C#8 nullable references implementation

[jeremyVignelles]

Description of Change

This changes allows to use LVS with nullable annotations.

• Added a TFM for .net standard 2.1
• Updated LangVersion to 8.0
• Added <Nullable>enable</Nullable> in csprojs
• Implemented nullable references where applicable
• Fixed possible null references exceptions
• I also started to fix the coding style errors I was having, but once I opened a new files, a hundred more would come, so I gave up.

This is currently a Work In Progress, only LibVLCSharp and LibVLCSharp.Forms has been implemented.

Issues Resolved

• fixes https://code.videolan.org/videolan/LibVLCSharp/issues/78

API Changes

Not sure if there is any. Intent will be explained more clearly.

Platforms Affected

Not applicable

Behavioral/Visual Changes

When using LVS with nullable references enabled, Visual studio can warn you about a wrong usage of the API regarding null values.

Before/After Screenshots

Not applicable

Testing Procedure

Call every API and see if it works? 😄

PR Checklist

• Rebased on top of the target branch at time of PR
• Changes adhere to coding standard

[mfkl]

Whether a libvlc API can or cannot return NULL is not part of the libvlc API contract, e.g:

• it's not documented,
• if that changes (i.e an API that can return null becomes an API that can never return null, or the opposite), it would not be considered a breaking change by the libvlc devs and versioning. Which makes our ability to keep the binding correct very hard (not just surface API changes to track anymore).

I say we should consider all strings (and possibly any pointers) possibly null. I understand it makes the C# 8 nullable support less appealing for us, but that's just the reality of native code interop.

[mfkl]

What's left to be done here?

• Rebase
• use nullable for all projects

[jeremyVignelles]

Just

• Rebased
• Applied what was discussed : a pointer that comes from the native is considered nullable.

I might have missed some though...

When you say "all projects", do you mean the samples as well?

[mfkl]

When you say "all projects", do you mean the samples as well?

No need to convert the sample projects, but maybe adding a small sample to libvlcsharp-samples displaying what calling a few libvlcsharp APIs with the netstandard2.1 target looks like?

[mfkl]

Is this good to merge? @jeremyVignelles

[kakone]

We can now remove the #nullable enable/restore lines I added in LibVLCSharp/Shared/MediaPlayerElement files waiting this PR.

[jeremyVignelles]

done. I don't think there's anything left on my side. I'm ok for merging

[mfkl]

🎉