使用 DTO 查看 - ASP.NET MVC

Question

I'm fairly new to MVC and I've been trying to create a view using a DTO as the Model Class, but it seems to be using the Data Context class I use for my Models, even though I am clearing the selection when I am creating the view.

This issue seems to be causing a NullReferenceException which is caused by the following exception being thrown and the view not having any returned to it.

ITSSkillsDatabase.Models.PersonSkillSetsDTO: : EntityType 'PersonSkillSetsDTO' has no key defined. Define the key for this EntityType.

PersonSkillSets: EntityType: EntitySet 'PersonSkillSets' is based on type 'PersonSkillSetsDTO' that has no keys defined.

My DTO:

namespace ITSSkillsDatabase.Models
{
    public class PersonSkillSetsDTO
    {
        public int IDSkillset { get; set; }
        public int IDCategory { get; set; }
        public string Product { get; set; }
        public string P_Version { get; set; }
        public string Notes { get; set; }
        public int PersonSkillsID { get; set; }
        public int IDPerson { get; set; }
        public int Score { get; set; }
        public DateTime ScoreDate { get; set; }
        public int TargetScore { get; set; }
        public DateTime TargetDate { get; set; }
        public DateTime RefresherDate { get; set; }
    }
}

Controller method:

public ActionResult SkillSets(int? id)
{
    try
    {
        if (id == null)
        {
            return HttpNotFound();
        }
        var viewModel = (from a in db.SkillSets
                         join c in db.PersonSkills on a.IDSkillset equals c.IDSkillSet
                         where c.IDPerson == id
                         select new Models.PersonSkillSetsDTO
                         {
                             IDSkillset = a.IDSkillset,
                             IDCategory = a.IDCategory,
                             Product = a.Product,
                             P_Version = a.P_Version,
                             Notes = a.Notes,
                             PersonSkillsID = c.PersonSkillsID,
                             IDPerson = c.IDPerson,
                             Score = c.Score,
                             ScoreDate = c.ScoreDate,
                             TargetScore = c.TargetScore,
                             TargetDate = c.TargetDate,
                             RefresherDate = c.RefresherDate
                         }).ToList();
        return View(viewModel);
    }
    catch
    {
        return View(); //this is where the NullReferenceException is thrown
    }
}

These are the settings when I'm creating the view:

I realise I can get rid of the NullReferenceException by checking for null values, but I don't have any idea how to fix the issue with my DTO.

Answer 1

I am going to try to explain using a ViewModel/DTO to create a Form and POST back.

ViewModels are outside of the Database Context, So if you are using a ViewModel you have to Map your data from ViewModel to Model and Model to ViewModel.

So if you are reading from Database

1. Create DBContext
2. read data you want to read
3. Map to a ViewModel
4. Pass ViewModel to the View or API

If you are writing to the database

1. POST ViewMdoel from View to Controller (You can use Ajax)
2. Create DBContext
3. Map from ViewModel to Model
4. Save Model to Database

let's say you have a DTO,

   public class CountryDTO
    {

        public int CountryId { get; set; }

        [Display(Name = "Country Name")]
        [Required(ErrorMessage = "This field is required")]
        public string CountryName { get; set; }

        [Display(Name = "Latitude")]
        [Required(ErrorMessage = "This field is required")]
        public double CentralLat { get; set; }

        [Display(Name = "Longitude")]
        [Required(ErrorMessage = "This field is required")]
        public double CentralLang { get; set; }

        [Display(Name = "GMT Offset")]
        [Required(ErrorMessage = "This field is required")]
        public double GMTOffSet { get; set; }

        [Display(Name = "Currency")]
        [Required(ErrorMessage = "This field is required")]
        public string Currency { get; set; }
    }

Create a controller ie CountryController and you have a Views Folder Country , Right Click Country Folder Add --> View , Name it CreateCountry and Select Model to be CountryDTO

You can't select DataContext here , because DTO is not part of the Context

This will create your view with Fields from the DTO.

Now in your Controller you need 2 Actions

1. GET method to return the View

2. POST method to POST back the form

   public ActionResult CreateCountry() { return PartialView(new CountryDTO()); }

Now in the POST method you will Pass the DTO, Let's assume you have a Country table in your Database, you will have to create a New Country type Object and add to the Context

    [HttpPost]
    public ActionResult CreateCountry(CountryDTO model)
    {
        if (ModelState.IsValid)
        {
            // Model State is Valid
            // here you will create Context

            using (var dbContext = new DATBASE_CONTEXT())
            {
                var newCountry = new Country() // Country is a Model from Database
                {
                    CountryName = model.CountryName,
                    CentralLat = model.CentralLat,
                    // Map All Properties from View Model to Model
                };

                // Add the New Country to the Countries 
                dbContext.Countries.Add(newCountry);

                // Save Database Changes
                dbContext.SaveChanges();
            }
        }
        return PartialView(model);
    }

IF you want to display this Country:

    public ActionResult CountryDetails(int id)
    {
        var model = new CountryDTO();

        using (var dbContext = new DATABASE_CONTEXT())
        {
            var country = dbContext.Country.First(s => s.CountryId == id);
            model.CountryName = country.CountryName;
            // Same for other Properties
            // You can use AutoMapper Library to Map from Model to DTO/ViewModel

        }
        return View(model);
    }

Answer 2

try
{
    // <...>
    return View(viewModel);
}
catch
{
    return View(); //this is where the NullReferenceException is thrown
}

You get NullReferenceException because your view expects the model and doesn't perform null checks in Razor.

To verify this, you can create a dummy model and pass it into your return View(/* model */); call.

Answer 3

As I understood the exception trouble is not with DbContext but with lame model: "No key defined". I think checking data-annotations might help.

Answer 4

I created a new View Model which contains everything I need from the two models.

I then set the values for the View Model within the controller.

Which gave me the result I wanted.