c# - 如何使用 LINQ 连接三个数据源？

Question

我有以下内容：

var questions = questionService.Details(pk, rk);
var topics = contentService.GetTitles("0006000")
                .Where(x => x.RowKey.Substring(2, 2) != "00");
model = (
    from q in questions
    join t in topics on q.RowKey.Substring(0, 4) equals t.RowKey into topics2
    from t in topics2.DefaultIfEmpty()
    select new Question.Grid {
        PartitionKey = q.PartitionKey,
        RowKey = q.RowKey,
        Topic = t == null ? "No matching topic" : t.Title,
        ...

现在我需要添加以下内容：

var types = referenceService.Get("07");

questions.type和之间有联系的地方types.RowKey

如果类型表中没有任何匹配项，有没有办法可以链接这第三个数据源并让它给出一条消息“没有匹配的类型”？我的问题是我不太确定如何进行下一次加入。

Answer 1

        var questions = questionService.Details(pk, rk);
        var topics = contentService.GetTitles("0006000")
                        .Where(x => x.RowKey.Substring(2, 2) != "00");
        var types = referenceService.Get("07");
        model = (from q in questions
            join t in topics on q.RowKey.Substring(0, 4) equals t.RowKey into topics2
            from t in topics2.DefaultIfEmpty())
            select new Question.Grid {
                PartitionKey = q.PartitionKey,
                RowKey = q.RowKey,
                Topic = t == null ? "No matching topic" : t.Title,
                ...
            }).Union
            (from a in types
             join q in questions on q.type equals a.RowKey 
             where a.ID != null
             select new Question.Grid {
                    PartitionKey = q.PartitionKey,
                    RowKey = q.RowKey,
                    Topic = t == null ? "No matching topic" : t.Title,
                    ...
            })

Answer 2

我相信您可以将第一次连接的结果与第三个数据源连接起来：

var questions = questionService.Details(pk, rk);
var topics = contentService.GetTitles("0006000")
                .Where(x => x.RowKey.Substring(2, 2) != "00");
var  types = referenceService.Get("07");

model = (
    from q in questions
    join t in topics on q.RowKey.Substring(0, 4) equals t.RowKey into topics2
    from t in topics2.DefaultIfEmpty()
    join type in types on q.type equals type.RowKey into types2
    from type in types2.DefaultIfEmpty()
    select new Question.Grid {
        PartitionKey = q.PartitionKey,
        RowKey = q.RowKey,
        Topic = t == null ? "No matching topic" : t.Title,
        Type = type == null ? "No matching type" : type.Title,
        ...