MongoDB C＃2.0驱动程序-如何计算服务器上的聚合计数？

您应该再用$ sum添加一个组

var result = await collection
        .Aggregate()
        .Group(groupby)
        .Match(query)
        .Group(new BsonDocument
        {
            { "_id", "_id" },
            {"count", new BsonDocument("$sum", 1)}
        })
        .FirstAsync();
        var count = result["count"].AsInt32;