What does Array.BinarySearch mean when it says an array with negative indexes?

Question

The documentation for Array.BinarySearch in .NET says that it does not work for an array that has negative indexes. As far as I know, .NET only has arrays with positive indexes and you are not allowed to inherit from the System.Array type.

Why does the documentation state this and how is it possible?

This method does not support searching arrays that contain negative indexes. array must be sorted before calling this method.

You can create array with any lower bound you want, just not obvious in C#. Check out blog.csharphelper.com/2010/12/22/…, or search for more - bing.com/search?q=c%23+array+negative+lower+bound — Alexei Levenkov
– Alexei Levenkov, Commented Jun 24, 2014 at 17:05
var negativeIndexesArray = Array.CreateInstance(typeof (int), new[] {10}, new[] {-5}); — Atomosk
– Atomosk, Commented Jun 24, 2014 at 17:06
You must realize what the documentation is for. The doc is for .NET, not C# specifically. Negative indexes is a completely legitimate thing in C++ which is also referenced in the linked ref. — Daniel
– Daniel, Commented Jun 24, 2014 at 17:09
Also in C#, such arrays can occur when you interop with a COM component. Albeit that the more typical mishap is their first element is at index 1. — Hans Passant
– Hans Passant, Commented Jun 24, 2014 at 17:24

Sergey Kalinichenko · Accepted Answer · 2014-06-24 17:14:08Z

5

You can create an array that lets you supply negative indexes to your array:

var grid = (int[,])Array.CreateInstance(typeof(int),new[] {7,7}, new[] {-3,-3});
for (int r = -3 ; r <= 3 ; r++) {
    for (int c = -3 ; c <= 3 ; c++) {
        grid[r,c] = 10+r + c;
    }   
}

Demo on ideone.

You can make an array with a single dimension, too, but you wouldn't be able to convert it to int[] because CLR uses a special type for one-dimension arrays. However, you could use such array with negative indexes through the Array API. The documentation says that you are not allowed to pass such arrays to the BinarySearch method.

answered Jun 24, 2014 at 17:14

Sergey Kalinichenko

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famousgarkin · Accepted Answer · 2014-06-25 05:52:46Z

There is a way to create an Array with negative indexes in .NET, using this overload of Array.CreateInstance factory method, where you can specify the lower bound of the index:

public static Array CreateInstance(
    Type elementType,
    int[] lengths,
    int[] lowerBounds
)

A 1-dimensional array with length of 10 and index starting at -10:

var a = Array.CreateInstance(typeof(string), new[] { 10 }, new[] { -10 });
a.SetValue("test", -5);
Console.WriteLine(a.GetValue(-5));
Console.WriteLine(a.GetLowerBound(0));

// yields:
// test
// -10

Also note that a 1-dimensional array with negative index lower bound cannot be cast to a vector, such as int[], which must be 0-based indexed. This kind of array is of different type than vector or 0-based indexed array:

Console.WriteLine((Array.CreateInstance(typeof(int), new[] { 1 }, new[] { -1 })).GetType());
Console.WriteLine((Array.CreateInstance(typeof(int), new[] { 1 }, new[] { 0 })).GetType());
Console.WriteLine((new int[] {}).GetType());

// yields:
// System.Int32[*]
// System.Int32[]
// System.Int32[]

(int[])Array.CreateInstance(typeof(int), new[] { 1 }, new[] { -1 })

// throws:
// System.InvalidCastException

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