.Single() must return a single record, or it will throw an error if no records found, or more than one record found.

using System;
using System.Linq;

class Program
{
static void Main()
{
  // Find only element > 999.
  int[] array1 = { 1, 3, 1000, 4, 5 };
  int value1 = array1.Single(element => element > 999);

  // Ensure only one element.
  int[] array2 = { 4 };
  int value2 = array2.Single();

  Console.WriteLine(value1);
  Console.WriteLine(value2);

  // See exception when more than one element found.
  try
  {
   int value3 = array1.Single(element => element > 0);
  }
  catch (Exception ex)
  {
   Console.WriteLine(ex.GetType());
  }
}
}

Output
------
1000
4
System.InvalidOperationException