Question

I deserialized json string to List<ClassB> and now I want to cast it to List<ClassA> before I return it from
BindModel method. I need casting because the methods expects to get List<ClassA>.

Why I get error while casting? After all, ClassB inherits from ClassA. What should I do?

P.S. this question is extended from this post. In line new DataContractJsonSerializer(typeof(List<ClassB>)); instead of List<ClassB> the type will be constructed at runtime.

    public override object BindModel(...)
    {
          var serializer = new DataContractJsonSerializer(typeof(List<ClassB>));
          MemoryStream ms = new MemoryStream(Encoding.UTF8.GetBytes("[{\"id\":\"1\",\"name\":\"name\"}]"));
          var list = serializer.ReadObject(ms);

          return (List<ClassA>)list;
    }

    [KnownType(typeof(ClassA))]
    [DataContract]
    public class ClassA
    {
        public ClassA();
    }

    [KnownType(typeof(ClassB))]       
    [DataContract]
    public class ClassB : ClassA
    {
        [DataMember(Name = "id")]
        public int Id { get; set; }
        [DataMember(Name = "name")]
        public string CategoryName { get; set; }
    }

Answer 1

You can use

 Cast<T>()

For example:

 List<A> listOfA = new List<B>().Cast<A>();

This in fact is inferior to Linq and is implemented on IEnumerable rather than IEnumerable<T> but still is useful. It is not efficient since as I said, it tries to cast it.

Remember List does not allow for covariance which is a nuisance. It is preferable to use IEnumerable<T> as interface rather than List.

You can say:

 IEnumerable<B> listOfB = new List<B>();
 IEnumerable<A> listOfA = listOfB; // no casting required