Question

I have code like this:

        User rValue = null;
        var userByID = new Func<User, bool>(x => x.UserId.Equals(userID, StringComparison.InvariantCultureIgnoreCase));

        if (this._context.Users.Any(userByID))
        {
            var user = this._context.Users
                .Include("nested.Associations")
                .Single(userByID);

            rValue = user;
        }

        return rValue;

I started profiling the query, and noticed that EF is NOT applying the Func<> to the SQL query, but to the collection in-mem after returning the entire .Users set.

Interestingly, the following generates the query with the correct where clause filtering the UserID:

        User rValue = null;
        //var userByID = new Func<User, bool>(x => x.UserId.Equals(userID, StringComparison.InvariantCultureIgnoreCase));

        if (this._context.Users.Any(x => x.UserId.Equals(userID, StringComparison.InvariantCultureIgnoreCase)))
        {
            var user = this._context.Users
                .Include("nested.Associations")
                .Single(x => x.UserId.Equals(userID, StringComparison.InvariantCultureIgnoreCase));

            rValue = user;
        }

        return rValue;

1. Why will EF not build the predicate var into the generated SQL?
2. Is there another way to re-use the predicate so that my code can stay clean and readable, and also have EF stuff it into the generated SQL?

Answer 1

1. EF cannot interpret compiled IL code (which your Func essentially is). It would need to guess the original code like Reflector does. That’s not an attractive design choice for a framework so it doesn’t work that way. When you inline the lambda it is not a Func. It is an expression which is analyzable. Search for “C# expression trees” to find out more.

2. Yes: Use Expression<Func<User, bool>> for your variable.