var server = MongoServer.Create("mongodb://username:password@localhost");
var database = server["test"];
var collection = database["test"];

using (database.RequestStart())
{
    collection.Insert(new BsonDocument("x", 1));
    var result = server.GetLastError();
}

System.InvalidOperationException: Connection requires credentials.
   at MongoDB.Driver.Internal.MongoConnection.CheckAuthentication(MongoDatabasedatabase) in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Internal\MongoConnection.cs:line 243
   at MongoDB.Driver.MongoServer.AcquireConnection(MongoDatabase database, Boolean slaveOk) in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Core\MongoServer.cs:line 1126
   at MongoDB.Driver.MongoCursorEnumerator`1.AcquireConnection() in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Core\MongoCursorEnumerator.cs:line 210
   at MongoDB.Driver.MongoCursorEnumerator`1.GetFirst() in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Core\MongoCursorEnumerator.cs:line 223
   at MongoDB.Driver.MongoCursorEnumerator`1.MoveNext() in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Core\MongoCursorEnumerator.cs:line 141
   at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source)
   at MongoDB.Driver.MongoCollection.FindOneAs[TDocument](IMongoQuery query) in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Core\MongoCollection.cs:line 513
   at MongoDB.Driver.MongoCollection`1.FindOne(IMongoQuery query) in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Core\MongoCollection.cs:line 1660
   at MongoDB.Driver.MongoDatabase.RunCommandAs(Type commandResultType, IMongoCommand command) in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Core\MongoDatabase.cs:line 875
   at MongoDB.Driver.MongoDatabase.RunCommandAs[TCommandResult](IMongoCommand command) in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Core\MongoDatabase.cs:line 852
   at MongoDB.Driver.MongoServer.GetLastError() in C:\work\10gen\mongodb\mongo-csharp-driver\Driver\Core\MongoServer.cs:line 906
   at TestFree7051.Program.Main(String[] args) in c:\users\robert stam\documents\visual studio 2010\Projects\TestFree7051\Program.cs:line 23