Why does if Assigned(ListViewLayers1.Items) then Produce an Exception

Question

{ Clear ListViewLayers }
  try
    if Assigned(ListViewLayers1.Items) then
      ListViewLayers1.Clear;
  except
    on E: Exception do
    begin
      ShowMessage(E.ClassName + #10#13 + E.Message);
    end;
  end;

How can you determine if the listview is empty before trying to call Clear?

The Exception is EAccessViolation. But I do not understand why when the ListViewLayers1 was created at design time and can be accessed at other times throughout the life of the app.

EurekaLog 7.0.7.1  

Exception:
--------------------------------------------------------------------------------------------------------------
  2.2 Address: 00D2E79E
  2.5 Type   : EAccessViolation
  2.6 Message: Access violation at address 00D2E79E in module 'FaceBookDesigner.exe'. Read of address 00000000
  2.7 ID     : 20970000
  2.11 Sent  : 0

User:
-------------------------
  3.2 Name : Windows User
  3.3 Email: 

Steps to reproduce:
------------
  8.1 Text: 


Call Stack Information:
-------------------------------------------------------------------------------------------------------------------------------------------------------------
|Methods |Details|Stack   |Address |Module              |Offset  |Unit            |Class              |Procedure/Method                          |Line      |
-------------------------------------------------------------------------------------------------------------------------------------------------------------
|*Exception Thread: ID=10000; Parent=0; Priority=0                                                                                                          |
|Class=; Name=MAIN                                                                                                                                          |
|DeadLock=0; Wait Chain=                                                                                                                                    |
|Comment=                                                                                                                                                   |
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Processes Information:
----------------------


Assembler Information:
---------------------------------------------------------------------------------------------------------------------------
; Base Address: $D2E000, Allocation Base: $400000, Region Size: 94208
; Allocation Protect: PAGE_EXECUTE_WRITECOPY, Protect: PAGE_EXECUTE_READ
; State: MEM_COMMIT, Type: MEM_IMAGE
; 
;
; Unit1.TForm1.UpdateGUI (Line=1803 - Offset=8)
; ---------------------------------------------
00D2E738  8B45FC        MOV  EAX, [EBP-4]
00D2E73B  8B80A0030000  MOV  EAX, [EAX+$000003A0]
00D2E741  8B90D0060000  MOV  EDX, [EAX+$000006D0]
00D2E747  8B45FC        MOV  EAX, [EBP-4]
00D2E74A  8B80A0030000  MOV  EAX, [EAX+$000003A0]
00D2E750  E86FBEE6FF    CALL -$194191                   ; ($00B9A5C4) imageenview.TImageEnView.GetLayer
00D2E755  8B8080000000  MOV  EAX, [EAX+$00000080]
00D2E75B  BA20EAD200    MOV  EDX, $00D2EA20             ; ($00D2EA20) Unit1.TForm1.UpdateGUI (Line=1829) ANSI: 'Border'
00D2E760  E81FD26DFF    CALL -$922DE1                   ; ($0040B984) System._LStrEqual
00D2E765  7518          JNZ  +$18                       ; ($00D2E77F) Unit1.TForm1.UpdateGUI (Line=1806)
;
; Line=1804 - Offset=55
; ---------------------
00D2E767  BA34EAD200    MOV  EDX, $00D2EA34             ; ($00D2EA34) Unit1.TForm1.UpdateGUI (Line=1829) UNICODE: 'Replace'
00D2E76C  8B45FC        MOV  EAX, [EBP-4]
00D2E76F  8B8018040000  MOV  EAX, [EAX+$00000418]
00D2E775  8B08          MOV  ECX, [EAX]
00D2E777  FF9120010000  CALL DWORD PTR [ECX+$00000120]
00D2E77D  EB16          JMP  +$16                       ; ($00D2E795) Unit1.TForm1.UpdateGUI (Line=1808)
;
; Line=1806 - Offset=79
; ---------------------
00D2E77F  BA50EAD200    MOV  EDX, $00D2EA50             ; ($00D2EA50) Unit1.TForm1.UpdateGUI (Line=1829) UNICODE: 'Border'
00D2E784  8B45FC        MOV  EAX, [EBP-4]
00D2E787  8B8018040000  MOV  EAX, [EAX+$00000418]
00D2E78D  8B08          MOV  ECX, [EAX]
00D2E78F  FF9120010000  CALL DWORD PTR [ECX+$00000120]
;
; Line=1808 - Offset=101
; ----------------------
00D2E795  8B45FC        MOV  EAX, [EBP-4]
00D2E798  8B80B0030000  MOV  EAX, [EAX+$000003B0]
;
; Line=1808 - Offset=110
; ----------------------
00D2E79E  8B10          MOV  EDX, [EAX]                 ; <-- EXCEPTION
00D2E7A0  FF9200010000  CALL DWORD PTR [EDX+$00000100]
00D2E7A6  8BD8          MOV  EBX, EAX
00D2E7A8  8B45FC        MOV  EAX, [EBP-4]
00D2E7AB  8B80B0030000  MOV  EAX, [EAX+$000003B0]
00D2E7B1  8B80B4020000  MOV  EAX, [EAX+$000002B4]
00D2E7B7  E8282B95FF    CALL -$6AD4D8                   ; ($006812E4) Vcl.TListItems.GetCount
00D2E7BC  48            DEC  EAX
00D2E7BD  3BD8          CMP  EBX, EAX
00D2E7BF  7D17          JGE  +$17                       ; ($00D2E7D8) Unit1.TForm1.UpdateGUI (Line=1808)
00D2E7C1  8B45FC        MOV  EAX, [EBP-4]
00D2E7C4  8B80B0030000  MOV  EAX, [EAX+$000003B0]
00D2E7CA  8B80B4020000  MOV  EAX, [EAX+$000002B4]
00D2E7D0  E80F2B95FF    CALL -$6AD4F1                   ; ($006812E4) Vcl.TListItems.GetCount
00D2E7D5  48            DEC  EAX
00D2E7D6  7F04          JG   +4                         ; ($00D2E7DC) Unit1.TForm1.UpdateGUI (Line=1808)
00D2E7D8  33D2          XOR  EDX, EDX

Registers:
-----------------------------
EAX: 00000000   EDI: 0018FABC
EBX: 00D2C96F   ESI: 00000000
ECX: 0018FAB0   EBP: 0018FAC0
EDX: 00000000   ESP: 0018FAB8
EIP: 00D2E79E   FLG: 00010246
EXP: 00D2E79E   STK: 0018FAC0

Stack:               Memory Dump:
------------------   ---------------------------------------------------------------------------
0018FAFC: 0018FB28   00D2E79E: 8B 10 FF 92 00 01 00 00 8B D8 8B 45 FC 8B 80 B0  ...........E....
0018FAF8: 04641BC0   00D2E7AE: 03 00 00 8B 80 B4 02 00 00 E8 28 2B 95 FF 48 3B  ..........(+..H;
0018FAF4: 06091090   00D2E7BE: D8 7D 17 8B 45 FC 8B 80 B0 03 00 00 8B 80 B4 02  .}..E...........
0018FAF0: 0878A920   00D2E7CE: 00 00 E8 0F 2B 95 FF 48 7F 04 33 D2 EB 02 B2 01  ....+..H..3.....
0018FAEC: 00000000   00D2E7DE: 8B 45 FC 8B 80 58 04 00 00 8B 08 FF 91 80 00 00  .E...X..........
0018FAE8: 00000000   00D2E7EE: 00 8B 45 FC 8B 80 B0 03 00 00 8B 10 FF 92 00 01  ..E.............
0018FAE4: 00000000   00D2E7FE: 00 00 85 C0 7E 17 8B 45 FC 8B 80 B0 03 00 00 8B  ....~..E........
0018FAE0: 00000000   00D2E80E: 80 B4 02 00 00 E8 CC 2A 95 FF 48 7F 04 33 D2 EB  .......*..H..3..
0018FADC: 0878A920   00D2E81E: 02 B2 01 8B 45 FC 8B 80 5C 04 00 00 8B 08 FF 91  ....E...\.......
0018FAD8: 0091D1DC   00D2E82E: 80 00 00 00 8B 45 FC 8B 80 B0 03 00 00 8B 10 FF  .....E..........
0018FAD4: 08EEACA0   00D2E83E: 92 00 01 00 00 8B D8 8B 45 FC 8B 80 B0 03 00 00  ........E.......
0018FAD0: 0018FAFC   00D2E84E: 8B 80 B4 02 00 00 E8 8B 2A 95 FF 48 3B D8 7D 17  ........*..H;.}.
0018FACC: 0040A4CC   00D2E85E: 8B 45 FC 8B 80 B0 03 00 00 8B 80 B4 02 00 00 E8  .E..............
0018FAC8: 0018FB0C   00D2E86E: 72 2A 95 FF 48 7F 04 33 D2 EB 02 B2 01 8B 45 FC  r*..H..3......E.
0018FAC4: 00D2CB35   00D2E87E: 8B 80 60 04 00 00 8B 08 FF 51 50 8B 45 FC 8B 80  ..`......QP.E...
0018FAC0: 0018FAFC   00D2E88E: B0 03 00 00 8B 10 FF 92 00 01 00 00 85 C0 7E 17  ..............~.

Answer 1

Assuming the error is an access violation relating to the code that we can see, the only pointer dereferences in the if statement are when you dereference Self, and when you subsequently dereference ListViewLayers1. So, the conclusion is that either you called the method on an invalid object instance, or that ListViewLayers1 is an invalid object instance.

If the exception is not such an access violation, then it is an exception raised by the getter for the Items property.

Without any more code, or the actual exception details, it is difficult to say any more.

Answer 2

When you close a devexpress dockpanel, the default behaviour is that it gets destroyed. That will destroy your listview too, so if you try to access it after that, you'll get an exception.

So, make sure to check if listviewlayers1 is assigned before you try to do anything with its items. I don't even think you'll have to check if its items property is assigned, unless you're doing some weird hacks to your listview.