Linq没有命令，也没有区别

Question

I have an MVC 5 .net app, using Razor view engine. I am trying to populate a dropdown list from entity framework model. Here is my linq statement in the controller:

private void populateDistrictTypeDropDown(object selectedDistrict = null){
    var query = from d in db.roadways
                orderby d.district
                select d;
    ViewBag.District = new SelectList(query, "district", "District", selectedDistrict).Distinct();
}

but it doesn't seem to be ordered or only grabbing the distinct items, which it looks like to me it should be.

Here is what it looks like:

Here is my view:

@Html.DropDownListFor(xy => xy.SelectedDistrictTypeIds, new SelectList(Model.DistrictTypesList, "Id", "district")) 

And here is the model:

public class SearchPageModel
{
    public IEnumerable<lkp_roadwaytypes> FacilityTypes { get; set; } // use this to get roadway type, which = FacilityTypes
    public IEnumerable<survey> Surveys { get; set; } // use this to get period to search for from surveydate
    public IEnumerable<roadway> Roadways { get; set; }// use this to get Disctricts and Cost Centers

    // below is for drop down select list on search page
    public int[] SelectedDistrictTypeIds { get; set; }
    public List<roadway> DistrictTypesList { get; set; }

    public int[] SelectedPeriodIds { get; set; }
    public List<survey> PeriodList { get; set; }

    public MainContext myContext = new MainContext();

    public List<roadway>GetDistrictTypes(){
        return myContext.roadways.Where(a=>a.district != null).ToList();
    }
}

EDIT for Sami's answer:

created a new class with an Equals method as -

class District
{
    public int ID {get; set;}
    public int Name { get; set; }

    public bool Equals(District d1, District d2)
    {
        if(d1.ID == d2.ID){return true;}
        else{return false;}
    }
}

changed the query as -

private void populateDistrictTypeDropDown(object selectedDistrict = null){
    var query = (from d in db.roadways
                orderby d.district
                select new District {ID = d.ID, Name = d.district}).Distinct();
    ViewBag.District = new SelectList(query, "ID", "Name", selectedDistrict);

}

but I'm getting the same result, so I must be missing something you said. Am I supposed to call the equals method somehow, or is the .Distinct() doing that for me?

Edit 2:

I noticed I wasn't using the ViewBag in the view, so I changed it to this:

@Html.DropDownListFor(vx => vx.SelectedDistrictTypeIds, new SelectList(ViewBag.District))

but now my box looks like this:

Answer 1

Don't distinct the SelectList, instead distinct the query:

var query = (from d in db.roadways
            orderby d.district
            select d.district)
            .Distinct;

Also, as per Sami you need to order and distinct on District, not Roadway (see the changed select line)

Answer 2

You're currently using LINQ to select objects of type roadway . If you try to distinct those, you will always get all objects back, since they are distinct, unless you have defined a equality comparison function.

I guess you don't want to compare roadway s to each other always by district ID, so I would suggest you create a new class for storing district name and ID, return that type of object from your query, define comparison operations for it and then you can distinct it.

So your query would be something like

var query = (from d in db.roadways
                    orderby d.district
                    select new District { ID = d.Id, Name = d.district }).Distinct();

Then if you have defined an Equals method, your list should be distincted by whatever you define there (probably a comparison of Id?)

You will also have to bind the SelectList to use the new properties in the District class and not the roadway class.

Answer 3

Ok, not sure if this is the best way to do it, but it works for me:

put this in my Model:

// populate district drop down
    public IEnumerable<int> populateDistrictTypeDropDown(){
        var query = (from d in myContext.roadways
                    orderby d.district                        
                    select d.district).ToList().Distinct();            

        return query;
    }

this is my controller:

 public ActionResult Search()
        {   

            SearchPageModel spm = new SearchPageModel();       

            spm.DistrictTypesList = spm.populateDistrictTypeDropDown();

            return View(spm);
        }

and this is my view:

 @Html.DropDownListFor(xy => xy.SelectedDistrictTypeIds, new SelectList(Model.DistrictTypesList))