use entity frame work core in multy thread - asp.net-core

Question

i use async funcs with await Task.WhenAll inmy function.
and some times i get exception with this message "A second operation was started on this context before a previous operation completed. This is usually caused by different threads concurrently using the same instance of DbContext. For more information on how to avoid threading issues with DbContext".
public async Task<int> getCountAsync(long userId)
{
return await _appDbContenxt.onTimeRequests
.Where(i => (userId == 0 ? true : i.userId == userId))
.CountAsync()
;
}
public async Task<List<OnTimeRequest>> GetOnTimeRequestsAsync(int pageSize, int currentPage, long userId)
{
return await _appDbContenxt.onTimeRequests
.Where(i => (userId == 0 ? true : i.userId == userId))
.OrderByDescending(i => i.id)
.Skip((currentPage - 1) * pageSize)
.Take(pageSize)
.ToListAsync()
;
}
public async Task<OnTimePaginationDto> getUserOnTimeRequests(int pageSize, int currentPage, long userId)
{
Task<int> count = _onTimeRequestsRepository.getCountAsync(userId);
Task<List<OnTimeRequest>> values = _onTimeRequestsRepository.GetOnTimeRequestsAsync(pageSize, currentPage, userId);
await Task.WhenAll(count, values);
OnTimePaginationDto onTimePaginationDto = new OnTimePaginationDto
{
count = count.Result,
values = _mapper.Map<IList<ReadOnTimeRequestDto>>(values.Result)
};
return onTimePaginationDto;
}
this is my functions. getUserOnTimeRequests in on timeRequestService.
getCountAsync and GetOnTimeRequestsAsync in timeRequestRepository .
and this is my startup code
services.AddScoped<IMemberSheetRepository, MemberShiptRepository>();
services.AddScoped<IPackageHistoryRepository, PackageHistoryRepository>();
services.AddScoped<IOnTimeRequestsRepository, OnTimeRequestsRepository>();
services.AddScoped<IMemberShipService, MemberShipService>();
services.AddScoped<IPackageHistoryService, PackageHistoryService>();
services.AddScoped<IOnTimeRequestService, OnTimeRequestService>();
services.AddDbContext<AppDbContext>(options =>
options.UseQueryTrackingBehavior(QueryTrackingBehavior.NoTracking)
.UseSqlServer(Configuration.GetConnectionString("DbConnection")),ServiceLifetime.Transient
);
thanks for your helps.

Answer 1

only when efcore send the SQL command to your database that your LINQ commands
has these at the end of you commands like(.tolist or .count or ...)
that means LINQ query doesn't load (if you check that type, it is IQueryable), and when you ask it for data by .tolist or .count or ... it will be
sent the SQL commands to the database and many of LINQ commands like .remove
not async and you cannot use them as async