Unable to convert List> to return type IList>

Question

For level order traversal why does this exception occur? Following exception occurs:

Cannot implicitly convert type \'System.Collections.Generic

Answer 1

Pretty sure that if it compiles, doing a cast is a real bad idea. Here's why:

public class myStupidList : IList
{
//implementation unimportant
}

private void Button_Click(object sender, RoutedEventArgs e)
{
  var result = new List>();
  IList> imNotAListofLists = (IList>)result;
  imNotAListofLists.Add(new myStupidList());
  //result is not a very valid variable right now, is it?
}

As mentioned in my comment, these types of collection issues boil down to covariance and contravariance, and .NET does provide a lot of tools for dealing with them. (Such as various read-only collections and interfaces)

..Which, explains why you get your error as well. There is no implicit cast from List to List because that cast cannot succeed without breaking type-safety. (and as @Grax mentioned, neither derives from the other)