按日期时间过滤 LINQ 查询导致 System.NotSupportedException 错误

Question

I have a database table containing a list of Secondary School Surveys. I have a controller which displays a list of the entries in the DB table. I am trying to allow a user to search for specific survey entries on "dd-MM-yyyy" date.

I have tried the following, although the return View(surveys.ToPagedList(pagerNumber, pageSize)) throws the following error when I search for a date:

System.NotSupportedException: 'LINQ to Entities does not recognize the method 'System.String ToString(System.String)' method, and this method cannot be translated into a store expression.'

My Index Controller:

// GET: SecondarySchoolSurvey
        public ActionResult Index(string sortOrder, string currentFilter, string searchString, int? page)
        {
            ViewBag.CurrentSort = sortOrder;
            ViewBag.NameSortParm = String.IsNullOrEmpty(sortOrder) ? "name_desc" : "";

            //paging
            if (searchString != null)
            {
                page = 1;
            }
            else
            {
                searchString = currentFilter;
            }

            ViewBag.CurrentFilter = searchString;

            var surveys = from s in db.SecondarySchoolSurveys
                           select s;

            if (!String.IsNullOrEmpty(searchString))
            {
                surveys = db.SecondarySchoolSurveys.Where(s => s.OfficialSchoolName.Contains(searchString) || s.RollNumber.Contains(searchString) || s.CampDate.Value.ToString("dd/MM/yyyy").Contains(searchString));
            }


            switch (sortOrder)
            {
                case "name_desc":
                    surveys = surveys.OrderByDescending(s => s.OfficialSchoolName);
                    break;
                default:
                    surveys = surveys.OrderBy(s => s.OfficialSchoolName);
                    break;
            }



            int pageSize = 10;
            int pageNumber = (page ?? 1);
            return View(surveys.ToPagedList(pageNumber, pageSize));
        }

I could possibly use the SqlFunctions class ie SqlFunctions.StringConvert but how would I then convert DateTime to the correct format (dd-MM-yyyy)?

I also tried converting the search string to DateTime although kept getting a Sys.FormatException: "The string was not recognized as a valid DateTime".

DateTime dt = DateTime.Parse("dd-MM-yyyy", CultureInfo.InvariantCulture);
            if (!String.IsNullOrEmpty(searchString))
            {
                surveys = db.SecondarySchoolSurveys.Where(s => s.OfficialSchoolName.Contains(searchString) || s.RollNumber.Contains(searchString) || s.CampDate == dt);
        }

Answer 1

If I understand the question you could use DateTime.TryParse

Converts the specified string representation of a date and time to its DateTime equivalent and returns a value that indicates whether the conversion succeeded.

// Read the documentation on TryParse
var isDate = DateTime.TryParse(searchString, out var searchDate);

// the assumption is if it's a date, 
// then it's not going to satisfy any other search requirements 
if (isDate)
{
    // note, I use .Date as it's likely you don't want to compare time as well
    surveys = db.SecondarySchoolSurveys
                .Where(s => s.CampDate.Date = searchDate.Date);
} 
else if (!String.IsNullOrEmpty(searchString))
{
    surveys = db.SecondarySchoolSurveys
                .Where(s => 
                   s.OfficialSchoolName.Contains(searchString) || 
                   s.RollNumber.Contains(searchString))
}

_{Note : There are other variations of how to do this, you could have it all in a single query, though I think this is explicit and is simple to understand}