Here is my solution Nikole! :-)
using System;
class Task3
{
static void Main()
{
int n = int.Parse(Console.ReadLine());
Console.WriteLine("{0}{1}{0}",
new string('.', n),
new string('#', n));//1ST PART
Console.WriteLine("{0}{1}{2}{1}{0}",
new string('.', n - 2),
new string('#', 3),
new string('.', n - 2));//2ND PART
for (int i = 0; i < n / 2 - 1; i++)//3RD PART
{
Console.WriteLine("{0}{1}{2}{3}{4}{3}{2}{1}{0}",
new string('.', ((n - 4) - 2 * i)),
new string('#', 2),
new string('.', 2 + 2 * i),
new string('#', 1),
new string('.', n - 2));
}
for (int i = 0; i < n / 2 - 1; i++)//4TH PART
{
Console.WriteLine("{0}{1}{2}{3}{4}{3}{2}{1}{0}",
new string('.', (1 + (2 * i))),
new string('#', 2),
new string('.', (n - 3) - 2 * i),
new string('#', 1),
new string('.', n - 2));
}
Console.WriteLine("{0}{1}{2}{1}{0}",
new string('.', n - 2),
new string('#', 3),
new string('.', n - 2));//2ND PART
Console.WriteLine("{0}{1}{0}",
new string('.', n),
new string('#', n));//1ST PART
}
}