Adopt Nullable annotations in Polly.dll

[Barsonax]

Describe the bug

The xml docs imply that Result and Exception are nullable in https://github.com/App-vNext/Polly/blob/main/src/Polly/DelegateResult.cs but this is not clear from the typing.

Expected behavior

TResult should be TResult? and Exception should be Exception?

Actual behavior

TResult and Exception are non nullable

Steps to reproduce

No response

Exception(s) (if any)

We got a null ref on .Result since we thought it was non nullable

Polly version

8.4.2

.NET Version

8.0.403

Anything else?

No response

[peter-csala]

The DelegateResult class is part of the V7 API which is currently under maintenance. Your proposed solution regarding nullability is used in its successor class inside the V8's Outcome.

@martincostello I think this would cause breaking change. What do you think?

[martincostello]

The Polly assembly has not opted into nullable annotations at all (the project file has no Nullable property, compared to Polly.Core which has).

This issue isn't a bug - Polly pre-dates the concept of nullable reference types. This issue is instead a feature request to add them to Polly.

As Polly is effectively now legacy and exists for backwards compatibility and minimal future evolution, it's unlikely we'd do the work to add nullable reference types to it due to the effort involved.