MongoDB (server v 2.6.7) with C# driver 2.0: How to get the result from InsertOneAsync

Question

I am testing MongoDB (server v 2.6.7) with the C# driver 2.0.

When I am using the insert function InsertOneAsync for a document with an _id which exists I am expecting an error like the one you get from the Mongo shell:

WriteResult({
    "nInserted" : 0,
    "writeError" : {
            "code" : 11000,
            "errmsg" : "insertDocument :: caused by :: 11000 E11000 duplicate key error index: mydb.Commands.$_id_  dup key: { : 0.0 }"
    }})

But the problem is that the insert with the C# driver does not throw an exception and I can not find the WriteResult for the insert. When I look in the database it seems nothing have happened.

So my question is what to expect from InsertOneAsync when inserting an existing _id?

The code in Visual Studio:

IMongoCollection<BsonDocument> commandsCollection = db.GetCollection<BsonDocument>("Commands");
var bson = new BsonDocument
        {
            {"_id", i.Value},
            {"label", i.Key}
        };
commandsCollection.InsertOneAsync(bson);

Answer 1

If you're doing this within an async method, then Brduca's answer will work (and is preferrable), otherwise you can call Wait() on the Task returned from the InsertOneAsync call to ensure your application stays around long enough to see the duplicate key exception:

commandsCollection.InsertOneAsync(doc).Wait();

If the insert fails because of a duplicate key, the Wait() will throw an AggregateException that contains a MongoWriteException that contains the duplicate key details.

try
{
    commandsCollection.InsertOneAsync(doc).Wait();
}
catch(AggregateException aggEx)
{
    aggEx.Handle(x => 
    { 
        var mwx = x as MongoWriteException;
        if (mwx != null && mwx.WriteError.Category == ServerErrorCategory.DuplicateKey) 
        {
            // mwx.WriteError.Message contains the duplicate key error message
            return true; 
        }
        return false;
    });
}

Similarly, if you're using await, that will throw an AggregateException as well.

To avoid the added complexity of the AggregateException wrapping the mongo exception, you can call GetAwaiter().GetResult() instead of Wait():

try
{
    commandsCollection.InsertOneAsync(doc).GetAwaiter().GetResult();
}
catch(MongoWriteException mwx)
{
    if (mwx.WriteError.Category == ServerErrorCategory.DuplicateKey) 
    {
        // mwx.WriteError.Message contains the duplicate key error message
    }
}

Answer 2

This is an async Task, you're missing the await

await commandsCollection.InsertOneAsync(bson);

https://github.com/mongodb/mongo-csharp-driver/blob/master/README.md