在linq中对实体使用自定义方法

Question

I have a Person table in my database that has NationalId field. Is there any way to load all Persons with even NationalId , using Ef code first and Linq to entities , without loading all Person s to memory?

Somethings like:

public  bool IsEven(int number)
{
   return number % 2 == 0;
}

var context = new MyContext();
var personsWithEvenNationalId = context.Persons
                                       .Where(x=> IsEven(x.NationalId))
                                       .ToList();

Answer 1

You would have to do your check inline

var personsWithEvenNationalId = context.Persons
                                       .Where(x=> x.NationalId%2 == 0)
                                       .ToList();

Linq to Entities doesn't know how to translate your custom method into SQL basically. If you do need to use a custom method you would have to get Persons as enumerable and then use your custom method, ie

var personsWithEvenNationalId = context.Persons
                                       .AsEnumerable()
                                       .Where(x=> IsEven(x.NationalId))
                                       .ToList();

But that's hardly ideal as it would load all Persons and then filter on IsEven

Edit: Thinking about it you could also create an extension method for IQueryable<Person> if you don't want to have to write it inline every time. Something like this where you build an Expression

    public static IQueryable<Person> WhereEven(this IQueryable<Person> source, Expression<Func<Person, int>> property)
    {
        var expression = Expression.Equal(
            Expression.Modulo(
                property.Body,
                Expression.Constant(2)),
            Expression.Constant(0));

        var methodCallExpression = Expression.Call(typeof (Queryable),
            "where",
            new Type[] {source.ElementType},
            source.Expression,
            Expression.Lambda<Func<Person, bool>>(expression, property.Parameters));

        return source.Provider.CreateQuery<Person>(methodCallExpression);
    }

And to use it:

context.Persons.WhereEven(x => x.NationalId).ToList();

Answer 2

Rather than a function that does what you want, you'll need to have an function (or property, or field) that provides an Expression that does the projection that you want:

public static Expression<Func<int, bool>> IsEven()
{
    return number => number % 2 == 0;
}

You can now write:

using(var context = new MyContext())
{
    var personsWithEvenNationalId = context.Persons
        .Select(x=> x.NationalId)
        .Where(IsEven())
        .ToList();
}