有关float类型的问题

先声明：菜鸟，别砍我@-@

            float j = 1;
            F(i++,i++,++i);
            Console.WriteLine(1 is int);
            Console.WriteLine(1 is float);
            Console.WriteLine(1.0 is float);
            Console.WriteLine(1.0 is double);
            Console.WriteLine(j is float);

得到    TRUE
        false
        false
        TRUE
        TRUE
Console.WriteLine(1 is float)和Console.WriteLine(1.0 is float)为什么是False?!    是不是C#中未输入小数点的数字皆默认为int，带有小数点的默认为Double类型？