list-empty-return

🌳 Sustainable Code - the cost of returning an empty collection

The return of empty collections is practically part of every code. But which variants cost how much - and wouldn't zero be even better?

🔥 Benchmark

BenchmarkDotNet v0.14.0, Windows 10 (10.0.19045.5131/22H2/2022Update)
AMD Ryzen 9 9950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 9.0.100
  [Host]   : .NET 9.0.0 (9.0.24.52809), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI
  .NET 7.0 : .NET 7.0.20 (7.0.2024.26716), X64 RyuJIT AVX2
  .NET 8.0 : .NET 8.0.11 (8.0.1124.51707), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI
  .NET 9.0 : .NET 9.0.0 (9.0.24.52809), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI


| Method              | Runtime  | Mean      | StdDev    | Gen0   |
|-------------|--------- |----------:|----------:|-------:|
| List()      | .NET 7.0 | 2.6872 ns | 0.0125 ns | 0.0019 |
| List()      | .NET 8.0 | 1.7082 ns | 0.0100 ns | 0.0019 |
| List()      | .NET 9.0 | 1.6796 ns | 0.0065 ns | 0.0019 |
|             |          |           |           |        |
| List(0)     | .NET 7.0 | 2.6740 ns | 0.0157 ns | 0.0019 |
| List(0)     | .NET 8.0 | 1.6931 ns | 0.0095 ns | 0.0019 |
| List(0)     | .NET 9.0 | 1.6590 ns | 0.0066 ns | 0.0019 |
|             |          |           |           |        |
| List []     | .NET 7.0 | 2.6605 ns | 0.0096 ns | 0.0019 |
| List []     | .NET 8.0 | 1.7156 ns | 0.0118 ns | 0.0019 |
| List []     | .NET 9.0 | 1.7171 ns | 0.0088 ns | 0.0019 |
|             |          |           |           |        |
| Array.Empty | .NET 7.0 | 0.1821 ns | 0.0086 ns |      - |
| Array.Empty | .NET 8.0 | 0.1678 ns | 0.0040 ns |      - |
| Array.Empty | .NET 9.0 | 0.1686 ns | 0.0040 ns |      - |
|             |          |           |           |        |
| Array []    | .NET 7.0 | 0.1776 ns | 0.0060 ns |      - |
| Array []    | .NET 8.0 | 0.1733 ns | 0.0055 ns |      - |
| Array []    | .NET 9.0 | 0.1735 ns | 0.0052 ns |      - |
|             |          |           |           |        |
| HashSet()   | .NET 7.0 | 2.1399 ns | 0.0215 ns | 0.0038 |
| HashSet()   | .NET 8.0 | 2.0574 ns | 0.0273 ns | 0.0038 |
| HashSet()   | .NET 9.0 | 2.0175 ns | 0.0180 ns | 0.0038 |
|             |          |           |           |        |
| HashSet []  | .NET 7.0 | 2.1432 ns | 0.0156 ns | 0.0038 |
| HashSet []  | .NET 8.0 | 2.0484 ns | 0.0248 ns | 0.0038 |
| HashSet []  | .NET 9.0 | 1.9895 ns | 0.0223 ns | 0.0038 |

🏁 Remarks

• The new notation [] is an alias for all collections in order to create the collection in the most efficient way
• The return of Array.Empty is by far the fastest and most efficient way; the array is only created once in the background and then statically retained. This is not possible with other collections.

Conclusion

It is very useful to follow the current recommendation and use [].

⌨️ Run this sample

dotnet run -c Release --framework net9.0

Updates

• 2023/11 - Add .NET 8
• 2024/11 - Add .NET 9