CODEWITHSUNDEEP
c#mongodb
zrbecker
zrbecker

Reputation: 1592

MongoDB C# Driver 2.0 and ForEachAsync

I am not very familiar with async in C#, but I am trying to understand it better. I am trying to process a list of documents in Mongo, and I am not sure if I have a race condition or if async is special causing this to be fine. Here is my example.

var docs = new Dictionary<string, BsonDocument>();
var result = db.GetCollection<BsonDocument>("mycollection").Find(new BsonDocument());
result.ForEachAsync((bsonDoc) =>
{
    string name = bsonDoc.GetValue("name").AsString;
    if (!docs.ContainsKey(name))
    {
        docs[name] = bsonDoc;
    }
});

It seems like if two documents had the same name, then that if statement could be executed twice for the same name.

I am not sure why there doesn't seem to be a straight forward was to do a synchronous for each with the new mongo driver.

Can someone tell me how I should structure the above code, or why it isn't a race condition if I am wrong?

Thank you.

Upvotes: 1

Views: 5344

Answers (1)

Colin
Colin

Reputation: 4135

According to this:

http://mongodb.github.io/mongo-csharp-driver/2.0/reference/driver/crud/reading/

Finding all the documents in a collection is done with an empty filter and the method Find. Once we have a cursor (of type IAsyncCursor), we can iterate it like we manually iterate an IEnumerable.

So Find() is returning an async cursor, which only ever points to one document at a time. The documentation doesn't say explicitly that ForEachAsync() won't move the cursor position till after your lambda finishes, but that's presumably how it works (otherwise, the point of a cursor is defeated since you're loading up more than one record at a time into memory).

Thus, you should be safe doing what you are, and shouldn't encounter a race condition.

Upvotes: 2

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