CreateMap and CreateProjection for same source / target types

[nstdspace]

As the upgrade guide clearly states and several stack overflow questions point out, it is not possible to define CreateMap and CreateProjection for the same pair of source- and target-types.

Why is that?

We have often the case, that a mapping is done sometimes "inbetween some service logic", where a direct instance to be mapped is given (i.e., only Map<> can be used), and sometimes directly succeeding a database query (i.e., ProjectTo<> would provide a significat performance boost but we are using Map<> instead).
Is this not a common requirement?

[lbargaoanu]

It is. And that's what CreateMap does :) So if you need Map, you have no use for CreateProjection.

[jbogard]

And to clarify why, CreateMap builds the Map expression tree at config-time. The projection expression tree is created with ProjectTo then cached. If you never call Map then that extra expression tree, which is way more complicated, wastes resources.

CreateProjection was added for just that case.