Question

I have to be missing something obvious here. I don’t get why this cast of the results of a linq query returns null and not the typed list I’m requesting.

IList<IMyDataInterface> list = query.ToList() as IList<IMyDataInterface>; 

The complete code to run this is below. This is a knowledge gap I need to bridge. I have tried all kinds of permutations of casts to get it to work. I get no Exceptions, just a null. Of note is that the Linq query is selecting its results into instances of my custom ‘MyDataClass’ which implements IMyDataInterface

class Program {     static void Main(string[] args)     {         IMyFunctionalInterface myObject = new MyClass();           //myObject.Get() returns null for some reason...         IList<IMyDataInterface> list = myObject.Get();          Debug.Assert(list != null, 'Cast List is null');     } }  public interface IMyFunctionalInterface {     IList<IMyDataInterface> Get(); }  public class MyClass : IMyFunctionalInterface {     public IList<IMyDataInterface> Get()     {         string[] names = { 'Tom', 'Dick', 'Harry', 'Mary', 'Jay' };          var query = from n in names                     where n.Contains('a')                     select new MyDataClass                     {                         Name = n.ToString()                     };          //There IS data in the query result         Debug.Assert(query != null, 'result is null');         //but the cast here makes it return null         IList<IMyDataInterface> list = query.ToList() as IList<IMyDataInterface>;          return list;     }  } public interface IMyDataInterface {     string Name { get; set; } }  public class MyDataClass : IMyDataInterface {     public string Name { get; set; } } 

Answer 1

The problem here is one of covariance.

First, your example is a bit too complicated. I have removed some fluff. Also, I’ve added some diagnostics that illuminate the problem.

class Program {     static void Main(string[] args)     {         var names = new[] { 'Tom', 'Dick', 'Harry', 'Mary', 'Jay' };          var query = from n in names                     select new C                     {                         S = n                     };          //There IS data in the query result         Debug.Assert(query != null, 'result is null');          //but the conversion here makes it return null         var list = query.ToList() as IList<I>;         Console.WriteLine(query.ToList().GetType());          // this assert fires.         Debug.Assert(list != null, 'Cast List is null');     } }  interface I {     string S { get; set; } }  class C : I {     public string S { get; set; } } 

The output of this program is:

 System.Collections.Generic.List`1[C] 

Note that we’re trying to cast a List<C> to List<I> which doesn’t work in C# 3.0.

In C# 4.0 you should be able to do this, thanks to the new co- and contra-variance of type parameters on generic interfaces.

Also, your original question asked about IQueryable but that’s not relevant here: the query expression you supplied creates an IEnumerable<string> not an IQueryable<string>.

EDIT: I want to point out that your ‘cast’ using the as operator is technically not a cast, but is a ‘type conversion’. If you had use a cast, you would have gotten an exception with useful information. If I change to:

    var list = (IList<I>)query.ToList(); 

I get an InvalidCastException with:

 Additional information: Unable to cast object of type ‘System.Collections.Generic.List1[C]' to type 'System.Collections.Generic.IList1[I]’.