LINQ Union with Constant Values

问题

Very primitive question but I am stuck (I guess being newbie). I have a function which is supposed to send me the list of companies : ALSO, I want the caller to be able to specify a top element for the drop-down list as well.. (say for "None"). I have following piece of code, how I will append the Top Element with the returning SelectList?

    public static SelectList GetCompanies( bool onlyApproved, FCCIEntityDataContext entityDataContext, SelectListItem TopElement )
    {
        var cs = from c in entityDataContext.Corporates
                 where ( c.Approved == onlyApproved || onlyApproved == false )
                 select new
                 {
                     c.Id,
                     c.Company
                 };

        return new SelectList( cs.AsEnumerable(), "Id", "Comapny" );
    }

Thanks!

回答1:

This should work for you:

List<Corporate> corporates =
            (from c in entityDataContext.Corporates
            where (c.Approved == onlyApproved || onlyApproved == false)
            select c).ToList();
corporates.Add(new Corporate { Id = -1, Company = "None" });

return new SelectList(corporates.AsEnumerable(), "Id", "Comapny");

回答2:

This method has always worked for me.

public static SelectList GetCompanies( bool onlyApproved, FCCIEntityDataContext entityDataContext, SelectListItem TopElement )
    {
            var cs = from c in entityDataContext.Corporates
                             where ( c.Approved == onlyApproved || onlyApproved == false )
                             select new SelectListItem {
                                     Value = c.Id,
                                     Text = c.Company
                             };

            var list = cs.ToList();
            list.Insert(0, TopElement);

            var selectList = new SelectList( list, "Value", "Text" );
            selectList.SelectedValue = TopElement.Value;

            return selectList;
    }

Update forgot the lesson I learned when I did this. You have to output the LINQ as SelectListItem.

回答3:

cs.ToList().Insert(0, new { TopElement.ID, TopElement.Company });

回答4:

You could convert it to a list as indicated or you could union the IQueryable result with a constant array of one element (and even sort it):

    static void Main(string[] args)
    {
        var sampleData = new[] {
            new { Id = 1, Company = "Acme", Approved = true },
            new { Id = 2, Company = "Blah", Approved = true }
        };

        bool onlyApproved = true;

        var cs = from c in sampleData
                 where (c.Approved == onlyApproved || onlyApproved == false)
                 select new
                 {
                     c.Id,
                     c.Company
                 };

        cs = cs.Union(new [] {new { Id = -1, Company = "None" }}).OrderBy(c => c.Id);

        foreach (var c in cs)
        {
            Console.WriteLine(String.Format("Id = {0}; Company = {1}", c.Id, c.Company));
        }

        Console.ReadKey();
    }

来源：https://stackoverflow.com/questions/1852442/linq-union-with-constant-values

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linq

drop-down-menu

selectlist