使用Npgsql for Postgresql的C#查询显示重复结果和缺少表数据

我正在检查PostgreSQL作为SQLServer的潜在替代品,我在PostgreSQL公共模式的测试数据库中创建了一个测试表,并在测试表中添加了两行数据.

现在的问题是当使用NpgSQL.dll从C#.net运行简单查询时,我得到重复的结果,并不是所有的表数据都显示出来.

这是我使用的代码：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data;
using Npgsql;


namespace PlayingWithPostgres
{
    class Program
    {
        static void Main(string[] args)
        {
            // creating the connection string (Server, Port, User id, password, database)
            string conStr = "Server=127.0.0.1; Port=5432; User Id=postgres; Password=Sada1973; Database=CarsTestDB;";
            NpgsqlConnection conn = new NpgsqlConnection(conStr);
            string comStr = "Select * FROM \"CarsTable\";";
            NpgsqlCommand com = new NpgsqlCommand(comStr, conn);
            NpgsqlDataAdapter ad = new NpgsqlDataAdapter(com);
            DataTable dt = new DataTable();
            Console.WriteLine("Conection to server established successfuly \n");
            // check if connection is open or not
            if(conn != null && conn.State == ConnectionState.Open)
            {
                Console.WriteLine("Connection Open");
                conn.Close();
            }
            else
            {
                conn.Open();
            }

            // Fill data table with data and start reading
            ad.Fill(dt);
            NpgsqlDataReader dRead = com.ExecuteReader();

            try
            {
                Console.WriteLine("Contents of table in database: \n");
                while (dRead.Read())
                {
                    foreach(DataRow row in dt.Rows)
                    {
                        for (int i = 0; i < dt.Rows.Count; i++)
                        {
                            Console.Write("{0} \t \n", row[i].ToString());
                        }
                    }
                }
            }
            catch (NpgsqlException ne)
            {
                Console.WriteLine("Problem connecting to server, Error details {0}", ne.ToString());
            }
            finally
            {
                Console.WriteLine("Closing connections");
                dRead.Close();
                dRead = null;
                conn.Close();
                conn = null;
                com.Dispose();
                com = null;
            }
        }
    }
}

解决方法:

重复内容的问题是由使用While(dRead.Read())的循环和使用foreach的表的DataRows循环引起的.这有效地在您的数据上循环两次.

如果要使用DataReader循环遍历记录,则不需要DataRow和DataTable,而是使用DataReader的FieldCount属性和DataReader索引器作为当前记录.

// Not needed
// ad.Fill(dt);
NpgsqlDataReader dRead = com.ExecuteReader();

while (dRead.Read())
{
   for(int i = 0; i < dRead.FieldCount; i++)
       Console.Write("{0} \t \n", dRead[i].ToString());
}

相反,如果你想循环遍历DataTable及其行,则需要使用Columns.Count进行循环

ad.Fill(dt);
// Not needed
// NpgsqlDataReader dRead = com.ExecuteReader();
foreach(DataRow row in dt.Rows)
{
    for (int i = 0; i < dt.Columns.Count; i++)
    {
        Console.Write("{0} \t \n", row[i].ToString());
    }
}

标签：c,database,postgresql,datasource,npgsql
来源： https://codeday.me/bug/20190624/1275939.html