Class vs. Struct performance in .NET

In addition to classes that represent reference types, there are also structs that represent value types.

But how do the two different structures behave in terms of performance, and when can a class or a struct be used at all?

The key differences

• Structs are value types, allocated on the stack (or inline in containing types, based on compiler optimizations).
• Classes are reference types, allocated on the heap and managed by the garbage collector.
• Handling of structs is cheaper than handling classes, if we talk about allocations.
• Since structs are values, you copy those values (which costs time too!) instead of using an existing reference.

Performance comparison

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.19044.1415 (21H2)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK=6.0.101
  [Host]     : .NET 6.0.1 (6.0.121.56705), X64 RyuJIT
  DefaultJob : .NET 6.0.1 (6.0.121.56705), X64 RyuJIT


|       Method |      Mean |    Error |    StdDev |  Gen 0 | Allocated |
|------------- |----------:|---------:|----------:|-------:|----------:|
|  SmallStruct |  39.16 ns | 0.069 ns |  0.061 ns |      - |         - |
| MediumStruct |  39.30 ns | 0.254 ns |  0.238 ns |      - |         - |
|   SmallClass | 215.30 ns | 3.048 ns |  2.994 ns | 0.1433 |   2,400 B |
|  MediumClass | 487.58 ns | 9.713 ns | 15.959 ns | 0.2389 |   4,000 B |

Source: https://github.com/BenjaminAbt/SustainableCode

Benchmark Results

• 🔋 Both struct samples produce no allocations!
• 🚀 Struct has a better performance over all!

Remarks!

• The use of struct or class is part of the software architecture.
• Not in all scenarios struct makes sense or can be used (e.g. serialization)!
• The change from class to struct is a breaking change!
• It's all a matter of perspective, whether these are small or large samples here - names are smoke and mirrors.