错误-LINQ to Entities无法识别方法'System.String ToString（System.IFormatProvider）'

Question

I am converting MVC3 linq-to-sql to MVC5 Entity Framework 6.1; I used the following helper for dropdown list:

public static string ExDropDownList (this HtmlHelper helper, string name, IEnumerable<SelectListItem> selectList, bool readOnly, object htmlAttributes)
{
    string html = "";
    if (readOnly)
    {
        foreach (SelectListItem item in selectList)
        {
            if (item.Selected)
            {
                html = String.Format("<label for='{0}'>{1}</label>", name, item.Text);
                break;
                }
            }
        }
    else
        html = AddEmptyOption(System.Web.Mvc.Html.SelectExtensions.DropDownList(helper, name, selectList, htmlAttributes).ToString());
    return html;
}

I generate this list using:

var proviences = lookupRepository.GetProviences();
IEnumerable<SelectListItem> selectListProvience =  from p in proviences
   select new SelectListItem
   {
      Text = p.ProvinceName,
      Value = p.ProvinceID.ToString(CultureInfo.InvariantCulture)
    };

I get the following error:

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

Answer 1

Entity Framework is trying to convert this to SQL, which fails because it can't handle ToString() . Try adding a ToList() instead:

var proviences = lookupRepository.GetProviences().ToList();

This causes the query to execute, but this is fine if you're selecting every item in the list anyway.

Answer 2

You need to materialise the query before you invoke ToString().

var proviences = lookupRepository.GetProviences();
IEnumerable<SelectListItem> selectListProvience = (from p in proviences).AsEnumerable()
  .Select(p => new SelectListItem
  {
     Text = p.ProvinceName,
    ....

but it could be just

var proviences = lookupRepository.GetProviences();
IEnumerable<SelectListItem> selectListProvience = proviences.Select(p => new SelectListItem
{
  Text = p.ProvinceName,

or easier still

SeelctList selectListProvience = new SelectList(proviences, "ProvinceID", "ProvinceName");

Answer 3

I had same issue when separated my model project I removed using System.Data.Objects.SqlClient; and replaced it with using System.Data.Entity.SqlServer; and issue was gone