Casting generic type in linq query

Question

So I have class which accepts a generic type parameter and does a little special handling if the type parameter is a subclass of a given type.

IEnumerable<T> models = ...

// Special handling of MySpecialModel
if (filterString != null && typeof(MySpecialModel).IsAssignableFrom(typeof(T)))
{
    var filters = filterString.Split(...);
    models = 
        from m in models.Cast<MySpecialModel>()
        where (from t in m.Tags
               from f in filters 
               where t.IndexOf(f, StringComparison.CurrentCultureIgnoreCase) >= 0
               select t)
              .Any()
        select (T)m;
}

But I'm getting an exception on the last line

Cannot convert type 'MySpecialModel' to 'T'

If I change the code to use as instead of casting, I get this error.

The type parameter 'T' cannot be used with the 'as' operator because it does not have a class type constraint nor a 'class' constraint.

What am I missing here?

Update

This class needs can take any type parameter, including structs and built-in types, so a generic constraint would not be a suitable solution in my case.

Answer 1

Do Select(x => (MySpecialModel)x)

The LINQ Cast<T> method will only work for casting elements to that the element already is (such as a base type, derived type, or interface). It is not intended to cast objects that are able to be cast to a target type. (e.g. new List<int>{1,2,3}.Cast<long>() will throw an exception as well.

The above answer wasn't wrong, but it doesn't address the question.

Just because you have proved with reflection that a generic parameter is bound to a given type, doesn't mean that the compiler knows that it is. In order to make this work, you will need to cast your T instance to a common type (e.g. object), then cast it to the specific type. e.g. (changing the last line in your query to select (T)(object)m should do the trick.

Answer 2

Try the following

select (T)(object)m;

At runtime you've verified that T is a subtype of MySpecialModel but the compiler doesn't have access to this information at compile time. It just sees an attempted conversion between 2 unrelated types: T and MySpecialModel.

To work around this you need to use object as a middle man. The compiler understands how to convert MySpecialModel to object and to go from object to T.

Answer 3

The most straightforward fix is to cast to object first before the cast to T:

select (T)(object)m;

The problem is your check occurs at runtime, but the compiler doesn't know that T must be an instance of MySpecialModel within the if statement. Therefore it just sees you are trying to cast to some arbitrary type T from MySpecialModel which is not safe, hence the error.