How is null represented in .NET

int i;
i++;
console.writeline(i);

int? i;
i++;
console.writeline(i);

int? i;
if (int == 0)
{
//do something
}

public int? AddNumbers(int? x, int? y)
{
    if (x == null || y == null)
        return null;
    if (x == 0)
        return y;
    if (y == 0)
        return x;

    return x + y;
}