Question

我有一个搜索框，我已将数据分解为单词。我现在需要将每个单词与数据库中的几个字段进行比较。这是我的代码：

List<string> searchValues = searchString.Split(delimiters, StringSplitOptions.RemoveEmptyEntries).ToList();

using (DatabaseEntities context = new DatabaseEntities())
{
    IQueryable<DatabaseType> returnValue =
    context.DatabaseType.Where(y =>
        y.Field1.Contains(searchValues[0]) ||
        y.Field1.Contains(searchValues[0]));

    searchValues.Skip(1).ToList().ForEach(x =>
    {
        returnValue = returnValue.Where(y =>
            y.Field1.Contains(x) ||
            y.Field2.Contains(x));
    });

    return returnValue.ToList();

我的目标是在将项目实际加载到内存之前构建完整查询。但是，当我尝试运行它时，我收到以下错误：

System.NotSupportedException: LINQ to Entities does not recognize the method &#39;System.String get_Item(Int32)&#39; method, and this method cannot be translated into a store expression.

我原以为这种类型的错误意味着我尝试的操作在转换为SQL时不起作用，但我很确定我之前使用过Contains作为LIKE语句。此外，数据库中的所有信息都是nvarchars，因此这些信息等同于我的Entity中的字符串。有没有人知道问题是什么？使用IQueryable是最好的方法吗？如果可能的话，我宁愿懒得加载所有这些。

Answer 1

get_Item（Int32）调用将成为列表的索引。在你的情况下，他们是searchValues [0]。使用：

var searchValue = searchValues[0];

IQueryable<DatabaseType> returnValue =
context.DatabaseType.Where(y =>
    y.Field1.Contains(searchValue) ||
    y.Field1.Contains(searchValue));

Answer 2

我使用下面的扩展方法为Linq to Entities构建Contains类型表达式（在注释中引用原始源代码）。我认为它也适用于这种情况。

    /// <summary>
    /// Extension method that enables .Contains(obj) like functionality for Linq to Entities.
    /// 
    /// Source: http://www.velocityreviews.com/forums/t645784-linq-where-clause.html
    /// </summary>
    /// <typeparam name="TElement">The element being evaluated by the Where clause</typeparam>
    /// <typeparam name="TValue">The value to match</typeparam>
    /// <param name="valueSelector">Lamda for selecting matching values</param>
    /// <param name="values">IEnumerable of the values</param>
    /// <returns>Expression consumable by Linq to Entities that reflects semantics of .Contains(value)</returns>
    /// <remarks>
    /// Usage:
    /// 
    /// Replace expression like 
    /// 
    /// where ChildrenIDs.Contains(items.CategoryID)
    /// 
    /// with
    /// 
    /// .Where((BuildContainsExpression<Item, int>(item => item.CategoryID, ChildrenIDs))
    /// 
    /// NOTE: If the item collection is large, the SQL query will be as well.
    /// </remarks>
    static public 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();
        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);
    }

示例用法：

var exprWithContains = 
    LinqToEntitiesUtil.BuildContainsExpression<List<int?>, int?>
        (p => p.ProductYear, productYears);

var result = (from p in products select p).Where(exprWithContains);

Linq to Entity通过循环数据构建

2 个答案: