Linq查詢結果不支持類型'System.Linq.IQueryable`1 [System.Int32]的比較運算符

Question

我有以下功能：

public List<DTO.ArticleDTO> GetRecentArticles(int portalID, int moduleID, int domainID, int rootKnowledgeTypeID, int count)
{
    var articles =
        (from
            article in _Context.Articles.LatestArticles(_Context.ArticleVersions, portalID, moduleID)
            where _GetRootDomainsForArticle(article.ArticleID, article.Version, rootKnowledgeTypeID).Contains(domainID)
        orderby article.UpdatedOn descending
            select new ArticleDTO
            {
                ArticleID = article.ArticleID,
                Title = article.Title,
                Summary = article.Introduction,
                Content = article.Content,
                UpdatedOn = article.UpdatedOn,
                Version = article.Version,
                KnowledgeTypeText = (from
                                        category in _Context.Categories
                                    join
                                        categoryVersion in _Context.ArticleCategoryVersions
                                    on
                                        category.CategoryID equals categoryVersion.CategoryID
                                    where
                                        categoryVersion.Version == article.Version &&
                                        categoryVersion.ArticleID == article.ArticleID &&
                                        category.ParentID == rootKnowledgeTypeID
                                    select
                                        category.Name
                                ).First()
            });

    return articles.Take(count).ToList();
}

我想做的是獲取所有屬於根類別（域ID）的文章。 類別被認為是“域/子域”，每篇文章可以屬於一個或多個“域和子域”。 所以這就是為什么我在上面的linq查詢的where子句中使用以下方法的原因。

private IQueryable<int> _GetRootDomainsForArticle(int articleID, int version, int rootKnowledgeTypeID)
{
    return from c in _Context.Categories
                    join cv in _Context.ArticleCategoryVersions on c.CategoryID equals cv.CategoryID
                    where cv.ArticleID == articleID
                    where cv.Version == version
                    where c.ParentID == null
                    where c.CategoryID != rootKnowledgeTypeID
                    select c.CategoryID;
}

我得到的錯誤是：“類型'System.Linq.IQueryable'1 [System.Int32]'不支持比較運算符。”

我認為是因為執行延遲，但我不太了解正確的執行方式。

編輯：

基於@SivaGopal注釋，我將查詢從_GetRootDomainsForArticle移到了主查詢中，並且它起作用了：

public List<DTO.ArticleDTO> GetRecentArticles(int portalID, int moduleID, int domainID, int rootKnowledgeTypeID, int count)
{
    var articles =
        (from
            article in _Context.Articles.LatestArticles(_Context.ArticleVersions, portalID, moduleID)
            where (from c in _Context.Categories
                join cv in _Context.ArticleCategoryVersions on c.CategoryID equals cv.CategoryID
                where cv.ArticleID == article.ArticleID
                where cv.Version == article.Version
                where c.ParentID == null
                where c.CategoryID != rootKnowledgeTypeID
                select c.CategoryID).Contains(domainID)
            orderby article.UpdatedOn descending
            select new ArticleDTO
            {
                ArticleID = article.ArticleID,
                Title = article.Title,
                Summary = article.Introduction,
                Content = article.Content,
                UpdatedOn = article.UpdatedOn,
                Version = article.Version,
                KnowledgeTypeText = (from
                                category in _Context.Categories
                                    join
                                categoryVersion in _Context.ArticleCategoryVersions
                            on
                                category.CategoryID equals categoryVersion.CategoryID
                                    where
                                categoryVersion.Version == article.Version &&
                                categoryVersion.ArticleID == article.ArticleID &&
                                category.ParentID == rootKnowledgeTypeID
                                    select
                                category.Name
                        ).First()
            });

    return articles.Take(count).ToList();
}

我對Linq的有限了解使我相信，可以將查詢分解為一種查詢組合形式的方法，從而使它可重用。 似乎並非如此，但是肯定有一種方法可以這樣做，因為它將大大減少代碼，特別是在查詢的某些部分經常被重用的情況下。

Answer 1

嘗試進一步拆分功能，可以使用冗長而深入的Linq方法，但對於可讀性而言，它不是那么好，並且可能會引入您正在談論的此類錯誤。 我尚未對此進行測試，但它應該有助於解決您的問題：

public List<DTO.ArticleDTO> GetRecentArticles(int portalID, int moduleID, int domainID, int rootKnowledgeTypeID, int count)
{
    var allArticles = _Context.Articles.LatestArticles(_Context.ArticleVersions, portalID, moduleID);
    var filteredArticles = new List<DTO.ArticleDTO>();

    foreach (DTO.ArticleDTO article in allArticles)
    {
        var domains = _GetRootDomainsForArticle(article.ArticleID, article.Version, rootKnowledgeTypeID);
        if (domains.Contains(domainID)
        {
            filteredArticles.Add(article);
        }
    }

    var articles = filteredArticles.OrderBy(article.UpdatedOn).Descending().Select(article => new DTO.ArticleDTO {
        ArticleID = article.ArticleID,
        Title = article.Title,
        Summary = article.Introduction,
        Content = article.Content,
        UpdatedOn = article.UpdatedOn,
        Version = article.Version,
        KnowledgeTypeText = (from category in _Context.Categories
                                          join categoryVersion in _Context.ArticleCategoryVersions
                                          on category.CategoryID equals categoryVersion.CategoryID
                                          where
                                            categoryVersion.Version == article.Version &&
                                            categoryVersion.ArticleID == article.ArticleID &&
                                            category.ParentID == rootKnowledgeTypeID
                                          select
                                            category.Name
                                        ).First()
    }).Take(count).ToList();
}

您還絕對需要從_GetRootDomainsForArticle函數返回IQueryable <>嗎？ 如果可以解決問題，我將嘗試返回IEnumerable <>。

幸田