$count (aggregation stage)（聚合阶段）

Create a collection named scores with these documents:使用以下文档创建一个名为scores的集合：

db.scores.insertMany( [
   { "_id" : 1, "subject" : "History", "score" : 88 },
   { "_id" : 2, "subject" : "History", "score" : 92 },
   { "_id" : 3, "subject" : "History", "score" : 97 },
   { "_id" : 4, "subject" : "History", "score" : 71 },
   { "_id" : 5, "subject" : "History", "score" : 79 },
   { "_id" : 6, "subject" : "History", "score" : 83 }
] )

The following aggregation operation has two stages:以下聚合操作分为两个阶段：

1. The $match stage excludes documents that have a score value of less than or equal to 80 to pass along the documents with score greater than 80 to the next stage.$match阶段不包括得80小于或等于80的文档，将80大于80的文档传递到下一阶段。
2. The $count stage returns a count of the remaining documents in the aggregation pipeline and assigns the value to a field called passing_scores.$count阶段返回聚合管道中剩余文档的计数，并将该值分配给名为passing_scores的字段。

db.scores.aggregate( [
   { $match: { score: { $gt: 80 } } },
   { $count: "passing_scores" }
] )

The operation returns this result:该操作返回以下结果：

{ "passing_scores" : 4 }

If the input dataset is empty, $count doesn't return a result. The following example doesn't return a result because there are no documents with scores greater than 99:如果输入数据集为空，$count不会返回结果。以下示例不返回结果，因为没有得分大于99的文档：

db.scores.aggregate( [
   { $match: { score: { $gt: 99 } } },
   { $count: "high_scores" }
] )

The C# examples on this page use the sample_mflix database from the Atlas sample datasets. 本页上的C#示例使用Atlas示例数据集中的sample_mflix数据库。To learn how to create a free MongoDB Atlas cluster and load the sample datasets, see Get Started in the MongoDB .NET/C# Driver documentation.要了解如何创建免费的MongoDB Atlas集群并加载示例数据集，请参阅MongoDB .NET/C#驱动程序文档中的入门。

The following Movie class models the documents in the sample_mflix.movies collection:以下Movie类对sample_mflix.movies集合中的文档进行建模：

public class Movie
{
    public ObjectId Id { get; set; }

    public int Runtime { get; set; }
    
    public string Title { get; set; }

    public string Rated { get; set; }

    public List<string> Genres { get; set; }

    public string Plot { get; set; }
    
    public ImdbData Imdb { get; set; }

    public int Year { get; set; }

    public int Index { get; set; }
    
    public string[] Comments { get; set; }
   
    [BsonElement("lastupdated")]
    public DateTime LastUpdated { get; set; }
}

var camelCaseConvention = new ConventionPack { new CamelCaseElementNameConvention() };
ConventionRegistry.Register("CamelCase", camelCaseConvention, type => true);

To use the MongoDB .NET/C# driver to add a $count stage to an aggregation pipeline, call the Count() method on a PipelineDefinition object.要使用MongoDB .NET/C#驱动程序向聚合管道添加$count阶段，请在PipelineDefinition对象上调用Count()方法。

The following example creates a pipeline stage that counts the number of input documents and returns a document with the count as its value:以下示例创建了一个管道阶段，该阶段对输入文档的数量进行计数，并返回一个以计数为值的文档：

var pipeline = new EmptyPipelineDefinition<Movie>()
    .Count();

The Node.js examples on this page use the sample_mflix database from the Atlas sample datasets. 本页上的Node.js示例使用Atlas示例数据集中的sample_mflix数据库。To learn how to create a free MongoDB Atlas cluster and load the sample datasets, see Get Started in the MongoDB Node.js driver documentation.要了解如何创建免费的MongoDB Atlas集群并加载示例数据集，请参阅MongoDB Node.js驱动程序文档中的入门。

To use the MongoDB Node.js driver to add a $count stage to an aggregation pipeline, use the $count operator in a pipeline object.要使用MongoDB Node.js驱动程序向聚合管道添加$count阶段，请在管道对象中使用$count运算符。

The following example creates a pipeline stage that counts the number of input documents from the sample_mflix.movies collection and returns a document containing the count. 以下示例创建了一个管道阶段，该阶段对sample_mflix.movies集合中的输入文档数量进行计数，并返回一个包含该计数的文档。The example then runs the aggregation pipeline:然后，该示例运行聚合管道：

const pipeline = [{ $count: "movies" }];

const cursor = collection.aggregate(pipeline);
return cursor;