System.Runtime.CompilerServices.Range

[I-Omnibus]

There seems to be no real way of reversing the behaviour of indices, i.e. where the set of indices range from high to low, e.g. a bit pattern <UInt32> - bit31..bit0

A solution would be for System.Index to have, say, an Inferred property to indicate that the Start/End index was not present ('..' 'start..' '..end'), rather than blindly setting Index.Start.Value to zero.

using System;
using static System.Console;
using System.Runtime.InteropServices;

WriteLine(0b0.BitRange<int>(31..0));	// hoping for "0..31" and got "0..31"
WriteLine(0b0.BitRange<int>(^0..^31)); 	// hoping for "0..31" and got "0..31"
WriteLine(0b0.BitRange<int>(..)); 	// hoping for "0..31" but got "31..0"
WriteLine(0b0.BitRange<int>(31..));	// hoping for "0..31" but got "31..31"
WriteLine(0b0.BitRange<int>(..0)); 	// hoping for "0..31" but got "0..0"

static class Extension {

	internal static Range Invert(Range range, int length) {

		var rightmost = range.End.IsFromEnd ?
			length - 1 - range.End.Value
			:
			range.End.Value;

		var leftmost = range.Start.IsFromEnd ?
				length - 1 - range.Start.Value
				:
				range.Start.Value;

		return new Range(new Index(rightmost), new Index(leftmost));
	}

	internal static string BitRange<T>(this T data, Range range)
		where T : struct =>
		Invert(range, Marshal.SizeOf<T>() << 3).ToString();

}

[CyrusNajmabadi]

Moving to Runtime. The behavior of Range is defined there.

[I-Omnibus]

Here's an updated sample with suggested change to System.Index faked in IndexRefactored class:

using System;
using static System.Console;
using System.Runtime.InteropServices;

const int _whitespace_ = int.MaxValue;

// All should return "0..31"
// Suggestion: IsInferred property implemented in System.Index
// as per IndexRefactored class below
WriteLine(0b0.BitRange<int>(31..0));
WriteLine(0b0.BitRange<int>(^0..^31));
// Whitespace faked
WriteLine(0b0.BitRange<int>(_whitespace_..^_whitespace_));
WriteLine(0b0.BitRange<int>(31..^_whitespace_));
WriteLine(0b0.BitRange<int>(_whitespace_..0));
// Undesired results with _whitespace_ kludge removed
WriteLine(0b0.BitRange<int>(..));   // Returns "31..0"
WriteLine(0b0.BitRange<int>(31..)); // Returns "31..31"
WriteLine(0b0.BitRange<int>(..0));  // Returns "0..0"

public class IndexRefactored {
	// Suggestion for System.Index with IsInferred property
	public IndexRefactored(int value, bool fromEnd = false, int _whitespace_ = int.MaxValue)
		=> (IsFromEnd, IsInferred, Value) = (
		IsFromEnd = fromEnd,
		IsInferred = value == _whitespace_,
		Value = value == _whitespace_ ?
			0
		:
			value
		);
	public bool IsInferred { get; init; }
	public bool IsFromEnd { get; init; }
	public int Value { get; init; }
}
static class Extension {
	internal static Range Invert(Range range, int length) {
		var start = /*range.Start*/ Extension.Kludge(range.Start);
		var end = /*range.End*/ Extension.Kludge(range.End);

		var rightmost = end.IsInferred ?
			0
			:
			end.IsFromEnd ?
				length - 1 - end.Value
				:
				end.Value;

		var leftmost = start.IsInferred ?
			length - 1
			:
			start.IsFromEnd ?
				length - 1 - start.Value
				:
				start.Value;

		return new Range((Index)rightmost, (Index)leftmost);
	}

	public static IndexRefactored Kludge(Index index)
	=> new IndexRefactored(index.Value, fromEnd: index.IsFromEnd);

	internal static Range BitRange<T>(this T data, Range range)
		where T : struct =>
		Invert(range, Marshal.SizeOf<T>() << 3);

}