Fallback to ILBasedSerializer when BinaryFormatter is disabled

[ReubenBond]

Fix for #7028 & #6805.
Developers can enable BinaryFormatter on .NET 5.0+ using the workaround here: #6805 (comment)
Otherwise, ILBasedSerializer will be used as the fallback on .NET 5.0+

[turowicz]

@ReubenBond which formatter should we prefer? The unsafe one?

[ReubenBond]

I would stick with what works for now. They are only used as a fallback, eg for exceptions.

[SebastianStehle]

What is the long term plan here?

I thought ILBasedSerializer has been marked as obsolete and a new serializer has been developed a while ago but is not stable?

And how do these serializers with exceptions that have custom properties?

[ReubenBond]

@SebastianStehle yes, it was a roller coaster: ILBasedSerializer was developed in response to BinaryFormatter's removal from early .NET Core versions (1.0?). It was eventually marked [Obsolete] when the .NET team reversed that decision, since we didn't want another serializer just for the sake of it. BinaryFomatter was selected as the default. Now that BinaryFormatter has actually been disabled for .NET 5.0 and above, we are back to using ILBasedSerializer where necessary. We should probably remove the [Obsolete] attribute from it, since it's really the only option for Orleans 3.x on .NET 5.0 without explicitly opting-in to allowing it.

The long term plan is already in-place in main:

• A new, version-tolerant serializer has been added, which supports exceptions with a fallback exception type
• ILBasedSerialier is entirely removed
• All other serializers are entirely removed except for OrleansJsonSerializer, which is still there to configure Newtonsoft.Json.
• (in-progress) Grain persistence supports per-provider configurable serializers

We will likely need some options for developers who are serializing grain state using the current serialization framework, at some point. 4.x is not wire-compatible with 3.x, so rolling upgrades are not supported between those major versions.

[SebastianStehle]

Thanks a lot of your good explanation.