Why am I not being warned about too much in C # (and other questions)?

I am learning C # myself and solving project Euler problems for practice. I wrote the code below that solves problem 8, which asks to find the largest product with 13 consecutive digits in a long sequence of digits. I read this sequence of numbers in a program from a file.

First, I started by defining the numeric variables "maxProduct" and "productTmp" as being of type "int", but I didn't get the right answer because it is larger than the maximum range of that type. What confuses me is that I didn't get an error or warning stating that I was trying to manipulate numbers that were too large when dealing with my targets. Instead, he seemed to "wrap up" his range, which seems useless. Is this how it should be?

Second, the part where it reads the file is what I found on the internet. For some reason, however, I cannot write just "IO.StreamReader", but I have to include "System". in front of it, although I have enabled "use the system"; line. I thought that in the statement being used, everything in "System" is available for reference without the System prefix.

And third, in the while loop, where it reads in the file, what's going on? I am assuming that (line = file.ReadLine ()) is a boolean operator, but in this case it doesn't make sense that it could be zero. Also, I didn't give a value for "string", so the statement looks like it is comparing the null value of a string to something else.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace problem8
{
class Program
{
    static void Main(string[] args)
    {
        int noAdjacentDigits = 13;

        long maxProduct = 0;
        string numberSequence = "", line;


        System.IO.StreamReader file = new System.IO.StreamReader("C:/problem8.txt");
        while ((line = file.ReadLine()) != null)
        {
            numberSequence = numberSequence + line;
        }

        for (int i = 0; i <= numberSequence.Length-noAdjacentDigits;i++)
        {
            string substrTmp = numberSequence.Substring(i, noAdjacentDigits);
            long productTmp = 1;
            for (int j = 0; j <= noAdjacentDigits-1; j++)
            {
                productTmp = productTmp * int.Parse(substrTmp.Substring(j,1));
            }
            if (productTmp > maxProduct) maxProduct = productTmp;
        }

        Console.WriteLine(maxProduct);
        Console.ReadLine();
    }
}
}