bboys

DataSet ds = null;

instead of DataSet ds = null; use Dataset ds=new Dataset();

I have created method as follows

[OperationContract]
        public DataSet GetData()
        {
            DataSet ds = new DataSet();

            return ds;
        }

and i called like as follows 

ServiceRef.Service1Client cleint = new ServiceRef.Service1Client();
            DataSet ds=cleint.GetData();

I didn't get any Error while accessing from client,its working fine.

thanks