Beengie Beengie - 1 year ago 145
C# Question

populate dropdownlistfor mvc

I having a problem understanding some of the datatypes and datasets with MVC and LINQ. I am trying to populate a dropdownlist.

I receive the following error (noted below in the code)

"Unable to create a constant value of type 'vps_intranet.Models.Part'. Only primitive types or enumeration types are supported in this context."


private List<Part> partsNotPartOfStructure(int partID)
Part mainPart = db.Parts.Find(partID);
//var alreadySelected = db.Parts.Select(p => p.PartStructures_comp).Distinct();
List<Part> parts = new List<Part>();

List<PartStructures> excludeList = db.PartStructures1
.Where(p => p.mainPart_id == partID).ToList();

parts = db.Parts.Where(c => c.PartStructures_comp
.Except((List<PartStructures>) excludeList)).ToList();
//The line above gives the error...
//Unable to create a constant value of type 'vps_intranet.Models.Part'.
//Only primitive types or enumeration types are supported in this context.**

return parts;

public async Task<ActionResult> Details(int? id)
if (id == null)
return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
Part part = await db.Parts.FindAsync(id);
if (part == null)
return HttpNotFound();
ViewData["AvailableParts"] = partsNotPartOfStructure(id.Value);
return View(part);


@model vps_intranet.Models.Part

var fullList = (IEnumerable< vps_intranet.Models.Part >) ViewData["AvailableParts"];
var availableParts = fullList.Select(p => new SelectListItem { Text = p.partNo.ToString(), Value = });


@Html.DropDownListFor(model => model.PartStructures_comp, availableParts));

What do I need to change?

Answer Source

You are passing in a list of PartStructures class objects to the Except method. Except method uses the default equality comparer to compare the values.

If you are passing a custom class (not a simple value type like int) , You should implement IEqualityComparer<T> interface methods such as Equals and GetHashCode.

If you do not prefer to do that, You can get the Ids of your PartStructures collection and use that with the Contains method.

var excludeIdList = db.PartStructures1.Where(p => p.mainPart_id == partID)

var parts = db.Parts
          .Where(c => !c.PartStructures_comp.Any(g=>excludeIdList.Contains(g.Id)))
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