Why is this an issue?

When working with LINQ in C#, it is recommended to pay attention to the order in which methods are chained, especially when using Where and OrderBy methods. It is advised to call the Where method before OrderBy because Where filters the elements of the sequence based on a given condition and returns a new sequence containing only the elements that satisfy that condition. Calling OrderBy before Where, may end up sorting elements that will be later discarded, which can lead to inefficiency. Conversely, calling Where before OrderBy, will first filter the sequence to include only the elements of interest, and then sort them based on the specified order.

What is the potential impact?

We measured at least 2x improvement in execution time. For more details see the Benchmarks section from the More info tab.

How to fix it

The issue can be fixed by calling Where before OrderBy.

Code examples

Noncompliant code example

Public Function GetSortedFilteredList(data As IEnumerable(Of Integer)) As IEnumerable(Of Integer)
    Return data.OrderBy(Function(x) x).Where(Function(x) x Mod 2 = 0)
End Function

Compliant solution

Public Function GetSortedFilteredList(data As IEnumerable(Of Integer)) As IEnumerable(Of Integer)
    Return data.Where(Function(x) x Mod 2 = 0).OrderBy(Function(x) x)
End Function

Resources

Documentation

• Enumerable.OrderBy
• Enumerable.Where

Articles & blog posts

• Jon Skeet’s explanation on Stack Overflow

Benchmarks

The results were generated by running the following snippet with BenchmarkDotNet:

private IList<int> data;
private static readonly Random Random = new Random();

[Params(1_000_000)]
public int NumberOfEntries;

[GlobalSetup]
public void Setup() =>
    data = Enumerable.Range(0, NumberOfEntries).Select(x => Random.Next(0, NumberOfEntries)).ToList();

[Benchmark(Baseline = true)]
public void OrderByThenWhere() =>
    _ = data.OrderBy(x => x).Where(x => x % 2 == 0 ).ToList();  // OrderBy followed by Where

[Benchmark]
public void WhereThenOrderBy() =>
    _ = data.Where(x => x % 2 == 0 ).OrderBy(x => x).ToList();  // Where followed by OrderBy

Hardware configuration:

BenchmarkDotNet=v0.13.5, OS=Windows 10 (10.0.19045.2846/22H2/2022Update)
12th Gen Intel Core i7-12800H, 1 CPU, 20 logical and 14 physical cores
.NET SDK=7.0.203
  [Host]               : .NET 7.0.5 (7.0.523.17405), X64 RyuJIT AVX2
  .NET 7.0             : .NET 7.0.5 (7.0.523.17405), X64 RyuJIT AVX2