comparing “string” and “null” in linq to entity

问题

My code:

string name;
db.Entities.Where(m=>m.name==(name??m.name))
throw:Should be CHAR, but get NCLOB.

//----------------

  decimal? id;
  db.Entities.Where(m=>m.id==(id??m.id))
  The code is working.  

my database is oracle.
What should I do to make the code working.?

Thanks.

回答1:

Beware that in Oracle an empty string is NULL.

回答2:

I don't know about Oracle + EF and type mappings. But this is not the way to make predicates optional anyway, because now the part x ?? x.y will always be part of the expression that is translated into SQL. This may hit query optimization and performance.

The way to do this is:

IQueryable<Entity> query = db.Entities;
if (!string.IsNullOrWhiteSpace(name))
{
    query = query.Where(m => m.Name == name);
}

来源：https://stackoverflow.com/questions/27200600/comparing-string-and-null-in-linq-to-entity