vs2015c#查询oracle10g乱码

引用Oracle.ManagedDataAccess.dll
数据库为32位10g，字符集为AMERICAN_AMERICA.WE8ISO8859P1
可以查询到数据，不过汉字部分都是乱码。
求解决办法，谢谢。
目前代码如下：
public string getItemInfo(string Code)
        {
            _putenv("NLS_LANG = AMERICAN_AMRICA.WE8ISO8859P1");//没用
            string returnstr = "";
            Environment.SetEnvironmentVariable("NLS_LANG", "AMERICAN_AMERICA.WE8ISO8859P1", EnvironmentVariableTarget.Process);//没用

            //Create a connection to Oracle
            OracleConnection con = new OracleConnection();
            //Web.config连接字符串
            con.ConnectionString = System.Configuration.ConfigurationManager.ConnectionStrings["conn"].ConnectionString;
            try
            {
                con.Open();

                OracleGlobalization sessionGlob = con.GetSessionInfo();//没用
                sessionGlob.Language = "AMERICAN";//没用
                con.SetSessionInfo(sessionGlob);//没用

                OracleCommand cmmd = new OracleCommand("select * from VIEW_ITEMINFO where code='"+Code + "'", con);
                OracleDataAdapter oa = new OracleDataAdapter(cmmd);

                System.Data.DataSet ds = new System.Data.DataSet();
                oa.Fill(ds, "ItemInfo");

                foreach (System.Data.DataRow row in ds.Tables[0].Rows)
                {
                    foreach (System.Data.DataColumn column in ds.Tables[0].Columns)
                    {
                        returnstr = returnstr + row[column] + "\t";
                    }
                }
                return returnstr;
            }
            catch
            {
                return "打开失败！";
            }
            finally
            {
                con.Close();
                con.Dispose();
            }
        }