Support for DbBatch

[gliljas]

Observations

A DbBatch looks like a DbCommand, but just on the surface.
Unlike batching which uses an underlying DbCommand, DbBatch will require special handling for things such as Prepare, Enlist etc. While the DbBatch could theoretically be wrapped in a DbCommand "adapter" or even a NH specific "DbCommandOrBatch", since DbBatch is now a part of the core .NET 6+ API, I think the route forward is to be explicit about it, e.g. having both Enlist(DbCommand) and Enlist(DbBatch).

This means that a transparent move is harder, since custom derivatives of e.g MicrosoftDataSqlClientDriver may have vital implementations of e.g AdjustCommand that will have to be duplicated for DbBatch.

No really good way to create a DbBatchCommand from a DbCommand
If I'm not missing something, I think this is a bit of an oversight on the part of the DbBatch creators, especially since they already use "CanDoXxxx" properties.
All the parameters in a DbCommand will have to be recreated for the DbBatchCommand, since they can't be attached to two DbParameterCollections at the same time. The good ol' SqlCommandSet uses a bit of trickery for that, but I just opted to make sure that the DbParameter is ICloneable, and used that. Perhaps the value should be explicitly cloned too, if it's ICloneable. Making it possible to override this logic is probably a good idea.

ReflectionBasedDriver doesn't use DbProviderFactoryDriveConnectionCommandProvider on .NET 5 and above
Any reason for this, or just an effect of NETSTANDARD2_1_OR_GREATER not including NET5 etc.?

[hazzik]

Just a thought for the discussion (do not need to implement straight away). The .NET Core since 3 supports default interface members. With it we can add methods to interfaces without introducing breaking changes (it is similar to adding a virtual method for the class in prior versions). Since the DbBatch is .NET 6 and above we could use default methods to extend interfaces instead of doing our usual shimming. This should simplify code a little bit.

[gliljas]

Just a thought for the discussion (do not need to implement straight away). The .NET Core since 3 supports default interface members. With it we can add methods to interfaces without introducing breaking changes (it is similar to adding a virtual method for the class in prior versions). Since the DbBatch is .NET 6 and above we could use default methods to extend interfaces instead of doing our usual shimming. This should simplify code a little bit.

Yes! I wanted to use that but didn't realize the compiler directives actually meant that I could. Fixed.

[gliljas]

We could make it select the DbBatchBatcher automatically when possible, but perhaps that is something we should/could wait with.

[gliljas]

@hazzik Thanks for helping out!

A couple of ideas.

1. Maybe we should add the commands to a List instead of directly to the batch. It would make reuse of e.g existing AdjustCommand implementations easier (we'd call both AdjustCommand and AdjustBatch), and if the batch only contains one command, there no need to add the overhead of creating a batch and cloning the command. Perhaps the IDriver method should be CreateDbBatchFromDbCommands, and it will be called just before executing the batch.
2. The CanCreateBatch naming is maybe a bit off, since it answers the question "Can you create batches and can you create batch commands from dbcommands"