How to Filter Data in Linq to Entities Using a List of IDs Without \'Contains()\'?

Linq to Entities 'Contains()' Alternative for ID Filtering

In working with Silverlight ADO.Net Data Services, the absence of 'Contains()' in Linq to Entities poses a challenge when filtering data using a list of IDs. As an alternative, 'Any()' can be employed:

<code class="csharp">List<long?> txnIds = new List<long?>();
// Fill list

var q = from t in svc.OpenTransaction
        where txnIds.Any<long>(tt => tt == t.OpenTransactionId)
        select t;</code>

However, this approach yields an "The method 'Any' is not supported" error.

Workaround using Custom Method

To overcome this limitation, a custom LINQ method, 'WhereIn()', can be defined:

<code class="csharp">public static IQueryable<TEntity> WhereIn<TEntity, TValue>
  (
    this ObjectQuery<TEntity> query,
    Expression<Func<TEntity, TValue>> selector,
    IEnumerable<TValue> collection
  )
{
  if (selector == null) throw new ArgumentNullException("selector");
  if (collection == null) throw new ArgumentNullException("collection");
  if (!collection.Any()) 
    return query.Where(t => false);

  ParameterExpression p = selector.Parameters.Single();

  IEnumerable<Expression> equals = collection.Select(value =>
     (Expression)Expression.Equal(selector.Body,
          Expression.Constant(value, typeof(TValue))));

  Expression body = equals.Aggregate((accumulate, equal) =>
      Expression.Or(accumulate, equal));

  return query.Where(Expression.Lambda<Func<TEntity, bool>>(body, p));
}</code>

This method accepts a selector expression and a collection of values for comparison. It generates a series of equality expressions that are then combined using an 'OR' operator. The resulting expression is used to filter the query.

Usage

The 'WhereIn()' method can be used as follows:

<code class="csharp">var contacts = context.Contacts.WhereIn(c => c.Name, GetContactNames());</code>

This code filters the 'Contacts' table by matching names in the 'GetContactNames()' collection.

Update (EF 4 and Later)

It's worth noting that Entity Framework versions 4 and later directly support the 'Contains()' method, making this workaround obsolete.

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