Entity framework WhereInExtension

摘自 http://www.cnblogs.com/ejiyuan/archive/2009/07/20/1527224.html

    public static class WhereInExtension
    {
       private static Expression<Func<TElement, bool>> BuildWhereInExpression<TElement, TValue>(Expression<Func<TElement, TValue>> propertySelector, IEnumerable<TValue> values)
       {
           ParameterExpression p = propertySelector.Parameters.Single();
           if (!values.Any())
               return e => false;

           var equals = values.Select(value => (Expression)Expression.Equal(propertySelector.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);
       }

       public static IQueryable<TElement> WhereIn<TElement, TValue>(this IQueryable<TElement> source, Expression<Func<TElement, TValue>> propertySelector, params TValue[] values)
       {
           return source.Where(BuildWhereInExpression(propertySelector, values));
       }
    }