MongoDB.Driver.MongoQueryException was unhandled
  HResult=-2146233088
  Message=QueryFailure flag was unrecognized command: $readPreference (response was {"$err" : "unrecognized command: $readPreference", "code" : 13390 }).
  Source=MongoDB.Driver
  StackTrace:
       at MongoDB.Driver.Internal.MongoReplyMessage`1.ReadFrom(BsonBuffer buffer, IBsonSerializationOptions serializationOptions) in c:\mongodb-mongo-csharp-driver-v1.6.0.4624-4-g10826e1\mongodb-mongo-csharp-driver-10826e1\Driver\Internal\MongoReplyMessage.cs:line 99
       at MongoDB.Driver.Internal.MongoConnection.ReceiveMessage[TDocument](BsonBinaryReaderSettings readerSettings, IBsonSerializationOptions serializationOptions) in c:\mongodb-mongo-csharp-driver-v1.6.0.4624-4-g10826e1\mongodb-mongo-csharp-driver-10826e1\Driver\Internal\MongoConnection.cs:line 475
       at MongoDB.Driver.MongoCursorEnumerator`1.GetReply(MongoConnection connection, MongoRequestMessage message) in c:\mongodb-mongo-csharp-driver-v1.6.0.4624-4-g10826e1\mongodb-mongo-csharp-driver-10826e1\Driver\Core\MongoCursorEnumerator.cs:line 296
       at MongoDB.Driver.MongoCursorEnumerator`1.GetFirst() in c:\mongodb-mongo-csharp-driver-v1.6.0.4624-4-g10826e1\mongodb-mongo-csharp-driver-10826e1\Driver\Core\MongoCursorEnumerator.cs:line 253
       at MongoDB.Driver.MongoCursorEnumerator`1.MoveNext() in c:\mongodb-mongo-csharp-driver-v1.6.0.4624-4-g10826e1\mongodb-mongo-csharp-driver-10826e1\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:\mongodb-mongo-csharp-driver-v1.6.0.4624-4-g10826e1\mongodb-mongo-csharp-driver-10826e1\Driver\Core\MongoCollection.cs:line 557
       at MongoDB.Driver.MongoCollection`1.FindOne(IMongoQuery query) in c:\mongodb-mongo-csharp-driver-v1.6.0.4624-4-g10826e1\mongodb-mongo-csharp-driver-10826e1\Driver\Core\MongoCollection.cs:line 1734
       at MongoDB.Driver.MongoDatabase.RunCommandAs(Type commandResultType, IMongoCommand command) in c:\mongodb-mongo-csharp-driver-v1.6.0.4624-4-g10826e1\mongodb-mongo-csharp-driver-10826e1\Driver\Core\MongoDatabase.cs:line 973
       at MongoDB.Driver.MongoDatabase.RunCommandAs[TCommandResult](IMongoCommand command) in c:\mongodb-mongo-csharp-driver-v1.6.0.4624-4-g10826e1\mongodb-mongo-csharp-driver-10826e1\Driver\Core\MongoDatabase.cs:line 950
       at MongoDB.Driver.MongoDatabase.RunCommand(IMongoCommand command) in c:\mongodb-mongo-csharp-driver-v1.6.0.4624-4-g10826e1\mongodb-mongo-csharp-driver-10826e1\Driver\Core\MongoDatabase.cs:line 928
       at MongoDB.Driver.MongoCollection.Count(IMongoQuery query) in c:\mongodb-mongo-csharp-driver-v1.6.0.4624-4-g10826e1\mongodb-mongo-csharp-driver-10826e1\Driver\Core\MongoCollection.cs:line 158
       at MongoDB.Driver.MongoCollection.Count() in c:\mongodb-mongo-csharp-driver-v1.6.0.4624-4-g10826e1\mongodb-mongo-csharp-driver-10826e1\Driver\Core\MongoCollection.cs:line 143
       at MongoTest.Program.Main(String[] args) in c:\Users\kennyi\Documents\Visual Studio 2012\Projects\MongoTest\MongoTest\Program.cs:line 18
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

mongod.exe --shardsvr --replSet rs_a --port 27020 --dbpath c:\data\rs_a\n1
mongod.exe --shardsvr --replSet rs_a --port 27021 --dbpath c:\data\rs_a\n2
mongod.exe --shardsvr --replSet rs_a --port 27022 --dbpath c:\data\rs_a\n3

mongod.exe --configsvr --port 27030 --dbpath c:\data\config\c1
mongod.exe --configsvr --port 27031 --dbpath c:\data\config\c2
mongod.exe --configsvr --port 27032 --dbpath c:\data\config\c3

mongos.exe --configdb my-machine:27030,my-machine:27031,my-machine:27032

cfg = {
    _id : "rs_a",
    members : [
        {_id : 0, host : "my-machine:27020", priority : 1},
        {_id : 1, host : "my-machine:27021", priority : 1},
        {_id : 2, host : "my-machine:27022", priority : 0}
    ]}
rs.initiate(cfg)
db.adminCommand( { addShard : "rs_a/my-machine:27020,my-machine:27021,my-machine:27022" } )

            var server = MongoServer.Create("mongodb://my-machine");
            var db = server.GetDatabase("test");
            Console.WriteLine("Found {0} transactions", db.GetCollection("test").Count());