public static string HashPassword(string password)
{
  if(password is null)
  {
    throw new ArgumentNullException(nameof(password));
  }
  ...
}

public static class Guard
{
  public static void NotNull(object param, string paramName)
  {
    if (param is null)
    {
      throw new ArgumentNullException(paramName);
    }
  }

  public static void NotNullOrEmpty(string param, string paramName)
  {
    NotNull(param, paramName);
    if (param == string.Empty)
    {
      throw new ArgumentException($"The string can not be empty.", paramName);
    }
  }

  public static void NotNullOrEmpty<T>(IEnumerable<T> param, string paramName)
  {
    NotNull(param, paramName);
    if (param.Count() == 0)
    {
      throw new ArgumentException("The collection can not be empty.", paramName);
    }
  }
  ...
}

public static string HashPassword(string password)
{
  Guard.NotNull(password, nameof(password));
  ...
}

public static IEnumerable<TSource> DistinctBy<TSource, TKey>(
  this IEnumerable<TSource> source,
  Func<TSource, TKey> keySelector)
{
  Guard.NotNullOrEmpty(source, nameof(source));
  ...
}

public static string HashPassword(string password!)
{
  ...
}

public static class ApiGuard
{
  public static void EnsureNotNull(object param, string paramName)
  {
    if (param == null) throw new BadRequestException($"{paramName} can not be null.");
  }

  public static void EnsureNotEmpty<T>(IEnumerable<T> collection, string paramName)
  {
    if (collection == null || collection.Count() == 0)
      throw new BadRequestException($"{paramName} can not be null or empty.");
  }

  public static void EnsureExist(object value, string message = "Not found")
  {
    if (value == null) throw new BadRequestException(message);
  }

  public static void EnsureNotExist(object value, string message = "Already existed")
  {
    if (value != null) throw new BadRequestException(message);
  }
  ...
}

public async Task UpdateAsync(long id, BookUpdateDto dto)
{
  ApiGuard.EnsureNotNull(dto, nameof(dto));
  ApiGuard.EnsureNotEmpty(dto.TagValues, nameof(dto.TagValues));

  var book = await DbSet
    .Include(x => x.BookTags)
    .FirstOrDefaultAsync(x => x.Id == id);
  ApiGuard.EnsureExist(book);

  Mapper.Map(dto, book);

  ...
}

private Task HandleExceptionAsync(HttpContext context, Exception exception)
{
  ApiResult result;
  if (exception is BadRequestException)
  {
    result = ApiResult.Error(exception.Message, 400);
  }
  else if (exception is NotFoundException)
  {
    message = string.IsNullOrEmpty(message) ? "Not Found" : message;
    result = ApiResult.Error(message, 404);
  }
  else if (exception is UnauthorizedAccessException)
  {
    message = string.IsNullOrEmpty(message) ? "Unauthorized" : message;
    result = ApiResult.Error(message, 401);
  }
  ...
}