关于linq中的返回类型

Question

I have a table that has multiple fields which I just need one field. The table called ZipCompare. I usually use IQueryable<> as returned type of a linq query. But for the following code, an error comes out said "can not implicitly convert type System.Linq.IQueryable<AnonymousType#1> to ZipCompare. Then what return type I should use? I use this function to fill the dropdownlist control My code is :

public IQueryable<ZipCompare> GetStates()
    {
        VettingDataContext dc = new VettingDataContext(_connString);
        dc.DeferredLoadingEnabled = true;
        var query = (from c in dc.ZipCompares
                     select new { States = c.State }).Distinct();
        return query;
    }

Front end code:

ddl_BilState.DataSource = zipDAL.GetStates();
        ddl_BilState.DataTextField = "States";
        ddl_BilState.DataValueField = "States";
        ddl_BilState.DataBind();

This is a .net web application, I write in c#.

Answer 1

Look at your query:

var query = (from c in dc.ZipCompares
             select new { States = c.State }).Distinct();

That isn't selecting a ZipCompare - it's selecting an anonymous type. It's not clear what you are trying to do, but if you want to return an IQueryable<ZipCompare> your select clause will need to be select a ZipCompare .

If you don't want to return a ZipCompare... well, in this case it looks like you don't need an anonymous type. Just use:

var query = (from c in dc.ZipCompares
             select c.State).Distinct();

Or rather more succinctly:

return dc.ZipCompares.Select(c => c.State).Distinct();

You then change your data binding to bind to the value itself (use "." as the field perhaps? Or perhaps the empty string? Not sure).

Answer 2

return IEnumerable<TypeOfStateProperty> , and change your query not to use an anonymous type. For instance if State is a StateEnum value:

public IEnumerable<StateEnum> GetStates()
{
    VettingDataContext dc = new VettingDataContext(_connString);
    dc.DeferredLoadingEnabled = true;
    var query = (from c in dc.ZipCompares select c.State ).Distinct();
    return query;
}

Answer 3

Perhaps you want to return a List of string? State looks like a string...

Answer 4

Change your return type to List<string> and do this:

foreach(var item in query)
{
   myList.Add(item.States);
}