C#

int num = 12700000;
int answer = num;
while (--answer > 1) {
    if (num % answer == 0) {
        break;
    }
}
Console.WriteLine ((answer > 1 ) ?
                   string.Format ("The largest number that divides {0} is {1}.", num, answer) :
                   string.Format ("Sorry, {0} is a prime number.", num));
}

// use a nullable int result to distinguish
// between case that d is a prime of n
// and the case that it was necessary to find a nearest divisor
private bool findNearestPrimeDivisor(int n, int d, out int? result)
{
    int mod = n % d;

    if (mod == 0)
    {
        result = null;
        return true;
    }

    bool primeFound = false;

    int prime = 0;

    for (int i = mod; i < d + mod; i++)
    {
        if (n % i == 0)
        {
            primeFound = true;
            prime = i;
            if (i > d) break;
        }
    }

    if (primeFound)
    {
        result = prime;
        return true;
    }

    result = null;
    return false;
}

// test in some method or EventHandler:

int? factor;

int testn = 12700000;

int testd = 6269;

if (findNearestPrimeDivisor(testn, testd, out factor))
{
    if (factor == null)
    {
        Console.WriteLine("{0} is an integral divisor of {1}", testd, testn);
    }
    else
    {
        Console.WriteLine("{0} is the nearest integral divisor of {1} to {2}", factor, testn, testd);  
    }
}
else
{
    Console.WriteLine("No near prime divisor found.");
}

private int FindNearestDiviser(int n, int d)
{
    int positiveSearch = d;
    int negativeSearch = d;

    if (n % d != 0)
    {
        while (n % ++positiveSearch != 0
            && n % --negativeSearch != 0) ;

        if (n % positiveSearch == 0)
        {
            d = positiveSearch;
        }
        else if (n % negativeSearch == 0)
        {
            d = negativeSearch;
        }
    }

    return d;
}

SledgeHammer01 wrote:

only works on the main thread if the debugger is attached

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

namespace ConsoleApplication2
{
	class Program
	{
		static bool b = false;

		static void Main(string[] args)
		{
			Thread threadCurrent = Thread.CurrentThread;

			Task.Run(() =>
			{
				threadCurrent.Join();
				b = true;
			});

			Thread _threadWorker = new Thread(new ThreadStart(Worker));
			_threadWorker.SetApartmentState(ApartmentState.STA);
			_threadWorker.Start();

			Thread.Sleep(5000);
		}

		private static void Worker()
		{
			while (true)
			{
				Thread.Sleep(100);

				if (b)
					break;
			}
		}
	}
}