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Asked: 2026-05-16T07:45:13+00:00

I am confused about the BinarySearch method of List<T> in case when the item

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I am confused about the BinarySearch method of List<T> in case when the item does not exist.

I’ve got 

List<long> theList = {1, 3, 5, ...}.

theList.BInarySearch(0) returns 0, and theList.BInarySearch(3) returns 1, as expected.

However, theList.BinarySearch(1) returns -2, and not -1 as I’d expect. The MSDN manual says:
“Return value: The zero-based index of item in the sorted List, if item is found; otherwise, a negative number that is the bitwise complement of the index of the next element that is larger than item or, if there is no larger element, the bitwise complement of Count.”

A “bitwise complement”? What Am I missing here and why is it that theList.BinarySearch(1) != -1 ?

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1 Answer

  1. Editorial Team

    First – why would you expect -1? If the item is the first item, it cannot return -0 (for integers), so it stands to reason -2 will be returned for the second item.

    Next, you can easily get the right index by using ~-2 – the bitwise not operator.

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