public class ClassA
    {
    }
    public class classC
    {
        public ClassA GetClassA()
        {
            return new ClassA();
        }
    }
    public class ClassB<T1> where T1 : ClassA
    {
        void Foo()
        {
            var temp = new classC();
            T1 classA = temp.GetClassA();
        }
    }