LINQ to Entities仅支持无参数构造函数和初始器报错如何解决

错误原因 #

报错Only parameterless constructors and initializers are supported in LINQ to Entities的核心原因是：EF的LINQ to Entities需要将查询表达式翻译为SQL语句执行，无法直接识别并转换自定义类的带参数构造函数调用。你代码里new ParameterlessTuple<...>(conf, yes, no, cancel)用了带4个参数的构造方法，触发了这个限制。
你之前添加.ToList()无效，是因为你把调用放在了整个查询末尾，LINQ to Entities依然会先尝试把整个表达式翻译为SQL，带参构造的部分仍然无法被识别。

解决方法 #

方案1（优先选择，性能更好） #

如果需要保持属性返回IQueryable类型，且后续还要基于这个查询拼接数据库过滤条件，给ParameterlessTuple添加无参构造函数，然后改用对象初始化器赋值即可。假设ParameterlessTuple的四个属性对应为Item1/Item2/Item3/Item4，修改后的select部分如下：

select new ParameterlessTuple<tcDataCollectionConfiguration, tcDataCollectionConfiguration, tcDataCollectionConfiguration, tcDataCollectionConfiguration>
{
    Item1 = conf,
    Item2 = yes,
    Item3 = no,
    Item4 = cancel
}

方案2（适用于查询后不需要额外拼接数据库过滤的场景） #

先把匹配的数据库数据加载到内存，再执行Tuple构造，修改完整代码如下：

internal IQueryable<ParameterlessTuple<tcDataCollectionConfiguration, tcDataCollectionConfiguration, tcDataCollectionConfiguration, tcDataCollectionConfiguration>> DataCollectionAcessQuery
{
    get => (from conf in (from conf in this.Scope.Set<tcDataCollectionConfiguration>() where conf.DCC_ParameterName == "conf" select conf)
            join yes in (from yes in this.Scope.Set<tcDataCollectionConfiguration>() where yes.DCC_ParameterName == "YesCode" select yes) on conf.DCC_StationID equals yes.DCC_StationID
            join no in (from no in this.Scope.Set<tcDataCollectionConfiguration>() where no.DCC_ParameterName == "NoCode" select no) on conf.DCC_StationID equals no.DCC_StationID
            join cancel in (from cancel in this.Scope.Set<tcDataCollectionConfiguration>() where cancel.DCC_ParameterName == "cancel" select cancel) on conf.DCC_StationID equals cancel.DCC_StationID
            // 先将查询到的实体映射为匿名类型加载到内存，后续操作转为LINQ to Objects，不受EF转SQL的语法限制
            select new { conf, yes, no, cancel }).AsEnumerable()
            .Select(x => new ParameterlessTuple<tcDataCollectionConfiguration, tcDataCollectionConfiguration, tcDataCollectionConfiguration, tcDataCollectionConfiguration>(x.conf, x.yes, x.no, x.cancel))
            .AsQueryable();
}

注意：该方案后续对DataCollectionAcessQuery追加的过滤、排序等操作都会在内存执行，不会转成SQL到数据库侧执行，数据量较大时会有性能损耗。

内容的提问来源于stack exchange，提问作者Ken