如何使用linq将Selectone行添加到下拉列表

Question

I have a table called States and i am displaying values of states in a dropdown list using linq query Now i want to insert selection on the first place of id 0. I am using following query to display dropdown list:

var str = (from li in dbContext.states

               where li.state >= 1
               select new
               {

                   a=li.state

               }).ToArray();

The above query gives mean all the values present in my DB.How can i insert select one at first place? Any one please suggest any method or syntax. Thanks

Answer 1

You have to add one extra item to the beginning of the list.

var str = (from li in dbContext.states

               where li.state >= 1
               select new
               {

                   a=li.state

               }).ToList().Insert(0, "selectone");

Answer 2

See MajoBs answer.

Although if you're using the @Html.DropDownListFor() Helper, you could also specify the Id 0 row using the optionLabelArgument:

public static MvcHtmlString DropDownListFor<TModel, TProperty>(
    this HtmlHelper<TModel> htmlHelper,
    Expression<Func<TModel, TProperty>> expression,
    IEnumerable<SelectListItem> selectList,
    string optionLabel
)

Update:

I think the reason MajoB's example doesn't work is because you are using an anonymous type in the Linq query, but define a string in the .Insert() statement.

I believe it is not possible to reuse the same anonymous type that was used in the LINQ query in the .Insert() statement.

So you would have to use a regular type.

(from x in 
    (from li in dContext.states
    where li.state >= 0
        select new {
            value = li.state,
            text = li.something
        }).AsEnumerable()
    select new SelectListeItem {
        Value = x.value.ToString()
        Text = x.text
    }).ToList().Insert(0, new SelectListItem() { .. });

The nested query is necessary because the .ToString() can't be translated to sql. The .AsEnumerable() call created an in-memory list where .ToString will work on.

I am actually using a Extension method like this in our code base. But it requires LinqKit to work.

public static IList<SelectListItem> ToSelectList<T>(this IQueryable<T> query
        , Expression<Func<T, long>> value
        , Expression<Func<T, string>> text
        , string nullText = null)
    {
        var result = (from x in
                          (from y in query.AsExpandable()
                           select new
                           {
                               Value = value.Invoke(y),
                               Text = text.Invoke(y),
                           }).AsEnumerable()
                      select new SelectListItem
                      {
                          Value = x.Value.ToString(),
                          Text = x.Text.ToString()
                      }).ToList();

        AddNullText(nullText, result);
        return result;
    }

private static void AddNullText(string nullText, List<SelectListItem> result)
    {
        if (nullText != null)
        {
            result.Insert(0, new SelectListItem()
            {
                Value = "",
                Text = nullText,
            });
        }
    }

(+ the same Extension method with Expression<Func<T, string>> , etc.. as second parameter.)