[C#] Input-specific functions in IVariableLengthStruct

[badrishc]

In #270, we added an additional method to IVariableLengthStruct:

int GetLength<Input>(ref T t, ref Input input)

In addition, we already had this earlier:

int GetInitialLength<Input>(ref Input input)

With the moving of Input to client sessions (which is a huge improvement, btw), it is causing complications when users have sessions with different input types on the same FASTER store, as the above two functions cannot be typed specifically to a session.

The proposal would be to instead provide these two methods via ClientSession (internally, FasterSession) so that they may be customized per Input type.

CC @marius-klimantavicius as you authored the above-referenced PR and are in the best position to comment/contribute on this.

[badrishc]

Also cc @mito-csod

[marius-klimantavicius]

We could move out methods from IVariableLengthStruct to IFasterSession<...> what I am thinking about is how to handle Generic and Blittable cases (relatively easy to do) and how to allow the user to specify the required methods (without affecting fixed size cases). One idea is to have IVariableLengthStruct as is (or with <Input> methods removed) and introduce additional ISessionVariableLengthStruct (or similar, naming is difficult) which could be optionally provided when creating client session (for fixed sizes it would be either exception or not used).

[badrishc]

Makes sense. We could introduce IVariableLengthStruct<T, TInput> that could be provided per session. If not provided, we would default to fall through to existing IVariableLengthStruct.

[badrishc]

PR is out, will merge soon based on feedback if any.