Why is this an issue?

There’s no point in chaining multiple OrderBy calls in a LINQ; only the last one will be reflected in the result because each subsequent call completely reorders the list. Thus, calling OrderBy multiple times is a performance issue as well, because all of the sorting will be executed, but only the result of the last sort will be kept.

How to fix it

Instead, use ThenBy for each call after the first.

Code examples

Noncompliant code example

var x = personList
  .OrderBy(person => person.Age)
  .OrderBy(person => person.Name)  // Noncompliant
  .ToList();  // x is sorted by Name, not sub-sorted

Compliant solution

var x = personList
  .OrderBy(person => person.Age)
  .ThenBy(person => person.Name)
  .ToList();

Resources

Benchmarks

Glossary

• Mean
• Standard Deviation

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

public class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
    public int Size { get; set; }
}

private Random random = new Random(1);
private Consumer consumer = new Consumer();
private Person[] array;

[Params(100_000)]
public int N { get; set; }

[GlobalSetup]
public void GlobalSetup()
{
    array = Enumerable.Range(0, N).Select(x => new Person
    {
      Name = Path.GetRandomFileName(),
      Age = random.Next(0, 100),
      Size = random.Next(0, 200)
    }).ToArray();
}

[Benchmark(Baseline = true)]
public void OrderByAge() =>
    array.OrderBy(x => x.Age).Consume(consumer);

[Benchmark]
public void OrderByAgeOrderBySize() =>
    array.OrderBy(x => x.Age).OrderBy(x => x.Size).Consume(consumer);

[Benchmark]
public void OrderByAgeThenBySize() =>
    array.OrderBy(x => x.Age).ThenBy(x => x.Size).Consume(consumer);

Hardware configuration:

BenchmarkDotNet v0.14.0, Windows 10 (10.0.19045.5247/22H2/2022Update)
Intel Core Ultra 7 165H, 1 CPU, 22 logical and 16 physical cores
  [Host]               : .NET Framework 4.8.1 (4.8.9282.0), X64 RyuJIT VectorSize=256
  .NET 9.0             : .NET 9.0.0 (9.0.24.52809), X64 RyuJIT AVX2
  .NET Framework 4.8.1 : .NET Framework 4.8.1 (4.8.9282.0), X64 RyuJIT VectorSize=256