Csharp formatting an integer as a binary string representation (See related posts)

using System;

namespace testApp
{
	/// <summary>
	/// Summary description for Class1.
	/// </summary>
	class Class1
	{
		/// <summary>
		/// The main entry point for the application.
		/// </summary>
		[STAThread]
		static void Main(string[] args)
		{
			Class1 t = new Class1();
			long i = 12; // 
			Console.WriteLine(t.ConvertByteToBinaryString(i));
		}
		/// <summary>
		/// Format integer as a binary string with spaces between every 4 bits
		/// </summary>
		/// <param name="input">integer to format</param>
		/// <returns>string of the form "0000 0000 0000 0000 0000 0110 0001 1000"</returns>
		private string ConvertByteToBinaryString(long input)
		{
			string result = null;
			string bitstring = null;
			for(int i=31;i>=0;i--)
			{
				if (((input >> i) & 1) == 1) result = "1";
				else result = "0";
				bitstring += result;
				if ((i>0) && ((i)%4)==0)
				{
					bitstring += " ";
				}
			}
			return bitstring;
		}
	}
}