Low allocation re-implementation of LINQ-to-Objects
Switch branches/tags
Clone or download
Latest commit f8c85e6 Jun 10, 2018



A low allocation re-implementation of LINQ-to-Objects using some dubious techniques.


LinqAF aims to be "type inference compatible" with LINQ-to-Objects, a very weak form of source compatible I made up. Essentially, if your LINQ code has no type names in it then LinqAF will probably just work - this is common in LINQ code due to extensive use of anonymous delegates and var.

As an illustration, the following code will works seamlessly with LINQ-to-Objects and LinqAF.

var range  = Enumerable.Range(0, 100);
var expanded = range.SelectMany(x => new [] { x, x * 2 });
var reversed = expanded.Reverse();
var asString = reversed.Select(y => y.ToString());

foreach(var str in asString)

and could be written more concisely as

var e = 
  Enumerable.Range(0, 100)
  .SelectMany(x => new [] { x, x * 2 })
  .Select(y => y.ToString());
foreach(var str in e)

Several convenience types and extension methods are provided so LinqAF's IEnumerable<T>-ish types can be used in common ways without requiring .AsEnumerable()-calls all over the place.


  • LinqAFString provides implementations of System.String's Concat and Join methods that take all LinqAF enumerable types, and pass throughs for all other methods
  • LinqAFTask does likewise for System.Thread.Task's WaitAll, WaitAny, WhenAll, and WhenAny methods, and also provides pass through methods
  • LinqAFNew provides methods that are equivalent to enumerable consuming constructors for the builtin HashSet<T>, LinkedList<T>, List<T>, Queue<T>, SortedSet<T>, and Stack<T> types.
  • Extension methods for the following methods on HashSet<T>, ISet<T>, and SortedSet<T> that take all LinqAF enumerables are provided:
    • ExceptWith
    • IntersectWith
    • IsProperSubset
    • IsProperSuperset
    • IsSubsetOf
    • IsSupersetOf
    • Overlaps
    • SetEquals
    • SymmetricExceptWith
    • UnionWith
  • Likewise extension methods are provided for AddRange, and InsertRange on List<T>

Dealing with compatibility exceptions

There are cases where LinqAF is not a direct replacement for LINQ-to-Objects. Speaking broadly, these are:

  1. Where the IEnumerable<T> type appears
  2. When different kinds of enumerables need to be assigned or returned

Dealing with IEnumerable

There are a few fixes for the first case: use AsEnumerable() to create an IEnumerable<T> wrapper (this allocates), change the type to var, or change the type to the appropriate LinqAF enumerable.

This LINQ-to-Objects code, for example

IEnumerable<string> e = Enumerable.Repeat("foo", 3);
IEnumerable<char> f = e.Select((string x, int ix) => x[ix]);

could be rewritten as

var e = Enumerable.Repeat("foo", 3);
var f = e.Select((x, ix) => x[ix]);


RepeatEnumerable<string> e = Enumerable.Repeat("foo", 3);
SelectIndexedEnumerable<string, char, RepeatEnumerable<string>, RepeatEnumerator<string>> f = e.Select((x, ix) => x[ix]);

or (if allocations are acceptable)

IEnumerable<string> e = Enumerable.Repeat("foo", 3).AsEnumerable();
IEnumerable<char> f = e.Select((string x, int ix) => x[ix]).AsEnumerable();

obviously, the first option is the preferable one.

Coalescing different kinds of enumerables

All LINQ-to-Objects operators return IEnumerable<T> or a subtype of it, which means that you can assign the result of almost all operators to the same variables, parameters, or returns (modulo the generic type parameter T). In LinqAF every operator returns a different type, which breaks code like the following:

// this works with LINQ-to-Objects, but not LinqAF
int a = ...;
var e = a > 0 ? Enumerable.Repeat(1, 1) : Enumerable.Empty<int>();

There are two ways to fix this, either introduce calls to Box() or AsEnumerable(). AsEnumerable() returns an IEnumerable (and is thus ideal for interoperating with non-LinqAF code), while Box() returns a BoxedEnumerable<T> (there is also an explicit cast from all LinqAF enumerables to BoxedEnumerable<T>, but using .Box() lets you avoid typing out type names).

So the previous code would become either

// this works with both LINQ-to-Objects and LinqAF
int a = ...;
var e = a > 0 ? Enumerable.Repeat(1, 1).AsEnumerable() : Enumerable.Empty<int>().AsEnumerable();


// this works with LinqAF, but not LINQ-to-Objects
int a = ...;
var e = a > 0 ? Enumerable.Repeat(1, 1).Box() : Enumerable.Empty<int>().Box();


LinqAF's primary purpose (besides scratching an intellectual itch) is to minimize the number of allocations LINQ-like code involves. Accordingly almost all operations are zero-allocation, with the exceptions of the ToXXX, Distinct, Except, GroupJoin, Intersect, Join, OrderBy(Descending), ThenBy(Descending), Reverse, TakeLast, SkipLast, and Union operations (and even then, certain cases will be zero allocation).

LinqAF achieves this, in part, by representing all operations as structs and exploiting C#'s extensive duck-typing around LINQ and foreach. This blatently disregards the official guidance on struct use.

Note that LinqAF does not change any of C#'s LINQ -> method rewrite rules, so allocations introduced as part of those will still occur. It is for this reason that I encourage the direct use of methods rather than LINQ query keywords (in all cases, not just with LinqAF) as that syntax makes allocations from intermediate objects and delegates explicit.

LinqAF leverages the much more extensive type information it has available at compile time (compared to LINQ-to-Objects) to optimize numerous cases. For example, Enumerable.Range(0, 100).Reverse() is optimized into a ReverseRangeEnumerable that does no allocations.

LinqAF also has a few alternative collection implementations that aim to allocate less than what LINQ-to-Objects would.

LinqAF also enables custom allocation rules (enabling collection reuse, or allocation monitoring) via registering an IAllocator implementation with LinqAF.Config.Allocator.

More details can be found in an upcoming blog series.

Using LinqAF

LinqAF is available on Nuget.

Be aware that LinqAF is full of dubious ideas: large structs, mutable structs, lots of generic constraints, lots of generic parameters, and truly gargantuan amounts of generated code.

LinqAF can be useful, but it's not a painless or trivial dependency. Be very cautious in using it.

Also the LinqAF.dll is ~26MB, so that's a thing.

Building, testing, and benchmarking LinqAF

LinqAF is composed of several projects:

  • LinqAF - the template for generating the final libraries
    • Should always compile, but the produced DLL is non-functional
    • All enumerables are in the top level
    • Config/ contains non-LINQ-to-Objects replacing types
    • Templates/ contains the templates for each set of operators, these are used to generate all the type-specific implementations of each operator
    • Impl/ contains the actual code used to implement templates, as well as the interfaces for each operator (for static type checking in builds)
    • Overrides/ contains concrete, per-enumerable methods that replace the ones generated from templates.
  • LinqAF.Generator - a command line app that generates the final code
    • Must be run in the root directory of the solution
    • Looks for the LinqAF project to use as a template
    • Updates the LinqAF.Generated project as part of output
  • LinqAF.Tests - library containing tests for LinqAF, links against the LinqAF.Generated project
    • Uses the Microsoft.VisualStudio.TestTools.UnitTesting framework
    • Individual tests can run in Visual Studio
    • Attempting to run several tests will likely result in an OutOfMemoryException in Visual Studio's test runner
    • Any test that, by itself, results in an OutOfMemoryException should be split into multiple tests
  • LinqAF.TestRunner - command line app that runs all tests in LinqAF.Tests, links against the LinqAF.Tests project
    • Exists to work around the OutOfMemoryException issues noted above
    • Also produces code coverage statistics
  • LinqAF.Generated* - final library projects, updated by LinqAF.Generator
    • Only the LinqAF.Generated project contains files, all other projects reference those files instead
    • Each generated project is for a different target platform
    • All produce a LinqAF.dll
  • LinqAF.Benchmark - a command line app that runs a series of benchmarks
    • Uses the BenchmarkDotNet libary
    • Benchmarks are defined in Benchmarks/, but must be added to the Main() as well
    • All benchmarks needs a LINQ2Objects and a LinqAF method
    • the LINQ2Objects method should have Baseline = true as part of it's [Benchmark]
    • If a directory is provided as a command line parameter, results are logged to that directory


The initial motivation for LinqAF came from playing around with the Rust Language and its Iterators.

A great deal of my understanding of LINQ-to-Objects (and 600+ test cases) came from Jon Skeet's EduLinq blog series.

Thanks to Benjamin Hodgson for Box(), as he pointed out that explicit casting couldn't handle anonymous types and would be quite painful for long type names regardless.