Assume D is a subclass of C:

var query = collection.AsQueryable<C>().OfType<D>();

or using query syntax and type comparisons support either of the following:

var query = from c in collection.AsQueryable<C>() where c is D select c;
var query = from c in collection.AsQueryable<C>() where c.GetType() == typeof(D) select c;

The second example differs slightly in that it should not match subclasses of D.

These would be translated into queries against the "_t" discriminator value.