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suggest change 
var oneNumber = new[] {5};
var theOnlyNumber = oneNumber.Single();
Console.WriteLine(theOnlyNumber);  //5

var numbers = new[] {1,2,3,4,5};

var theOnlyNumberSmallerThanTwo = numbers.Single(n => n < 2);
Console.WriteLine(theOnlyNumberSmallerThanTwo);  //1

The following throws InvalidOperationException since there is more than one element in the sequence: 

var theOnlyNumberInNumbers = numbers.Single();
var theOnlyNegativeNumber = numbers.Single(n => n < 0);
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