Linq.IndexRange: The package to enable C# 8.0 index and range featues for LINQ and IEnumerable<T> types

LINQ operators to enable C# 8.0 index and range new features working with LINQ queries and any type that implements IEnumerable<T>.

dotnet add package Linq.IndexRange
var element = source1.ElementAt(index: `5);
var elements = source2.ElementsIn(range: 10..^10); // or Slice(10..^10)
var query1 = Enumerable.Range(10..20).Select().Where();
var query2 = (10..20).AsEnumerable().Select().Where();

Proposed these APIs to .NET Core dotnet/corefx/#35552.


Index/range are language level features. However, currently (v3.0.0-preview2/SDK 3.0.100-preview-010184), they

  • only work with array, not other types like List<T>
  • are compiled to array copy, apparently no deferred execution.

Rationale and usage

The goals of these LINQ APIs are:

  • Use index to locate an element in sequence.
  • Use range to slice a sequence. The usage should be consistent with array, but with deferred execution.
  • Use range to start fluent LINQ query.

This enables index and range langaguge features to work with any type that implements IEnumerable<T>.

LINQ already has ElementAt(int index) and ElementAtOrDefault(int index) query operator. It would be natural to have a overload for System.Index: ElementAt(Index index) and ElementAtOrDefault(Index index), and a new method ElementsIn(Range range) (or Slice(Range)), so that LINQ can seamlessly work with C# 8.0:

Index index = ...;
var element1 = source1.ElementAt(index);
var element2 = source2.ElementAtOrDefault(^5);
Range range = ...;
var slice1 = source3.ElementsIn(range); // or Slice(range)
var slice2 = source4.ElementsIn(2..^2) // or Slice(2..^2)
var slice2 = source5.ElementsIn(^10..); // or Slice(^10..)

The following Range overload and AsEnumerable overload for System.Range convert it to a sequence, so that LINQ query can be started fluently from c# range:

var query1 = Enumerable.Range(10..).Select(...);
Range range = ...;
var query1 = range.AsEnumerable().Select(...);
var query2 = (10..20).AsEnumerable().Where(...);


For LINQ to Objects:

namespace System.Linq
    public static partial class EnumerableExtensions
        public static TSource ElementAt<TSource>(this IEnumerable<TSource> source, Index index) { throw null; }

        public static TSource ElementAtOrDefault<TSource>(this IEnumerable<TSource> source, Index index) { throw null; }

        public static IEnumerable<TSource> ElementsIn<TSource>(this IEnumerable<TSource> source, Range range) { throw null; }

        public static IEnumerable<TSource> Range<TSource>(Range range) { throw null; }

        public static IEnumerable<TSource> AsEnumerable<TSource>(this Range source) { throw null; }

For remote LINQ:

namespace System.Linq
    public static partial class QueryableExtensions
        public static TSource ElementAt<TSource>(this IQueryable<TSource> source, Index index) { throw null; }

        public static TSource ElementAtOrDefault<TSource>(this IQueryable<TSource> source, Index index) { throw null; }

        public static IQueryable<TSource> ElementsIn<TSource>(this IQueryable<TSource> source, Range range) { throw null; }

Implementation details (and pull request)

The API review process says PR should not be submitted before the API proposal is approved. So currently I implemented these APIs separately :

So that anyone can start to use these APIs by adding a NuGet package:

dotnet add package Linq.IndexRange

If this proposal is doable, I can submit a PR quickly.

The implementation of ElementAt(Index), ElementAtOrDefault(Index) and ElementsIn(Range) for IQueryable<T> is straightforward. They just create an expression tree. ElementAt(Index) and ElementAtOrDefault(Index) for IEnumerable<T>are straightforward as well.

Open questions


Should it be called Slice? Currently I implement it as ElementsIn(Range range) to maintain the consistency with original ElementAt(int index), which could be natural for existing LINQ users.

What should we do when the range's start index and/or end index go off the boundaries of source sequence? There are 2 options:

  • Follow the behavior of range with array, and throw OverflowException/ArgumentEception/ArgumentOutOfRangeExceptionaccordingly.
  • Follow the behavior of current partitioning LINQ operators like Skip/Take/SkipLast/TakeLast, do not throw exception.

I implemented ElementsIn(Range) following the array behavior. See unit tests of ElementsIn. And I implemented Slicefollowing the LINQ behavior. See unit test of Slice.

ElementAt(Index) and Queryable

As @bartdesmet mentioned in the comments, LINQ providers may have issues when they see ElementAt having an Indexargument, etc. Should we have a new name for the operator instead of overload? For example, At(Index) and Slice(Range)?


For Range(Range) and AsEnumerable(Range), the question is: what does range's start index and end index mean, when the index is from the end? For example, 10..20 can be easily converted to a sequence of 10, 11,12, ... 19, but how about ^20...^10?

The easiest way is to disallow, and throw exception.

My current implementation attempts to make it flexible. Regarding Index's Value can be from 0 to int.MaxValue, I assume a virtual "full range" 0..2147483648, and any Range instance is a slice of that "full range". So:

  • Ranges .. and 0.. and ..^0 and 0..^0 are converted to "full sequence" 0, 1, .. 2147483647
  • Range 100..^47 is converted to sequence 100, 101, .. 2147483600
  • Range ^48..^40 is converted to sequence 2147483600, 2147483601 .. 2147483607
  • Range 10..10 is converted to empty sequence

etc. See unit tests of Range(Range).


Should this be provided to bridge range to LINQ? For me, at least (10..20).AsEnumerable().Select().Where() is intuitive and natural.


  • LINQ | ArgumentOutOfRangeException: Index was out of range

    for (int i = 0; i < intFeatureIDs.Count; i++) {
    slots_for = slots_for.Where(s => s.featureSlotMapping.Any(fsm => fsm.featureID == intFeatureIDs[i]));
    Here, intFeatureID.Count is 2. And throws below exception.

    Message=An exception was thrown while attempting to evaluate a LINQ query parameter expression. To show additional information call EnableSensitiveDataLogging() when overriding DbContext.OnConfiguring.

    Inner Exception 1: ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.

    If intFeatureIDs.Count == 1, there is no issue.

    Also, If I comment out the for loop, and manually run the code twice, it will work...

    slots = slots.Where(s => s.featureSlotMapping.Any(fsm => fsm.featureID == intFeatureIDs[1]));
    slots = slots.Where(s => s.featureSlotMapping.Any(fsm => fsm.featureID == intFeatureIDs[2]));
    Any comments are much appreciated.

  • great tnx for sharing

  • great i like that

  • Great tutorial! This has helped on my journey to use LINQ to iterate dictionaries. Cheers!

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