Linq ToDictionary Not Defined?

Question

I have this code

var contacts = dr.mktDoctorContacts
    .GroupBy(x => x.ContactType)
    .Select(zb => new 
     { 
         Key = zb.Key,
         GroupWiseContacts = zb.Select(x => x.Contact).ToList()
     })
    .ToDictionary<string,List<string>>(y => y.Key, y => y.GroupWiseContacts)

I don't know what is wrong with this code.

Compile time error msg says:System.Generic.IEnumerable does not contain definition of and best extension method overloads has some invalid arguments. i can see only two overloads of ToDictionary Method in my visual studio tooltip sort of documentation whereas i have come across more than two overloads of ToDictionary on the web
Edit Here is exact Error message at compile time

Error 13 'System.Collections.Generic.IEnumerable<AnonymousType#1>' does not contain a definition for 'ToDictionary' and the best extension method overload 'System.Linq.Enumerable.ToDictionary<TSource,TKey>(System.Collections.Generic.IEnumerable<TSource>, System.Func<TSource,TKey>, System.Collections.Generic.IEqualityComparer<TKey>)' has some invalid arguments

Answer 1

The compiler message makes the error clear: There is no method ToDictionary which can accept the arguments and types specified.

The mistake here is in specifying the type arguments on ToDictionary. The MSDN documentation defines the extension method as ToDictionary<TKey, TSource>. The source in your example is IEnumerable<AnonymousType#1>, but you have specified a type of List<string>.

To correct the error, omit the type arguments; the compiler will infer the correct types. You can additionally combine the Select and ToDictionary transformations into a single statement:

var contacts = dr.mktDoctorContacts
    .GroupBy(x => x.ContactType)
    .ToDictionary(
        y => y.Key, 
        y => y.Select(x => x.Contact).ToList());

Answer 2

Rewrote your code (and added .AsEnumerable()):

var dictionary = dr.mktDoctorContacts
    .GroupBy(x => x.ContactType)
    .AsEnumerable()
    .ToDictionary(
        i => i.Key, 
        i => i.Select(x => x.Contact).ToList()
    )
);

Answer 3

Don't run that group operation in the database, it'll cause the elements of each group to be fetched in separate roundtrips.

ILookup<string, string> contacts = dr.mktDoctorContacts
  .ToLookup<Contact, string, string>(x => x.ContactType, x => x.Contact);