Add support for MissingMemberHandling

[SigmundVik]

It would be great if Dahomey.Json could support MissingMemberHandling. This would help us build a robust API that does not let user mistakes slip by unnoticed (for our ASP.NET Core application).

Support for this feature might get added to System.Text.Json, but it is unclear if or when that will happen - see the 5.0 and Future milestone indicators here.

[mcatanzariti]

Please find an example here:
https://github.com/dahomey-technologies/Dahomey.Json/blob/master/src/Dahomey.Json.Tests/MissingMemberHandlingTests.cs

[SigmundVik]

Thank you so much for adding support for this!

However, there are two things that are problematic:

1. MissingMemberHandling.Error does not work for polymorphic types; an exception with message Missing member '<name of the discriminator property>' always gets thrown for such a type.
2. The exception message is probably going to be confusing to an end user ("but the member is not missing, it is right there!"). I think a message like Unrecognized property name '<name of property>' would help the user to more quickly realize what is wrong.

[SigmundVik]

@mcatanzariti, should we reopen this issue, or do you want me to file new issue(s)?

[mcatanzariti]

Yes let's reopen it

[mcatanzariti]

Thank you so much for adding support for this!

However, there are two things that are problematic:

1. MissingMemberHandling.Error does not work for polymorphic types; an exception with message Missing member '<name of the discriminator property>' always gets thrown for such a type.
2. The exception message is probably going to be confusing to an end user ("but the member is not missing, it is right there!"). I think a message like Unrecognized property name '<name of property>' would help the user to more quickly realize what is wrong.

@SiggyBar I think I need an example with sample code for this one!

[SigmundVik]

@mcatanzariti I created an example which produces the erroneous exception on this line.

[SigmundVik]

This works great now, thanks again!

I think it would make sense to add Support for MissingMemberHandling to the feature list in README.md 😁