Using the generic type System.Collections.Generic.List<T> requires 1 type arguments

Question

i upgraded my application .Net Framework version from 3.5 to 4.5.1 and also i changed the targeted version from 3.5 to 4.5.1. my application using Linq to SQL.. in my application i am using Class Library and in class library i need to pass multiple parameters in Generic list...

In Data Access Layer i have method where i am trying to pass a multiple class that generated by Linq to Sql. (tblaccount, vendorCategory)

internal class BasicDataDAO
{

  internal List<tblaccount, vendorCategory> GetAllvendor()
    {
       using (CommonDataContext VendorDC = new CommonDataContext(Settings.ConnectionString))
         {
             VendorDC.DeferredLoadingEnabled = true;

             return (from V in VendorDC.tblaccounts
                    join C in VendorDC.vendorCategories on V.CatID equals C.Vendor_CatID
                            where V.Sys_AC_type == 7
                            select new
                            {
                                V.ID,
                                V.ACCOUNT,
                                V.field2,
                                V.contactDetails,
                                V.Remarks,
                                V.field3,
                                V.field1,
                                V.email,
                                V.fax,
                                C.vendor_CategoryName
                            }).ToList();           
                }
            }
}

here how i am handling this in Business layer

public List<tblaccount, vendorCategory> GetAllVendors()       
{
            BasicDataDAO dao = new BasicDataDAO();
            return dao.GetAllvendor();            
}

but i am getting this error in both Data and Business Layers

Using the generic type System.Collections.Generic.List requires 1 type arguments

how to achieve this task, in which you can pass multiple classes generated by linq in Generic List?

although i have upgraded .Net Framework from 3.5 to 4.5.1 but still i am not able to reference the System.Tuple class in my Class Library

Answer 1

You cannot declare list with two type arguments.

You can use Tuple<T1, T2> instead: List<Tuple<tblaccount, vendorCategory>> or create your own class with 2 properties.

But looking at your code, it looks like you're trying to return anonymous type instance as method result. That's not what anonymous types are for:

You cannot declare a field, a property, an event, or the return type of a method as having an anonymous type . Similarly, you cannot declare a formal parameter of a method, property, constructor, or indexer as having an anonymous type. To pass an anonymous type, or a collection that contains anonymous types, as an argument to a method, you can declare the parameter as type object. However, doing this defeats the purpose of strong typing. If you must store query results or pass them outside the method boundary, consider using an ordinary named struct or class instead of an anonymous type .

from Anonymous Types (C# Programming Guide)

Follow the guidelines and create your own class to hold results of your query, and declare method as List<MyClass> .

Answer 2

use iQueryable Class to return an anonymous type

public IQueryable GetAllvendor()
        {
            CommonDataContext VendorDC = new CommonDataContext(Settings.ConnectionString);
            {
                VendorDC.DeferredLoadingEnabled = false;

                var List = (from V in VendorDC.tblaccounts
                            join C in VendorDC.vendorCategories on V.CatID equals C.Vendor_CatID
                            where V.Sys_AC_type == 7
                            select new
                            {
                                V.ID,
                                V.ACCOUNT,
                                V.field2,
                                V.contactDetails,
                                V.Remarks,
                                V.field3,
                                V.field1,
                                V.email,
                                V.fax,
                                C.vendor_CategoryName
                            });

                return List;
            }

        }

and on the calling site

 public IQueryable GetAllVendors()
        {

            BasicDataDAO dao = new BasicDataDAO();
            return dao.GetAllvendor();
        }