably see a certain function or sequence of functions repeated many times.
When you've found the repeating pattern, check the code to see why it's repeating. Look for a condition is supposed to have changed, but doesn't.
Related
A customer has reported a Range check error on a machine embedded in a factory running an older Delphi 7 application. We have not (yet) been able to reproduce the error. They sent us a photo:
The software can operate correctly for days on end, and we currently only have vague clues as to how this might be generated or reliably reproduced. My understanding is that this error occurs in two general scenarios:
(1) An array or string has been accessed outside its bounds
(2) The variable is assigned a value out-of-range for its type
An example of (1) is accessing array[50] if there are only 40 elements. An example of (2) is assigning the value "300" to an an unsigned BYTE.
This comes from SO question 1 and SO question 2. I've got lots of careful checking to do to try and identify the offending lines!
My question is how this error is generated in the first place. Both the above-mentioned questions refer to the {$R+} compiler directive. In Project Options > Compiler > Runtime errors, Range checking is off, and nowhere in the code is {$R+} (nor {$R-}) used. How does this error occur? Shouldn't the application crash or generate a different exception?
I'll answer the question:
Can a Range check error be generated without being specifically enabled?
However, the problem you're experiencing at your client's site will need further investigation for you to resolve it. Typically this kind of thing requires a combination of:
Exception logging: tools that produce a stack trace when exceptions occur to determine exactly what code was being called.
Tracing: logging messages that provide clues to the state of the application to assist in your investigation.
Yes, range check error can be generated without being specifically enabled.
Range check errors can be raised at any time with raise ERangeError.Create(...);. If called, this will raise the error regardless of the state of the range-check setting.
Note code can if so written, also honour the setting as follows:
{$IFOPT R+}
raise ERangeError.Create(...);
{$ENDIF}
But the point is that as soon as raise <SomeClass>.Create(...) is called, an exception will be raised. If you search the Delphi source code, you'll find a few places where ERangeError is raised. Loki's answer provides an example.
The second important thing to note is that the {$R} setting is not global. If you compile a project with this setting turn off, but link in DCUs which were compiled with the option turned on: then the setting will still be on for those DCUs.
Furthermore, the setting can be locally changed for specific sections of code. E.g.
{$IFOPT R-} {$R+} {$DEFINE TOGGLE_ROFF} {$ENDIF}
{ Ensure range-checking is on, but turn off again if it was already off.}
procedure MustUseRangeChecking(...);
begin
...
end;
{$IFDEF TOGGLE_ROFF} {$R-} {$ENDIF}
NOTE: You can use {$IFOPT} in your own code to check the state of the range-checking directive.
yes it's can be raise without having Range check error specifically enabled
exemple just look the TStream.SetSize function :
procedure TStream.SetSize(const NewSize: Int64);
begin
if (NewSize < Low(Integer)) or (NewSize > High(Integer)) then
raise ERangeError.CreateRes(#SRangeError);
SetSize(LongInt(NewSize));
end;
so it's will raise the exception with or without Range check error enabled. you have in delphi several function like this.
Solution found, see my comment below
D5, odbc to mysql database
This code:
with QryCmdPerf do begin
Close;
ParamByName('ACCTID').AsInteger:= AcctId;
ParamByName('FROMDT').AsString:= MySQLDate(FromDt);
ParamByName('TODT').AsString:= MySQLDate(ToDt);
Open;
first;
try
edit;
FieldByName('PnL').AsFloat:= 97979;
ApplyUpdates;
except
close;
end;
end; // with
(specifically the "ApplyUpdates") causes a popup to appear with the text "Update Failed" if the PnL field already has the value 97979, evidently because of this code:
procedure TUpdateSQL.ExecSQL(UpdateKind: TUpdateKind);
begin
with Query[UpdateKind] do
begin
Prepare;
ExecSQL;
if RowsAffected <> 1 then DatabaseError(SUpdateFailed);
end;
end;
in DBTables.pas. Anyway, I want to be able to issue ApplyUpdates, and not have to worry about a popup if it doesn't do any updating. But if "try...except" doesn't work, what will?
TIA
You're confusing the dialog displayed by the debugger with a dialog displayed by your program. Please see this article I wrote a few years ago:
Why do I continue getting error messages even after I have written an exception handler?
It describes several ways to avoid the debugger interfering:
Use "advanced breakpoints" to temporarily disable the debugger around the code that throws exceptions.
Configure the debugger to ignore certain exception types. (Read the debugger's message more carefully to see exactly what exception class you're dealing with.)
Configure the debugger not to interrupt on any exceptions.
Turn off integrated debugger entirely.
The short answer is, you have to set up an eventhandler for OnUpdateError or no amount of "try...except" blocks will block the popup. The long answer is it appears to be a bug with odbc. The repro is here: http://www.codeupload.com/3919 for anyone who wants to take a look at it. You can skip the MySQL stuff, any odbc database will do.
There are two things that can be going wrong here.
Option 1
First, some "very bad code" may be short-circuiting the unwinding of the call stack on exceptions. E.g. ApplyUpdates or one of its child routines may also have a try...except block that calls Application.HandleException directly.
To test this, if you put a breakpoint on QryCmdPerf.Close, do you reach it?
If not, then Application.HandleException (or worse Application.ShowException) has been called directly.
Solving this requires a custom exception handler hooked to the Application.OnException event. You may have to set temporary state to know when this particular exception can be ignored.
Yes it's messy, that why calling Application.HandleException directly, is "very bad code".
Option 2
If you do reach the breakpoint, but the exception is being raised again, then it should be a lot simpler to solve.
The Close method is probably attempting to save any pending changes, so is effectively applying the updates again. Rather than simply closing the data set, call CancelChanges or equivalent.
Background
We have a problem that sometimes the grid in Devexpress raise exception
"Raised EConvertError: Cannot assign a nil to a TFont".
But to trace the real cause of this we have changed Font in cxEdit to be a public property instead with a get and set method. Here I want to log the callstack.
My question
Normally JCL is used to log callstacks when exceptions appear. But how can I log the callstack without using raise exception and show a dialog for the user about this ?
I have found the lines:
var
GlobalStackList: TJclGlobalStackList;
in JclDebug but I fail to use it. If someone have a example how to get the callstack I would be happy.
Regards Roland Bengtsson
This answer shows how to do it with JCL by calling JclCreateStackList.
I am using the following code to copy text to the clipboard:
Clipboard.Open;
try
Clipboard.AsText := GenerateClipboardText;
finally
Clipboard.Close;
end;
Seemingly at random I get "Cannot open clipboard: Access Denied" errors. I'm guessing that these errors are caused by other application locking the clipboard, but I never seem to be doing anything with other applications that should cause the locks.
Strangely my users seem to be reporting more of the errors with Vista and Windows 7 than with XP.
Is there a way to check if the clipboard is locked before trying to access it?
This is not a Delphi problem. Because the clipboard can be locked any moment, even if you check, if the clipboard is currently not locked, it might become locked directly after the check.
You have two possibilities here:
Don't use the Delphi clipboard class. Instead use raw API functions, where you have a little more fine-grained control over possible error situations.
Expect your code to fail by adding an exception handler. Then add some retry code, i.e. retry to set the text three times, perhaps with exponential backoff, before throwing your own error.
I'd recommend the second solution, because it'd be the more Delphi-like approach and in the end will result in cleaner code.
var
Success : boolean;
RetryCount : integer;
begin
RetryCount := 0;
Success := false;
while not Success do
try
//
// Set the clipboard here
//
Success := True;
except
on E: EClipboardException do
begin
Inc(RetryCount);
if RetryCount < 3 then
Sleep(RetryCount * 100)
else
raise Exception.Create('Cannot set clipboard after three attempts');
end else
raise; // if not a clipboard problem then re-raise
end;
end;
Strangely my users seem to be
reporting more of the errors with
Vista and Windows 7 than with XP
This may have to do with how Vista/Win7 deal with clipboard viewer notification. While they still support the XP "clipboard viewer chain", which sends one notification message that must be re-sent to each listener in turn (and if one app fails to do this, the other apps aren't notified). Starting with Vista, apps are notified directly. And there's nothing to keep them from trying to access the clipboard all at once.
Analogy: I have 3 children. I have a cake. With XP rules, I tell the oldest child to have some cake, then tell the next oldest child to have a slice. She gets her slice, tells her brother, he gets his, and tells his brother, who gets his, and everything proceeds in an orderly fashion.
Problem: The middle child takes the cake to his room, doesn't tell the youngest, and the youngest misses out.
With Vista/Windows7, that system still exists. But newer apps can request to be notified immediately, by me, as soon as the cake arrives in the kitchen. I shout "cake is ready!" and they all show up at the same time and try to grab some. But there's only one serving knife, so they have to keep reaching for the knife, failing to get it, and waiting for the next opportunity.
Try to check GetClipboardOwner, if it's not null and not your Application.Handle, you cannot Open to modify it's content.
And even it seems good to go, it might not be anymore when you actually do it.
So add a try except in a loop until you get it or give up nicely (notifying the user for instance).
There is no way to check for something and then depending on the result do something else with the expectation that it could not fail, because unless the check and the action are one atomic operation there is always the possibility that another process or thread does the same in parallel.
This holds whether you try to open the clipboard, open a file, create or delete a directory - you should simply try to do it, maybe several times in a loop, and gracefully handle errors.
First of all please notice that this probably isn't a problem in your application. Other applications locked the clipboard or messed up the notification chain and now your application fails to access it. When I do have problems like this I restart the computer and they magically go away... well... at least until I run again the application that creates the problem.
This code (not checked in Delphi) may help you. It won't fix the problem is the notification chain is broken (nothing except a PC restart will ever fix it) but it will fix the problem if an application is locking the clipboard for a while. Increase the MaxRetries if that pesky application keeps the clipboard locked for A REALLY LONG time (seconds):
procedure Str2Clipboard(CONST Str: string; iDelayMs: integer);
CONST
MaxRetries= 5;
VAR RetryCount: Integer;
begin
RetryCount:= 0;
for RetryCount:= 1 to MaxRetries DO
TRY
inc(RetryCount);
Clipboard.AsText:= Str;
Break;
EXCEPT
on Exception DO
if RetryCount = MaxRetries
then RAISE Exception.Create('Cannot set clipboard')
else Sleep(iDelayMs)
END;
end;
Also, it may be a good idea to drop the 'raise' and convert it to a function and use it like this:
if not Str2Clipboard
then Log.AddMsg('Dear user, other applications are blocking the clipboard. We have tried. We really did. But it didn''t work. Try again in a few seconds.');
I guess you are running your app on Win 8 or higher.
Just right-click on your App .exe file, go to Compatibility tab and change compatibility mode on Windows XP or lower versions. It'll work, guaranteed!
I'm a member in a team that use Delphi 2007 for a larger application and we suspect heap corruption because sometimes there are strange bugs that have no other explanation.
I believe that the Rangechecking option for the compiler is only for arrays. I want a tool that give an exception or log when there is a write on a memory address that is not allocated by the application.
Regards
EDIT: The error is of type:
Error: Access violation at address 00404E78 in module 'BoatLogisticsAMCAttracsServer.exe'. Read of address FFFFFFDD
EDIT2: Thanks for all suggestions. Unfortunately I think that the solution is deeper than that. We use a patched version of Bold for Delphi as we own the source. Probably there are some errors introduced in the Bold framwork. Yes we have a log with callstacks that are handled by JCL and also trace messages. So a callstack with the exception can lock like this:
20091210 16:02:29 (2356) [EXCEPTION] Raised EBold: Failed to derive ServerSession.mayDropSession: Boolean
OCL expression: not active and not idle and timeout and (ApplicationKernel.allinstances->first.CurrentSession <> self)
Error: Access violation at address 00404E78 in module 'BoatLogisticsAMCAttracsServer.exe'. Read of address FFFFFFDD. At Location BoldSystem.TBoldMember.CalculateDerivedMemberWithExpression (BoldSystem.pas:4016)
Inner Exception Raised EBold: Failed to derive ServerSession.mayDropSession: Boolean
OCL expression: not active and not idle and timeout and (ApplicationKernel.allinstances->first.CurrentSession <> self)
Error: Access violation at address 00404E78 in module 'BoatLogisticsAMCAttracsServer.exe'. Read of address FFFFFFDD. At Location BoldSystem.TBoldMember.CalculateDerivedMemberWithExpression (BoldSystem.pas:4016)
Inner Exception Call Stack:
[00] System.TObject.InheritsFrom (sys\system.pas:9237)
Call Stack:
[00] BoldSystem.TBoldMember.CalculateDerivedMemberWithExpression (BoldSystem.pas:4016)
[01] BoldSystem.TBoldMember.DeriveMember (BoldSystem.pas:3846)
[02] BoldSystem.TBoldMemberDeriver.DoDeriveAndSubscribe (BoldSystem.pas:7491)
[03] BoldDeriver.TBoldAbstractDeriver.DeriveAndSubscribe (BoldDeriver.pas:180)
[04] BoldDeriver.TBoldAbstractDeriver.SetDeriverState (BoldDeriver.pas:262)
[05] BoldDeriver.TBoldAbstractDeriver.Derive (BoldDeriver.pas:117)
[06] BoldDeriver.TBoldAbstractDeriver.EnsureCurrent (BoldDeriver.pas:196)
[07] BoldSystem.TBoldMember.EnsureContentsCurrent (BoldSystem.pas:4245)
[08] BoldSystem.TBoldAttribute.EnsureNotNull (BoldSystem.pas:4813)
[09] BoldAttributes.TBABoolean.GetAsBoolean (BoldAttributes.pas:3069)
[10] BusinessClasses.TLogonSession._GetMayDropSession (code\BusinessClasses.pas:31854)
[11] DMAttracsTimers.TAttracsTimerDataModule.RemoveDanglingLogonSessions (code\DMAttracsTimers.pas:237)
[12] DMAttracsTimers.TAttracsTimerDataModule.UpdateServerTimeOnTimerTrig (code\DMAttracsTimers.pas:482)
[13] DMAttracsTimers.TAttracsTimerDataModule.TimerKernelWork (code\DMAttracsTimers.pas:551)
[14] DMAttracsTimers.TAttracsTimerDataModule.AttracsTimerTimer (code\DMAttracsTimers.pas:600)
[15] ExtCtrls.TTimer.Timer (ExtCtrls.pas:2281)
[16] Classes.StdWndProc (common\Classes.pas:11583)
The inner exception part is the callstack at the moment an exception is reraised.
EDIT3: The theory right now is that the Virtual Memory Table (VMT) is somehow broken. When this happen there is no indication of it. Only when a method is called an exception is raised (ALWAYS on address FFFFFFDD, -35 decimal) but then it is too late. You don't know the real cause for the error. Any hint of how to catch a bug like this is really appreciated!!! We have tried with SafeMM, but the problem is that the memory consumption is too high even when the 3 GB flag is used. So now I try to give a bounty to the SO community :)
EDIT4: One hint is that according the log there is often (or even always) another exception before this. It can be for example optimistic locking in the database. We have tried to raise exceptions by force but in test environment it just works fine.
EDIT5: Story continues... I did a search on the logs for the last 30 days now. The result:
"Read of address FFFFFFDB" 0
"Read of address FFFFFFDC" 24
"Read of address FFFFFFDD" 270
"Read of address FFFFFFDE" 22
"Read of address FFFFFFDF" 7
"Read of address FFFFFFE0" 20
"Read of address FFFFFFE1" 0
So the current theory is that an enum (there is a lots in Bold) overwrite a pointer. I got 5 hits with different address above. It could mean that the enum holds 5 values where the second one is most used. If there is an exception a rollback should occur for the database and Boldobjects should be destroyed. Maybe there is a chance that not everything is destroyed and a enum still can write to an address location. If this is true maybe it is possible to search the code by a regexpr for an enum with 5 values ?
EDIT6: To summarize, no there is no solution to the problem yet. I realize that I may mislead you a bit with the callstack. Yes there are a timer in that but there are other callstacks without a timer. Sorry for that. But there are 2 common factors.
An exception with Read of address FFFFFFxx.
Top of callstack is System.TObject.InheritsFrom (sys\system.pas:9237)
This convince me that VilleK best describe the problem.
I'm also convinced that the problem is somewhere in the Bold framework.
But the BIG question is, how can problems like this be solved ?
It is not enough to have an Assert like VilleK suggest as the damage has already happened and the callstack is gone at that moment. So to describe my view of what may cause the error:
Somewhere a pointer is assigned a bad value 1, but it can be also 0, 2, 3 etc.
An object is assigned to that pointer.
There is method call in the objects baseclass. This cause method TObject.InheritsForm to be called and an exception appear on address FFFFFFDD.
Those 3 events can be together in the code but they may also be used much later. I think this is true for the last method call.
EDIT7: We work closely with the the author of Bold Jan Norden and he recently found a bug in the OCL-evaluator in Bold framework. When this was fixed these kinds of exceptions decreased a lot but they still occasionally come. But it is a big relief that this is almost solved.
You write that you want there to be an exception if
there is a write on a memory address that is not allocated by the application
but that happens anyway, both the hardware and the OS make sure of that.
If you mean you want to check for invalid memory writes in your application's allocated address range, then there is only so much you can do. You should use FastMM4, and use it with its most verbose and paranoid settings in debug mode of your application. This will catch a lot of invalid writes, accesses to already released memory and such, but it can't catch everything. Consider a dangling pointer that points to another writeable memory location (like the middle of a large string or array of float values) - writing to it will succeed, and it will trash other data, but there's no way for the memory manager to catch such access.
I don't have a solution but there are some clues about that particular error message.
System.TObject.InheritsFrom subtracts the constant vmtParent from the Self-pointer (the class) to get pointer to the adress of the parent class.
In Delphi 2007 vmtParent is defined:
vmtParent = -36;
So the error $FFFFFFDD (-35) sounds like the class pointer is 1 in this case.
Here is a test case to reproduce it:
procedure TForm1.FormCreate(Sender: TObject);
var
I : integer;
O : tobject;
begin
I := 1;
O := #I;
O.InheritsFrom(TObject);
end;
I've tried it in Delphi 2010 and get 'Read of address FFFFFFD1' because the vmtParent is different between Delphi versions.
The problem is that this happens deep inside the Bold framework so you may have trouble guarding against it in your application code.
You can try this on your objects that are used in the DMAttracsTimers-code (which I assume is your application code):
Assert(Integer(Obj.ClassType)<>1,'Corrupt vmt');
It sounds like you have memory corruption of object instance data.
The VMT itself isn't getting corrupted, FWIW: the VMT is (normally) stored in the executable and the pages that map to it are read-only. Rather, as VilleK says, it looks like the first field of the instance data in your case got overwritten with a 32-bit integer with value 1. This is easy enough to verify: check the instance data of the object whose method call failed, and verify that the first dword is 00000001.
If it is indeed the VMT pointer in the instance data that is being corrupted, here's how I'd find the code that corrupts it:
Make sure there is an automated way to reproduce the issue that doesn't require user input. The issue may be only reproducible on a single machine without reboots between reproductions owing to how Windows may choose to lay out memory.
Reproduce the issue and note the address of the instance data whose memory is corrupted.
Rerun and check the second reproduction: make sure that the address of the instance data that was corrupted in the second run is the same as the address from the first run.
Now, step into a third run, put a 4-byte data breakpoint on the section of memory indicated by the previous two runs. The point is to break on every modification to this memory. At least one break should be the TObject.InitInstance call which fills in the VMT pointer; there may be others related to instance construction, such as in the memory allocator; and in the worst case, the relevant instance data may have been recycled memory from previous instances. To cut down on the amount of stepping needed, make the data breakpoint log the call stack, but not actually break. By checking the call stacks after the virtual call fails, you should be able to find the bad write.
mghie is right of course. (fastmm4 calls the flag fulldebugmode or something like that).
Note that that works usually with barriers just before and after an heap allocation that are regularly checked (on every heapmgr access?).
This has two consequences:
the place where fastmm detects the error might deviate from the spot where it happens
a total random write (not overflow of existing allocation) might not be detected.
So here are some other things to think about:
enable runtime checking
review all your compiler's warnings.
Try to compile with a different delphi version or FPC. Other compilers/rtls/heapmanagers have different layouts, and that could lead to the error being caught easier.
If that all yields nothing, try to simplify the application till it goes away. Then investigate the most recent commented/ifdefed parts.
The first thing I would do is add MadExcept to your application and get a stack traceback that prints out the exact calling tree, which will give you some idea what is going on here. Instead of a random exception and a binary/hex memory address, you need to see a calling tree, with the values of all parameters and local variables from the stack.
If I suspect memory corruption in a structure that is key to my application, I will often write extra code to make tracking this bug possible.
For example, in memory structures (class or record types) can be arranged to have a Magic1:Word at the beginning and a Magic2:Word at the end of each record in memory. An integrity check function can check the integrity of those structures by looking to see for each record Magic1 and Magic2 have not been changed from what they were set to in the constructor. The Destructor would change Magic1 and Magic2 to other values such as $FFFF.
I also would consider adding trace-logging to my application. Trace logging in delphi applications often starts with me declaring a TraceForm form, with a TMemo on there, and the TraceForm.Trace(msg:String) function starts out as "Memo1.Lines.Add(msg)". As my application matures, the trace logging facilities are the way I watch running applications for overall patterns in their behaviour, and misbehaviour. Then, when a "random" crash or memory corruption with "no explanation" happens, I have a trace log to go back through and see what has lead to this particular case.
Sometimes it is not memory corruption but simple basic errors (I forgot to check if X is assigned, then I go dereference it: X.DoSomething(...) that assumes X is assigned, but it isn't.
I Noticed that a timer is in the stack trace.
I have seen a lot of strange errors where the cause was the timer event is fired after the form i free'ed.
The reason is that a timer event cound be put on the message que, and noge get processed brfor the destruction of other components.
One way around that problem is disabling the timer as the first entry in the destroy of the form. After disabling the time call Application.processMessages, so any timer events is processed before destroying the components.
Another way is checking if the form is destroying in the timerevent. (csDestroying in componentstate).
Can you post the sourcecode of this procedure?
BoldSystem.TBoldMember.CalculateDerivedMemberWithExpression
(BoldSystem.pas:4016)
So we can see what's happening on line 4016.
And also the CPU view of this function?
(just set a breakpoint on line 4016 of this procedure and run. And copy+paste the CPU view contents if you hit the breakpoint). So we can see which CPU instruction is at address 00404E78.
Could there be a problem with re-entrant code?
Try putting some guard code around the TTimer event handler code:
procedure TAttracsTimerDataModule.AttracsTimerTimer(ASender: TObject);
begin
if FInTimer then
begin
// Let us know there is a problem or log it to a file, or something.
// Even throw an exception
OutputDebugString('Timer called re-entrantly!');
Exit; //======>
end;
FInTimer := True;
try
// method contents
finally
FInTimer := False;
end;
end;
N#
I think there is another possibility: the timer is fired to check if there are "Dangling Logon Sessions". Then, a call is done on a TLogonSession object to check if it may be dropped (_GetMayDropSession), right? But what if the object is destroyed already? Maybe due to thread safety issues or just a .Free call and not a FreeAndNil call (so a variable is still <> nil) etc etc. In the mean time, other objects are created so the memory gets reused. If you try to acces the variable some time later, you can/will get random errors...
An example:
procedure TForm11.Button1Click(Sender: TObject);
var
c: TComponent;
i: Integer;
p: pointer;
begin
//create
c := TComponent.Create(nil);
//get size and memory
i := c.InstanceSize;
p := Pointer(c);
//destroy component
c.Free;
//this call will succeed, object is gone, but memory still "valid"
c.InheritsFrom(TObject);
//overwrite memory
FillChar(p, i, 1);
//CRASH!
c.InheritsFrom(TObject);
end;
Access violation at address 004619D9 in module 'Project10.exe'. Read of address 01010101.
Isn't the problem that "_GetMayDropSession" is referencing a freed session variable?
I have seen this kind of errors before, in TMS where objects were freed and referenced in an onchange etc (only in some situations it gave errors, very difficult/impossible to reproduce, is fixed now by TMS :-) ). Also with RemObjects sessions I got something similar (due to bad programming bug by myself).
I would try to add a dummy variable to the session class and check for it's value:
public variable iMagicNumber: integer;
constructor create: iMagicNumber := 1234567;
destructor destroy: iMagicNumber := -1;
"other procedures": assert(iMagicNumber = 1234567)