Let's collect improvements we could/should make in our next major versions and are not allowed to apply now because of SemVer. Issue might be updated.

• Make RouteAction a read-only struct to optimize performance

• Return array instead of IList, so we can optimize contract

  NSubstitute/src/NSubstitute/Core/IThreadLocalContext.cs

  Line 27 in deea0ea

  IList<IArgumentSpecification> DequeueAllArgumentSpecifications();

• Refactor ICall and replace Get methods with properties
  

  NSubstitute/src/NSubstitute/Core/ICall.cs

  Lines 8 to 19 in a2462d5

  	public interface ICall
  	{
  	Type GetReturnType();
  	MethodInfo GetMethodInfo();
  	object[] GetArguments();
  	object[] GetOriginalArguments();
  	object Target();
  	IParameterInfo[] GetParameterInfos();
  	IList<IArgumentSpecification> GetArgumentSpecifications();
  	void AssignSequenceNumber(long number);
  	long GetSequenceNumber();
  	bool CanCallBase { get; }

• Make NSubstitute.Core.Argument a read-only struct, as it's a thin wrapper on top of ICall and index.

• Rework IAutoValueProvider interface to:

  • Have only bool TryCreateValue(Type type, out object result) method
  • Avoid cyclic dependency from provider to list of all providers
  • Expose single aggregate provider from factory rather than an array of providers.

• Make the ReflectionExtensions helper internal and remove OrNull suffix as we use nullability for that.

• Normalize Extension-like classes, as at the moment we have a plenty of them and it's a bit hard to navigate through them.

• Add class constraint to all the DSL extension methods receiving substitute. Example: ReceivedExtensions.Received()

• Review IPendingSpecification interface and potentially change it to Try pattern.

• Convert to a record once we drop .NET Standard 1.3 support:

  • ArgumentMatchInfo
  • Quantity descendants

• Do not expose internal classes from Core namespace, but instead expose interfaces.

• Make SubsitutionContext constructor private, so it can be instantiated via container only.

• Change return type of ArgumentMatchInfo.DescribeNonMatch to return null if cannot describe instead of an empty string.