c# - 动态+linq编译报错

Question

我会先声明，我正在使用动态数据上的 linq 做一些非常可怕的事情。 但是我不明白为什么这个查询编译失败:

错误 1 ​​属性 '<>h__TransparentIdentifier0' 不能与类型参数一起使用

public class Program
{
    public static void Main(string[] args)
    {
        var docs = new dynamic[0];
        var q = from doc in docs
                where doc["@metadata"]["Raven-Entity-Name"] == "Cases"
                where doc.AssociatedEntities != null
                from entity in doc.AssociatedEntities
                where entity.Tags != null // COMPILER ERROR HERE
                from tag in entity.Tags
                where tag.ReferencedAggregate != null
                select new {tag.ReferencedAggregate.Id, doc.__document_id};
    }
}

public static class LinqOnDynamic
{
    private static IEnumerable<dynamic> Select(this object self)
    {
        if (self == null)
            yield break;
        if (self is IEnumerable == false || self is string)
            throw new InvalidOperationException("Attempted to enumerate over " + self.GetType().Name);

        foreach (var item in ((IEnumerable) self))
        {
            yield return item;
        }
    }

    public static IEnumerable<dynamic> SelectMany(this object source,
                                                    Func<dynamic, int, IEnumerable<dynamic>> collectionSelector,
                                                    Func<dynamic, dynamic, dynamic> resultSelector)
    {
        return Enumerable.SelectMany(Select(source), collectionSelector, resultSelector);
    }

    public static IEnumerable<dynamic> SelectMany(this object source,
                                                    Func<dynamic, IEnumerable<dynamic>> collectionSelector,
                                                    Func<dynamic, dynamic, dynamic> resultSelector)
    {
        return Enumerable.SelectMany(Select(source), collectionSelector, resultSelector);
    }

    public static IEnumerable<dynamic> SelectMany(this object source,
                                                    Func<object, IEnumerable<dynamic>> selector)
    {
        return Select(source).SelectMany<object, object>(selector);
    }

    public static IEnumerable<dynamic> SelectMany(this object source,
                                                                    Func<object, int, IEnumerable<dynamic>> selector)
    {
        return Select(source).SelectMany<object, object>(selector);

    }
}

雪上加霜的是，以下工作:

var docs = new dynamic[0];
var q = from doc in docs
        where doc["@metadata"]["Raven-Entity-Name"] == "Cases"
        where doc.AssociatedEntities != null
        from entity in doc.AssociatedEntities
        where entity.Tags != null
        from tag in entity.Tags
        select new { tag.ReferencedAggregate.Id, doc.__document_id };

只有当我添加:

其中 tag.ReferencedAggregate != null

我在两行之前得到一个错误:

where entity.Tags != null//此处编译器错误

不知道发生了什么

Answer 1

如果我尝试将您的调用转换为:

var q = from doc in docs.Where(doc => doc["@metadata"]["Raven-Entity-Name"] == "Cases" || doc.AssociatedEntities != null)
        from entity in doc.AssociatedEntities.Where(entity => entity.Tags != null)

我得到一个不同的编译器错误，它可能揭示了正在发生的事情:

“如果不首先将 lambda 表达式转换为委托(delegate)或表达式树类型，则不能将其用作动态分派(dispatch)操作的参数”

所以我猜你必须重载 Where 运算符。