2 solution improvements
source | link

C#, 116116 110 bytes

s=>{string r="\n".PadLeft(s.Length+5,'#'),p="#".PadRight(s.Length+3,' ')+"#\n";return r+p+"# "+s+" #\n"+p+r;};

Ungolfed:

s=>
{
    string r = "\n".PadLeft(s.Length + 5, '#'),         //the first line, made of '#'s
        p = "#".PadRight(s.Length + 3, ' ') + "#\n";    //the second line
    return r + p + "# " + s + " #\n" + p + r;           //output is simmetrical
};

Initial version:

s=>{int l=s.Length;string r=new string('#',l+4)+"\n",p="#"+new string(' ',l+2)+"#\n";return r+p+"# "+s+" #\n"+p+r;};

Full program with test cases:

using System;

namespace SurroundStringWithHashes
{
    class Program
    {
        static void Main(string[] args)
        {
            Func<string,string>f= s=>{int l=s.Length;string r=new string('#',l+4)+"\n",p="#"+new string(' ',l+2)+"#\n";return r+p+"# "+s+" #\n"+p+r;};

            Console.WriteLine(f("Hello World"));
            Console.WriteLine(f("Programming Puzzles & Code Golf"));
        }
    }
}

C#, 116 bytes

s=>{int l=s.Length;string r=new string('#',l+4)+"\n",p="#"+new string(' ',l+2)+"#\n";return r+p+"# "+s+" #\n"+p+r;};

Full program with test cases:

using System;

namespace SurroundStringWithHashes
{
    class Program
    {
        static void Main(string[] args)
        {
            Func<string,string>f= s=>{int l=s.Length;string r=new string('#',l+4)+"\n",p="#"+new string(' ',l+2)+"#\n";return r+p+"# "+s+" #\n"+p+r;};

            Console.WriteLine(f("Hello World"));
            Console.WriteLine(f("Programming Puzzles & Code Golf"));
        }
    }
}

C#, 116 110 bytes

s=>{string r="\n".PadLeft(s.Length+5,'#'),p="#".PadRight(s.Length+3,' ')+"#\n";return r+p+"# "+s+" #\n"+p+r;};

Ungolfed:

s=>
{
    string r = "\n".PadLeft(s.Length + 5, '#'),         //the first line, made of '#'s
        p = "#".PadRight(s.Length + 3, ' ') + "#\n";    //the second line
    return r + p + "# " + s + " #\n" + p + r;           //output is simmetrical
};

Initial version:

s=>{int l=s.Length;string r=new string('#',l+4)+"\n",p="#"+new string(' ',l+2)+"#\n";return r+p+"# "+s+" #\n"+p+r;};

Full program with test cases:

using System;

namespace SurroundStringWithHashes
{
    class Program
    {
        static void Main(string[] args)
        {
            Func<string,string>f= s=>{int l=s.Length;string r=new string('#',l+4)+"\n",p="#"+new string(' ',l+2)+"#\n";return r+p+"# "+s+" #\n"+p+r;};

            Console.WriteLine(f("Hello World"));
            Console.WriteLine(f("Programming Puzzles & Code Golf"));
        }
    }
}
1
source | link

C#, 116 bytes

s=>{int l=s.Length;string r=new string('#',l+4)+"\n",p="#"+new string(' ',l+2)+"#\n";return r+p+"# "+s+" #\n"+p+r;};

Full program with test cases:

using System;

namespace SurroundStringWithHashes
{
    class Program
    {
        static void Main(string[] args)
        {
            Func<string,string>f= s=>{int l=s.Length;string r=new string('#',l+4)+"\n",p="#"+new string(' ',l+2)+"#\n";return r+p+"# "+s+" #\n"+p+r;};

            Console.WriteLine(f("Hello World"));
            Console.WriteLine(f("Programming Puzzles & Code Golf"));
        }
    }
}