Lambda Expression的新手，如何从一种类型转换为另一种类型

Question

I'm trying to return a list of countries, and assigning the return values to CountryCode and CountryName, I keep getting these errors:

Cannot implicitly convert type 'System.Collections.Generic.List to 'System.Linq.IQueryable'. An explicit conversion exists (are you missing a cast?)

and

Operator '==' cannot be applied to operands of type 'string' and 'System.Collections.Generic.List

Here is my code:

public class CountryViewModel
{
    public string CountryCode { get; set; }
    public string CountryName { get; set; }


    /*SELECT COUNTRIES */

    public static IQueryable<CountryViewModel> GetCountries()
    {
        DbEntities _context = new DbEntities();

        var featureCode = "PCLI";

        var countries = _context.country
                        .Where(c => c.Feature_Code == featureCode
                        .Select(n => new CountryViewModel
                        {
                            CountryCode = c.Country_Code,
                            CountryName = c.Name
                        }
                        ));

        return countries;

}

I couldn't find a proper example to help me along with this, any help would be appreciated.

Answer 1

I think the closing ")" for the .Where clause should be after "featureCode", just before .Select. The error message sounds like it's assuming the Select clause is to be part of the condition.

Answer 2

In you example the variable 'countries' you are returning is an IEnumerable not the IQueryable you have prototyped. I suppose you could return countries.AsQueryable(). If you want to return a list then return countries.ToList().

Answer 3

Quite a few things wrong with your code as far as I can see. Also I'm assuming DbEntities implements IDisposable so you should probably wrap that in a using statement.

public class CountryViewModel
{
    public string CountryCode { get; set; }
    public string CountryName { get; set; }

    public static IQueryable<CountryViewModel> GetCountries()
    {
        using (DbEntities _context = new DbEntities())
        {
            var featureCode = "PCLI";

            var countries = _context.country
                .Where(c => c.Feature_Code == featureCode)
                .Select(c => new CountryViewModel
                {
                    CountryCode = c.Country_Code,
                    CountryName = c.Name
                }
            );

            return countries;
        }
    }
}