A Range .. operator is proposed for C# 7.3. This tracks compiler work for that feature.

Proposals:

• https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/ranges.cs
• https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/index-range-changes.md

See also some discussion elsewhere

• Champion "slicing" / Range (16.3, Core 3) csharplang#185
• http://khyperia.com/range.html
• [Proposal] Syntactic ranges #7632
• Discussion: Range operator csharplang#198
• https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-07-05.md#slicing
• https://github.com/dotnet/csharplang/blob/master/meetings/2016/LDM-2016-11-01.md#slicing
• Proposal: Slicing #120
• Slicing pattern with range corefxlab#1306
• Generic implementation of Range. corefxlab#1859
• dotnet/corefx#25220
• https://github.com/terrajobst/designs/blob/27a6b1c7df08ae42baa2d5f13bcd9d0e259b47e2/accepted/system-range/system-range.md

Compiler

• Start work on a new feature branch
• New token ..
• Fit into operator precedence somewhere
• Parser for range expressions
• Built-in operator overloading
  • Range operator ..(int, int)
  • LongRange operator ..(long, long)
  • Requires some well-known APIs be present
• Permit user-defined operator
  • Accepting and binding the syntax
  • Decide on some underlying special name
• Overload the user-defined ones when applicable

API questions

• What is the name of the thing?
• Closed interval, half-open interval, or start and count?
• step?
• Implements IEnumerable pattern (using value types)?
• Conversions to array, List, IEnumerable, etc?
• Constructor vs static factory methods
• Is there a conversion (built-in or otherwise) from an int range to a long range?
• Were the types ported to mono? Add types supporting C# 8.0 Ranges mono/mono#12020

namespace System
{
    public struct Range
    {
        public Range(int left, int right) ...
    }
    public struct LongRange
    {
        public LongRange(long left, long right) ...
    }
}

• Supported in Span<T> and ReadonlySpan<T>?

Span<T> x = ...;
Span<T> y = x.Slice([1..0]);

IDE

• The ForEachToFor refactoring should recognize range syntax (not applicable to new design)

Open Issues

• Shall we support dropping one or more operands
  • 1..
  • ..10
  • ..
• Should we have two versions for half-open and closed intervals
  • Rust: .. for half-open and and ... for closed
  • Swift: ..< for half-open and ... for closed
• Do we want to do something with arrays for a[1..10]?
• Do we want to do something with pattern-matching?