MongoDB C# Aggregate Count

Question

I wish to use the C# Driver (V 2.7.0) to aggregate on a collection and then count the results.

Assuming I have an IMongoCollection col, I do the aggregate like this:

IAsyncCursor<BsonDocument> cursor = col.Aggregate()
    .Match(someFilterDefinition)
    .Project(someProjectionDefinition)
    .Unwind("someFieldName")
    .ToCursor();
while (cursor.MoveNext())
{
    foreach (BsonDocument doc in cursor.Current)
    {
        doStuff(doc);
    }
}

I wish to get the number of documents returned by the aggregation. in the shell you would do something like this

db.getCollection("someCollectionName").aggregate(
    [
        { 
            "$match" : {
                // some match filter
            }
        }, 
        { 
            "$project" : {
                "someField" : 1
            }
        }, 
        { 
            "$unwind" : "$someField"
        }, 
        { 
            "$count" : "count"
        }
    ], 
    { 
        "allowDiskUse" : false
    }
);

and you would get a document (or no document) with a single "count" field, having an int64 value of the number of elements from the stage before it.

There is an IAggregateFluent.Count() Function. However it returns an IAggregateFluent I want an AggregateCountResult or just a simple unsigned long.

How does one use the Aggregate.Count() function?

FalcoGer · Accepted Answer · 2019-02-05 09:30:37Z

13

Instead of just appending Count(), which gets you an aggregation stage, you must finish the aggregation with either .ToCursor or .FirstOrDefault or similar. The FirstOrDefault returns the AggregateCountResult, which has the Count property.

ulong count = col.Aggregate()
    .Match(someFilterDefinition)
    .Project(someProjectionDefinition)
    .Unwind("someFieldName")
    .Count()
    .FirstOrDefault() // returns AggregateCountResult
    .Count();         // returns ulong

edited Feb 5, 2019 at 9:30

answered Dec 19, 2018 at 13:49

FalcoGer

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2 Comments

Mathias Over a year ago

It should be .FirstOrDefault().Count; without brackets. In current version (2.9.1) it also returns a long instead of ulong.

Mathias Over a year ago

Be careful: When there are no entries to count then .Count() will return no element. Therefor .FirstOrDefault() is null and .Count will throw a NullReferenceException.

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