reviewed

Security/Service/Implementations/AuthService.cs

@@ -20,7 +20,7 @@ public class AuthService : IAuthService
     private readonly UserManager<ApplicationUser> _userManager;
     private readonly ITokenGenerator _tokenGenerator;
     private readonly IEmailService _emailService;
-    private readonly IHttpContextAccessor _httpContextAccessor;
+    private readonly IHttpContextAccessor _httpContextAccessor;//not needed
     private readonly ICurrentUserService _currentUserService;
     private readonly IWebHostEnvironment _webHostEnvironment;
     private readonly MailTemplatesConstants _mailTemplatesConstants;

TestingApi/Controllers/AnswerTemplatesController.cs

@@ -15,6 +15,7 @@ public class AnswerTemplatesController : ControllerBase
     private readonly IQuestionTmplService _questionTmplService;
     private readonly ILogger<AnswersController> _logger;
 
+    //logger, questionService and answerService are not needed. also very long string, better to split into several lines
     public AnswerTemplatesController(IQuestionService questionService, ILogger<AnswersController> logger, IAnswerService answerService, IAnswerTmplService answerTmplService, IQuestionTmplService questionTmplService)
     {
         _logger = logger;

TestingApi/Data/DataContext.cs

@@ -69,7 +69,7 @@ private void OnBeforeSaving()
                 switch (entry.State)
                 {
                     case EntityState.Modified:
-                        baseEntity.ModifiedTimestamp = DateTime.Now;
+                        baseEntity.ModifiedTimestamp = DateTime.Now;//better use utc time. it may cause data inconsistency after deployment to many availability zones
                         if(_currentUserService.UserId != null)
                             baseEntity.ModifiedBy = Guid.Parse(_currentUserService.UserId);
                         break;

TestingApi/Dto/FiltersDto.cs

@@ -6,7 +6,7 @@ public class FiltersDto
 {
     public string? SearchTerm { get; set; }
     public string? SortColumn { get; set; }
-    public string? SortOrder { get; set; }
+    public string? SortOrder { get; set; }//it can be boolean
 
     [Range(1, int.MaxValue, ErrorMessage = "Page must be greater than 0")]
     public int Page { get; set; }

TestingApi/Extensions/RegisterServices.cs

@@ -62,7 +62,7 @@ public static void ConfigureAuth(this IServiceCollection services, IConfiguratio
 
                     ValidateIssuerSigningKey = true,
                     ClockSkew = TimeSpan.Zero,
-                    IssuerSigningKey = new SymmetricSecurityKey(Encoding.ASCII.GetBytes(configuration["Auth:SecretKey"])),
+                    IssuerSigningKey = new SymmetricSecurityKey(Encoding.ASCII.GetBytes(configuration["Auth:SecretKey"])),//better to get the config model. this is safer
                 };
             });
     }

TestingApi/Services/Implementations/AnswerService.cs

@@ -20,15 +20,17 @@ public AnswerService(DataContext dataContext, ILogger<AnswerService> logger, IMa
         _logger = logger;
     }
 
-    public async Task<AnswerResponseDto?> GetAnswerById(Guid id, CancellationToken cancellationToken = default)
+    public async Task<AnswerResponseDto?> GetAnswerById(Guid id, CancellationToken cancellationToken = default)//no need to add word 'Answer' into the method name.
+                                                                                                               //it is already AnswerService
     {
         var answers = await _dataContext.Answers
             .FirstOrDefaultAsync(a => a.Id == id, cancellationToken);
 
         return _mapper.Map<AnswerResponseDto>(answers);
     }
 
-    public async Task<bool> AnswerExistsAsync(Guid id, CancellationToken cancellationToken = default)
+    public async Task<bool> AnswerExistsAsync(Guid id, CancellationToken cancellationToken = default)//looks like you need a generic repo to put common logic here
+                                                                                                     //I see at least several very similar methods in most of the services
     {
         return await _dataContext.Answers.AnyAsync(a => a.Id == id, cancellationToken);
     }

TestingApi/Services/Implementations/FileService.cs

@@ -9,11 +9,13 @@ public class FileService : IFileService
 {
     private readonly IConfiguration _configuration;
 
-    public FileService(IConfiguration configuration)
+    public FileService(IConfiguration configuration)//do not inject configuration. inject model instead
     {
         _configuration = configuration;
     }
-    public async Task RemoveFilesByNameIfExistsAsync(string fileName, CancellationToken cancellationToken = default)
+    public async Task RemoveFilesByNameIfExistsAsync(string fileName, CancellationToken cancellationToken = default)//there is no need in this method
+                                                                                                                    //old file is replaced by new one when created
+                                                                                                                    //read FileMode.Create description
     {
         var folderName = _configuration["FileStorage:FolderPath"];
         var pathToRemoveFiles = Path.Combine(Directory.GetCurrentDirectory(), folderName);

TestingApi/Services/Implementations/TestStatisticService.cs

@@ -19,7 +19,7 @@ public TestStatisticService(DataContext dataContext, IMapper mapper)
         _mapper = mapper;
     }
 
-    public async Task<TestStatisticResponseDto> GetTestStatistic(Guid testId,
+    public async Task<TestStatisticResponseDto> GetTestStatistic(Guid testId,//lines 29, 32, 36, 47 calls can be done in parallel
         CancellationToken cancellationToken = default)
     {
         var test = await _dataContext.Tests.Where(t => t.Id == testId).FirstAsync(cancellationToken);

TestingApi/Services/Implementations/UserService.cs

@@ -76,7 +76,8 @@ public async Task<bool> UserExistsAsync(Guid id, CancellationToken cancellationT
             .AnyAsync(e => e.Id == id, cancellationToken);
     }
 
-    public async Task<UserResponseDto> RegisterUserAsync(UserSignUpDto userSignUpDto,
+    public async Task<UserResponseDto> RegisterUserAsync(UserSignUpDto userSignUpDto,//this way of handling users is not the best
+                                                                                     //better to make security api visible to FE and do all the call directly
         CancellationToken cancellationToken = default, bool isAdmin = false)
     {
         var jsonContent = JsonSerializer.Serialize(userSignUpDto);

TestingApi/Services/Implementations/UserTestService.cs

@@ -202,7 +202,7 @@ public async Task<TestResultResponseDto> GetUserTestResults(Guid userId, Guid te
         };
     }
 
-    private IQueryable<QuestionResultResponseDto> GetAnsweredUserQuestionResultQuery(Guid userId, Guid testId)
+    private IQueryable<QuestionResultResponseDto> GetAnsweredUserQuestionResultQuery(Guid userId, Guid testId)//very big linq. better to split into private methods
     {
         var userAnswersDetailsQuery = _dataContext.UserAnswers
             .Include(ua => ua.Question)