int[] someArray = new int[5] { 1, 2, 3, 4, 5 };
int lastElement = someArray[^1]; // lastElement = 5
lastElement.Dump();

someArray[3..5].Dump();

someArray[1..^1].Dump();

someArray[1..].Dump();

someArray[..^1].Dump();

someArray[..2].Dump();

public readonly struct Index : IEquatable<Index>
{
 public Index(int value, bool fromEnd = false);

 /// <summary>Create an Index pointing at first element.</summary>
 public static Index Start => new Index(0);

 /// <summary>Create an Index pointing at beyond last element.</summary>
 public static Index End => new Index(~0);
 //
 // Summary:
 //  Gets a value that indicates whether the index is from the start or the end.
 //
 // Returns:
 //  true if the Index is from the end; otherwise, false.
 public bool IsFromEnd { get; }
 //
 // Summary:
 //  Gets the index value.
 //
 // Returns:
 //  The index value.
 public int Value { get; }

 //
 // Summary:
 //  Creates an System.Index from the end of a collection at a specified index position.
 //
 // Parameters:
 // value:
 //  The index value from the end of a collection.
 //
 // Returns:
 //  The Index value.
 public static Index FromEnd(int value);
 //
 // Summary:
 //  Create an System.Index from the specified index at the start of a collection.
 //
 // Parameters:
 // value:
 //  The index position from the start of a collection.
 //
 // Returns:
 //  The Index value.
 public static Index FromStart(int value);
 //
 // Summary:
 //  Returns a value that indicates whether the current object is equal to another
 //  System.Index object.
 //
 // Parameters:
 // other:
 //  The object to compare with this instance.
 //
 // Returns:
 //  true if the current Index object is equal to other; false otherwise.
 public bool Equals(Index other);
 //
 // Summary:
 //  Calculates the offset from the start of the collection using the given collection length.
 //
 // Parameters:
 // length:
 //  The length of the collection that the Index will be used with. Must be a positive value.
 //
 // Returns:
 //  The offset.
 public int GetOffset(int length);

 //
 // Summary:
 //  Converts integer number to an Index.
 //
 // Parameters:
 // value:
 //  The integer to convert.
 //
 // Returns:
 //  An Index representing the integer.
 public static implicit operator Index(int value);
}

private class TestCollection
{
 public IList<int> Data { get; init; }

 public int Count => Data.Count;
 public int this[int index] => Data[index];

 //public int this[Index index] => Data[index.GetOffset(Data.Count)];
}
var array = new TestCollection()
{
 Data = new[] { 1, 2, 3 }
};
Console.WriteLine(array[^1]);
Console.WriteLine(array[1]);

public readonly struct Range : IEquatable<Range>
{
 /// <summary>Represent the inclusive start index of the Range.</summary>
 public Index Start { get; }

 /// <summary>Represent the exclusive end index of the Range.</summary>
 public Index End { get; }

 /// <summary>Construct a Range object using the start and end indexes.</summary>
 /// <param name="start">Represent the inclusive start index of the range.</param>
 /// <param name="end">Represent the exclusive end index of the range.</param>
 public Range(Index start, Index end)
 {
  Start = start;
  End = end;
 }

 /// <summary>Create a Range object starting from start index to the end of the collection.</summary>
 public static Range StartAt(Index start) => new Range(start, Index.End);

 /// <summary>Create a Range object starting from first element in the collection to the end Index.</summary>
 public static Range EndAt(Index end) => new Range(Index.Start, end);

 /// <summary>Create a Range object starting from first element to the end.</summary>
 public static Range All => new Range(Index.Start, Index.End);

 /// <summary>Calculate the start offset and length of range object using a collection length.</summary>
 /// <param name="length">The length of the collection that the range will be used with. length has to be a positive value.</param>
 /// <remarks>
 /// For performance reason, we don't validate the input length parameter against negative values.
 /// It is expected Range will be used with collections which always have non negative length/count.
 /// We validate the range is inside the length scope though.
 /// </remarks>
 public (int Offset, int Length) GetOffsetAndLength(int length);
}

private class TestCollection
{
 public IList<int> Data { get; init; }
 //public int[] this[Range range]
 //{
 // get
 // {
 //  var rangeInfo = range.GetOffsetAndLength(Data.Count);
 //  return Data.Skip(rangeInfo.Offset).Take(rangeInfo.Length).ToArray();
 // }
 //}

 public int Count => Data.Count;

 public int[] Slice(int start, int length)
 {
  var array = new int[length];
  for (var i = start; i < length && i < Data.Count; i++)
  {
   array[i] = Data[i];
  }
  return array;
 }
}