FirstMethod.cs

Task.Run(() =>
                {
                    isDirectoryCopySuccessful = CopyHelper.CopyDirectory(sourceLocation, destinationLocation);
                    Win.Application.Current.Dispatcher.Invoke(() =>
                    {
                        BaseWindow.HideNotification();
                        ShowMessage?.Invoke("Files copied into destination location successfully", "Information");
                    });
                   
                }).ContinueWith((t) =>
                {
                    if(t.IsFaulted)
                    {
                        throw t.Exception;
                    }
                    else
                    {
                        if (isDirectoryCopySuccessful)
                        {
                            var description = "Copied To : " + selectedItem.GetFolderServerRelativeUrl();
                            CopyHelper.InsertAuditLog(
                                       sourceLocation,
                                       selectedItem.GetFolderServerRelativeUrl(),
                                       selectedItem.GetSiteUrl(),
                                       description,
                                       emailId,
                                       userFullName);
                        }
                    }
                }, TaskScheduler.FromCurrentSynchronizationContext());

SecondMethod.cs

foreach (var file in files)
                    {
                        var fileInfo = new FileInfo(file);
                        CopyFile(fileInfo, destinationLocation);

                        currentProgressValue += 1;

                        //Dispatcher.CurrentDispatcher.Invoke(() => {
                        //    BaseWindow.UpdateProgress(currentProgressValue, "Copying...");
                        //});
                        Win.Application.Current.Dispatcher.Invoke(() =>
                        {
                            BaseWindow.UpdateProgress(currentProgressValue, "Copying...");
                        });
                           
                    }

ThirdMethod.cs

 Win.Application.Current.Dispatcher.Invoke(() =>
                                {
                                    canOverWriteFile = (bool)GetOverWriteConfirmation?
                                                   .Invoke("A file name already exists in the destination location? Do you want to overwrite it?",
                                                   "Confirmation", "Yes");
                                });

                                //canOverWriteFile = (bool)GetOverWriteConfirmation?
                                //                   .Invoke("A file name already exists in the destination location? Do you want to overwrite it?",
                                //                   "Confirmation", "Yes");

                                if (!canOverWriteFile)
                                {
                                    // Log message with oldfile and new file name
                                    var oldFileName = fullPath.Replace(destinationLocation + "\\", "");
                                    fullPath = GetFinalFileNameWithPath(fullPath);
                                    string newName = fullPath.Replace(destinationLocation + "\\", "");

                                    var logMessgeBuilder = new StringBuilder();
                                    logMessgeBuilder.Append("Due to exsistance of another copy, File Name has been changed from");
                                    logMessgeBuilder.Append("" + oldFileName + " to " + newName + " on " + DateTime.Now.ToShortDateString() + "");
                                    // Log message that file name has been changed from old file to new file.
                                    LogHelper.Info(logMessgeBuilder.ToString());
                                }

Exception I got is below,

Unknown error: A Task's exception(s) were not observed either by Waiting on the Task or accessing its Exception property. As a result, the unobserved exception was rethrown by the finalizer thread.

Index was outside the bounds of the array.