How to do a “where in values” in LINQ-to-Entities 3.5

问题

Does anybody know how to apply a "where in values" type condition using LINQ-to-Entities? I've tried the following but it doesn't work:

var values = new[] { "String1", "String2" };  // some string values

var foo = model.entitySet.Where(e => values.Contains(e.Name));

I believe this works in LINQ-to-SQL though? Any thoughts?

回答1:

It is somewhat of a shame that Contains is not supported in Linq to Entities.

IN and JOIN are not the same operator (Filtering by IN never changes the cardinality of the query).

回答2:

Update: found out how to do this. And EF will generate the appropriate SQL on the database. I'm not sure if this is for EF4 only but I got the tip from Entity Framework 4.0 Recipes

var listOfIds=GetAListOfIds();
var context=CreateEntityFrameworkObjectContext();
var results = from item in context.Items
              where listOfIds.Contains(item.Category.Id)
              select item;
//results contains the items with matching category Ids

This query generates the correct in clause on the server side. I haven't tested it with EF 3.5 but it does work with EF4.

NB: The values passed into the in clause are NOT parameters so make sure you validate your inputs.

回答3:

Contains is not supported in EF at this time.

回答4:

FYI:

If you are using ESql you are able to use in operation. I don't have VS 2008 With me but code should be something like following:

var ids = "12, 34, 35";
using (context = new Entites())
{
    var selectedProducts = context.CreateQuery<Products>(
        String.Format("select value p from [Entities].Products as p 
                       where p.productId in {{{0}}}", ids)).ToList();
    ...
}

回答5:

For the cases when you want to use expressions when querying your data, you can use the following extension method (adapted after http://social.msdn.microsoft.com/forums/en-US/adodotnetentityframework/thread/095745fe-dcf0-4142-b684-b7e4a1ab59f0/):

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Data.Objects;

namespace Sample {
    public static class Extensions {
        public static IQueryable<T> ExtWhereIn<T, TValue>(this ObjectQuery<T> query,
                    Expression<Func<T, TValue>> valueSelector,
                    IEnumerable<TValue> values) {
            return query.Where(BuildContainsExpression<T, TValue>(valueSelector, values));
        }
        public static IQueryable<T> ExtWhereIn<T, TValue>(this IQueryable<T> query,
            Expression<Func<T, TValue>> valueSelector,
            IEnumerable<TValue> values) {
            return query.Where(BuildContainsExpression<T, TValue>(valueSelector, values));
        }
        private static Expression<Func<TElement, bool>> BuildContainsExpression<TElement, TValue>(
                Expression<Func<TElement, TValue>> valueSelector, IEnumerable<TValue> values) {
            if (null == valueSelector) { throw new ArgumentNullException("valueSelector"); }
            if (null == values) { throw new ArgumentNullException("values"); }
            ParameterExpression p = valueSelector.Parameters.Single();
            // p => valueSelector(p) == values[0] || valueSelector(p) == ...
            if (!values.Any()) {
                return e => false;
            }
            var equals = values.Select(value => (Expression)Expression.Equal(valueSelector.Body, Expression.Constant(value, typeof(TValue))));
            var body = equals.Aggregate<Expression>((accumulate, equal) => Expression.Or(accumulate, equal));
            return Expression.Lambda<Func<TElement, bool>>(body, p);
        }
    }
    class Program {
        static void Main(string[] args) {
            List<int> fullList = new List<int>();
            for (int i = 0; i < 20; i++) {
                fullList.Add(i);
            }

            List<int> filter = new List<int>();
            filter.Add(2);
            filter.Add(5);
            filter.Add(10);

            List<int> results = fullList.AsQueryable().ExtWhereIn<int, int>(item => item, filter).ToList();
            foreach (int result in results) {
                Console.WriteLine(result);
            }
        }
    }       
}

Using the extensions is really easy (as you can see in the sample). To use it on a database object, assuming you are filtering a table called "Product" by more than one id, you could do something like that:

class Product {
    public int Id { get; set; }
    /// ... other properties
}


List<Product> GetProducts(List<int> productIds) {    
    using (MyEntities context = new MyEntities()) {
        return context.Products.ExtWhereIn<Product, int>(product => product.Id, productIds).ToList();
    }
}

回答6:

Yes it does translate to SQL, it generates a standard IN statement like this:

SELECT [t0].[col1] FROM [table] [t0] WHERE [col1] IN ( 'Value 1', 'Value 2')

回答7:

Using the where method doesn't alway work

var results = from p in db.Products

             where p.Name == nameTextBox.Text

             select p;

来源：https://stackoverflow.com/questions/88473/how-to-do-a-where-in-values-in-linq-to-entities-3-5

标签

linq

linq-to-entities