-- VS 2019, C/NET 8.0.21

using MySql.Data.MySqlClient;
using System;
using System.Threading.Tasks;

namespace Bug100327
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
            string strConn = "server=(address=hod03.no.oracle.com:3333,priority=90),(address=localhost:3306,priority=100),(address=hod03.no.oracle.com:6666,priority=80);uid=root;password=mysql123!;connectiontimeout=60;defaultcommandtimeout=200;allowuservariables=True";

            ParallelOptions po = new ParallelOptions();
            po.MaxDegreeOfParallelism = Environment.ProcessorCount;

            Parallel.For(0, 1000, po, async j =>
            {
                Console.WriteLine(j);

                try
                {
                    using (var conn = new MySqlConnection(strConn))
                    {
                        await conn.OpenAsync(); //Errors here.

                        await conn.CloseAsync();
                    }
                }
                catch (Exception ex)
                {
                    Console.WriteLine(ex.Message + "\r\n" + ex.StackTrace);
                }
            });
        }
    }
}


-


Hello World!
0
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Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
634
508
2
134
884
758
258
381
635
135
3
885
259
509
759
382
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
136
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
383
260
4
886
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760
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510
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5
384
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761
511
138
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385
638
6
888
762
512
139
386
7
263
639
763
889
513
387
140
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
388
8
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890
764
141
265
389
9
641
515
765
142
891
266
10
642
516
390
766
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
143
391
892
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11
643
767
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392
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268
12
768
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894
13
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145
519
895
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
520
270
14
770
645
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896
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15
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897
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19
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901
20
152
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651
525
775
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
526
902
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21
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776
527
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528
904
155
23
778
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905
156
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24
405
655
906
157
281
530
780
25
656
406
907
531
781
26
657
908
407
782
532
27
658
408
783
909
533
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
659
910
28
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
409
911
784
534
660
29
410
785
912
535
661
158
30
282
913
Collection was modified; enumeration operation may not execute.
   at System.Collections.Generic.Dictionary`2.KeyCollection.Enumerator.MoveNext()
   at System.Data.Common.DbConnectionStringBuilder.get_ConnectionString()
   at System.Data.Common.DbConnectionStringBuilder.set_ConnectionString(String value)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
786
159
536
411
662
31
914
283
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537
787
412
663
32
915
284
161
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538
664
916
33
162
285
789
414
539
665
917
34
163
790
286
415
540
666
918
287
35
164
791
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541
667
919
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542
668
920
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37
921
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669
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167
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38
922
795
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923
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Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
549
171
800
926
42
674
295
550
424
172
927
801
43
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
802
296
675
551
173
425
803
928
676
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
929
44
297
174
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
552
45
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930
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936
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683
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938
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54
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436
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55
185
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687
941
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563
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
942
56
310
815
186
688
564
943
816
438
311
187
689
57
565
944
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
566
817
312
439
188
58
690
567
818
440
945
189
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59
691
819
568
946
441
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60
692
820
569
191
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947
61
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821
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948
62
694
822
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65
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66
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321
447
953
67
699
198
576
827
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
577
448
68
322
954
700
828
199
449
578
69
323
955
829
701
200
450
324
70
579
956
830
702
451
201
831
580
957
703
452
325
71
958
202
832
704
581
326
453
959
72
833
582
203
327
705
454
73
834
960
583
328
706
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
329
455
204
74
835
961
584
456
330
707
75
205
836
962
708
585
457
331
76
206
709
837
963
586
458
332
207
838
964
710
77
459
587
333
208
839
711
78
588
965
334
209
460
840
712
966
79
335
589
210
461
713
841
967
336
80
590
211
968
462
842
81
591
714
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
337
843
212
969
463
82
844
213
338
464
970
83
845
715
592
214
465
339
593
846
971
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
847
716
84
215
466
340
972
594
848
85
717
216
467
341
849
973
217
718
86
468
342
595
974
850
218
719
87
469
343
596
219
975
851
88
720
470
344
597
220
89
976
852
471
598
721
221
345
90
977
853
472
599
722
222
346
91
473
600
854
978
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
347
601
723
223
474
92
979
855
602
724
348
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
725
93
224
980
475
856
603
349
726
94
225
857
981
476
604
727
350
858
982
477
226
95
728
605
351
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
606
729
983
859
478
352
96
227
984
607
479
860
353
730
97
228
608
985
861
480
731
354
98
229
862
986
481
609
732
99
230
355
987
Object reference not set to an instance of an object.
   at MySql.Data.Failover.FailoverManager.AttemptConnection(MySqlConnection connection, String originalConnectionString, String& connectionString, Boolean mySqlPoolManager)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 27
356
863
610
482
733
100
231
357
988
864
483
611
101
734
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484
989
865
612
102
735
359
233
866
990
485
103
613
736
867
234
360
991
486
614
104
737
361
868
235
992
487
738
869
105
615
362
993
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488
739
870
106
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994
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871
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741
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742
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491
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240
743
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619
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367
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620
493
110
998
745
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111
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999
494
746
243
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622
112
495
Collection was modified; enumeration operation may not execute.
   at System.Collections.Generic.Dictionary`2.KeyCollection.Enumerator.MoveNext()
   at System.Data.Common.DbConnectionStringBuilder.get_ConnectionString()
   at System.Data.Common.DbConnectionStringBuilder.set_ConnectionString(String value)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 34
244
747
370
623
113
496
748
Collection was modified; enumeration operation may not execute.
   at System.Collections.Generic.Dictionary`2.KeyCollection.Enumerator.MoveNext()
   at System.Data.Common.DbConnectionStringBuilder.get_ConnectionString()
   at System.Data.Common.DbConnectionStringBuilder.set_ConnectionString(String value)
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at System.Data.Common.DbConnection.OpenAsync(CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at Bug100327.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in C:\Work\MySQLNet\source\Bug100327\Bug100327\Program.cs:line 34
371
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C:\Work\MySQLNet\source\Bug100327\Bug100327\bin\Release\netcoreapp3.1\Bug100327.exe (process 22392) exited with code 0.
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