使用Linq to Entities的“ Contains（）”解决方法？

Z时代
2024-01-10
分类：问答

我正在尝试创建一个查询，该查询使用Silverlight ADO.Net数据服务客户端api（并因此使用Linq To

Entities）在where子句中使用ID列表。有人知道不支持“包含”的解决方法吗？

我想做这样的事情：

List<long?> txnIds = new List<long?>();
// Fill list
var q = from t in svc.OpenTransaction
        where txnIds.Contains(t.OpenTransactionId)
        select t;

试过这个：

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

但是得到了“不支持方法’Any’”。

回答：

EF≥4 Contains直接支持（Checkout

Any），因此您不需要任何解决方法。

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));
}
//Optional - to allow static collection:
public static IQueryable<TEntity> WhereIn<TEntity, TValue>
  (
    this ObjectQuery<TEntity> query,
    Expression<Func<TEntity, TValue>> selector,
    params TValue[] collection
  )
{
  return WhereIn(query, selector, (IEnumerable<TValue>)collection);
}

用法：

public static void Main()
{
  using (MyObjectContext context = new MyObjectContext())
  {
    //Using method 1 - collection provided as collection
    var contacts1 =
      context.Contacts.WhereIn(c => c.Name, GetContactNames());
    //Using method 2 - collection provided statically
    var contacts2 = context.Contacts.WhereIn(c => c.Name,
      "Contact1",
      "Contact2",
      "Contact3",
      "Contact4"
      );
  }
}