csharp-int型について

using System;
 
public class Class1{
    public static int Main(string[] args){
 
        int i=123;
        string s = i.ToString();
        Console.WriteLine( "i={0}, s={1}", i, s );
 
        System.Int32 i1=123;
        System.String s1 = i.ToString();
        Console.WriteLine( "i={0}, s={1}", i1, s1);
 
        return 0;
    }
}