Why is this an issue?

Classes and records with either private or file access modifiers aren’t visible outside of their assemblies or files, so if they’re not extended inside their scope, they should be made explicitly non-extensible with the addition of the sealed keyword.

What is the potential impact?

We measured at least 4x improvement in execution time. For more details see the Benchmarks section from the More info tab.

How to fix it

The code can be improved by adding the sealed keyword in front of the class or record types that have no inheritors.

Code examples

Noncompliant code example

private class MyClass  // Noncompliant
{
  // ...
}

private record MyRecord  // Noncompliant
{
  // ...
}

file class MyClass  // Noncompliant
{
  // ...
}

file record MyRecord  // Noncompliant
{
  // ...
}

Compliant solution

private sealed class MyClass
{
  // ...
}

private sealed record MyRecord
{
  // ...
}

file sealed class MyClass
{
  // ...
}

file sealed record MyRecord
{
  // ...
}

Resources

Documentation

• The sealed keyword

Articles & blog posts

• Boosting Performance With Sealed Classes in .NET
• Performance Improvements in .NET 6

Benchmarks

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

[Params(1_000_000)]
public int Iterations { get; set; }

private readonly UnsealedClass unsealedType = new UnsealedClass();
private readonly SealedClass sealedType = new SealedClass();

[Benchmark(Baseline = true)]
public void UnsealedType()
{
    for (int i = 0; i < Iterations; i++)
    {
        unsealedType.DoNothing();
    }
}

[Benchmark]
public void SealedType()
{
    for (int i = 0; i < Iterations; i++)
    {
        sealedType.DoNothing();
    }
}

private class BaseType
{
    public virtual void DoNothing() { }
}

private class UnsealedClass : BaseType
{
    public override void DoNothing() { }
}

private sealed class SealedClass : BaseType
{
    public override void DoNothing() { }
}

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 5.0 : .NET 5.0.17 (5.0.1722.21314), X64 RyuJIT AVX2
  .NET 6.0 : .NET 6.0.16 (6.0.1623.17311), X64 RyuJIT AVX2
  .NET 7.0 : .NET 7.0.5 (7.0.523.17405), X64 RyuJIT AVX2