.net,mongodb: json reader was expecting a value but found 'db'

getting json reader was expecting a value but found 'db' .net error while passing a query from c# to mongodb.

below is the code.

mquery="db.getcollection('colname').find({mesgid:{$in:['m1','m2']}}, 
{"_id":1},{"mesgId":1})";
var cmd= new JsonCommand<BasonDocument>(mquery);
var monclient=new MongoClient(constring);
var res=monclient.GetDatabase("db_name").RunCommand(cmd);

when the debugger hits RunCommand, then it is throwing " json reader was expecting a value but found 'db'" error. can anyone help me out here.