Yes, thanks for your help Frederic

As written by Alexander on this PR upon your request, MappedAs can already specify the length.

First, does the parameter length causes any performance issue as the parameter type do? Maybe is it not worth it to adjust it.

Then, parameters in Linq queries are automatically extracted from literals found in the lambda. There are no places to adjust the resulting DbParameter, excepted with the MappedAs extension. So if you want some way to set a parameter length, the best place is currently to check how MappedAs works then add to it an overload taking the length and do required changes for having it working. MappedAs is processed in an expression tree visitor which detect it by reflection, so to find that in NHibernate sources, search it by name, not just by references.

If you want to add this functionality, please add a new Jira issue, since it would not be a fix but a new functionality giving a workaround. And check contributing guidelines.

Thanks again...
Do you known any way to set type/length in a Linq query Contains?
I am willing to modify the source to do that, but I dont known where I should look,

MappedAs does not currently allow to specify the type length/precision/scale. Maybe this could be a new feature, which would add some MappedAs overload for this.

MappedAs is for adjusting the type of what is converted to a query parameter: it does not try to infer anything from "nearby" entities. So those entities mappings are not taken into account for setting the parameter type characteristics, and this is by design.