Why does (string)int32 always throw: Cannot convert type 'int' to 'string'

public class Foo
    {
        private int FooID;
        public Foo()
        {
            FooID = 4;
            string s = (string)FooID; //throws compile error
            string sss = FooID.ToString(); //no compile error
        }
    }

string s = (string)70;

string s = Convert.ToString(-70, 10);

string s2 = 70.ToString("D");

int test1 = 34;
double test2 = (double)test1;

int test1 = 34;
string test2 = (string)test1;