获取表的主键

public static string GetPrimaryKey(string tableName)
  {
            string sql2=String.Format("exec sp_pkeys @table_name='{0}'",tableName);
   SqlDataReader reader=null;
   string key = "";
   try
   {
    reader = DBHelper.ExecuteReader(sql2);
    if(reader != null)
    {
     while (reader.Read())
     {
      key= reader["COLUMN_NAME"].ToString();
     }
    }
    return key;
   }
   catch(Exception ex)
   {
    throw ex;
   }
   finally{
    if(reader != null)
     reader.Close();
   }
  }