Entity Framework Base Repository

Question

I'm trying to create a base repository for use with Entity Framework 4.0 and having some trouble. In this code below, why is it not possible to do this in one line?

public IEnumerable<T> GetAll<T>(Expression<Func<T, bool>> filter)
{
    IEnumerable<T> allCustomers = this.GetAll<T>();
    IEnumerable<T> result = allCustomers.Where(filter.Compile());

    return result;
}

Won't this result in 2 SQL statements: one without a where clause that retrieves all rows, and one with a where clause that only retrieves the rows that match the predicate?

How can this be done with a single SQL statement? I can't get it to compile if I try to cast the filter.Compile() to Func<Customer, bool>.

Answer 1

Try this:

this.GetAll<T>().Where(filter);

If you want to add additional conditions and execute them on database side (using SQL), GetAll() should return IQueryable . IQueryable version of where takes Expression , so there is no need to call Compile() . EF will take expression and translate it to SQL.

Using IEnumerable version of Where executes query and retrieves all rows in the table before applying filter.