System.Data.SqlClient.SqlConnection: 用方法打开和关闭SqlConnection，在Main方法中读取和写入

Question

代码：

using System;
using System.Data.SqlClient;

namespace dbsql
{
    public class MainClass
    {
        public SqlConnection mssql = new SqlConnection();

        public static void Main(string[] args)
        {
            // Variables
            string nameVal = "";
            string dateVal = "";
            string catVal = "";
            double amntVal = 0;
            string sqlRec = "";
            int idVal = 0;
            int cnt = 1;

            // Configure and open SQL connection

            MainClass mc = new MainClass();

            mc.OpenSQLConnection("localhost", "trainning");

            //MainClass.OpenSQLConnection("localhost", "trainning");

            /*
             * Truncate the target table(s)
            */
            using (SqlCommand sqlDelete = new SqlCommand("DELETE FROM expenses_stage", mc.mssql))
            {
                sqlDelete.ExecuteNonQuery();
            }

            try
            {
                /*
                 * Retrieve records from source
                */
                SqlDataReader sqlReader = null;
                SqlCommand selectExp = new SqlCommand("SELECT * FROM expenses_source ORDER BY ID", mc.mssql);
                sqlReader = selectExp.ExecuteReader();

                while (sqlReader.Read())
                {
                    /*
                     * Read source data into vars, trim for clean strings
                    */
                    idVal = Convert.ToInt32(sqlReader["ID"]);
                    nameVal = sqlReader["Name"].ToString().Trim();
                    dateVal = sqlReader["Date"].ToString().Trim();
                    catVal = sqlReader["Category"].ToString().Trim();
                    amntVal = Convert.ToInt32(sqlReader["Amount"]);

                    sqlRec = "==> " + idVal + " | " + nameVal + " | " + dateVal + " | " + catVal + " | " + amntVal;

                    /*
                     * Check for blank, NULL, 0 values
                    */
                    if (idVal == 0)
                    {
                        Console.Write("ID is NULL : ");
                        Console.WriteLine(sqlRec);
                    }
                    else if (nameVal == "")
                    {
                        Console.Write("Name is NULL : ");
                        Console.WriteLine(sqlRec);
                    }
                    else if (dateVal == "")
                    {
                        Console.Write("Date is NULL : ");
                        Console.WriteLine(sqlRec);
                    }
                    else if (catVal == "")
                    {
                        Console.Write("Category is NULL : ");
                        Console.WriteLine(sqlRec);
                    }
                    else if (amntVal == 0)
                    {
                        Console.Write("Amount is NULL : ");
                        Console.WriteLine(sqlRec);
                    }
                    else
                        /*
                         * If food cost is over 0, reject
                        */
                        if (catVal == "Food" && amntVal > 200)
                        {
                            Console.Write("Food expense is too high : ");
                            Console.WriteLine(sqlRec);
                        }
                        else
                        {
                            Console.Write("Good record : ");
                            Console.WriteLine(sqlRec);

                            Console.WriteLine(" ====> AER FIRE--Inserting record!");

                            /*
                             * For every source record, insert two records to the target
                            */
                            for (cnt = 1; cnt <= 2; cnt++)
                            {
                                if (cnt == 1)
                                {
                                    SqlCommand insertExp = new SqlCommand("INSERT INTO expenses_stage (ID, Name, Date, Category, Amount) " +
                                        "VALUES (" + idVal + ", '" + nameVal + "', '" + dateVal + "', '" + catVal + "', " + amntVal + ")", mc.mssql);

                                    insertExp.ExecuteNonQuery();
                                }
                                else
                                {
                                    SqlCommand insertExp = new SqlCommand("INSERT INTO expenses_stage (ID, Name, Date, Category, Amount) " +
                                        "VALUES (" + idVal + ", '" + nameVal + "', '" + dateVal + "', 'AP', " + amntVal + ")", mc.mssql);

                                    insertExp.ExecuteNonQuery();
                                }
                            }
                        }
                }
            }
            catch (Exception e)
            {
                Console.WriteLine(e.ToString());
                Console.ReadKey();
            }

            mc.CloseSQLConnection();

            Console.ReadKey();
        } //Close main

        public void OpenSQLConnection(string sqlServer, string sqlDatabase)
        {
            string connString = "server=" + sqlServer + ";" +
            "Trusted_Connection=yes;" +
            "database=" + sqlDatabase + "; " +
            "connection timeout=30;" +
            "MultipleActiveResultSets=true";

            using (var mssql = new SqlConnection(connString))
            {
                // Open SQL connection
                try
                {
                    mssql.Open();
                }
                catch (Exception e)
                {
                    Console.WriteLine(e.ToString());
                }
            }

        }

        public void CloseSQLConnection()
        {
            MainClass mc = new MainClass();

            try
            {
                /*
                 * Close SQL connection
                */
                mc.mssql.Close();
            }
            catch (Exception e)
            {
                Console.WriteLine(e.ToString());
                Console.ReadKey();
            }
        }

    }
}

我正在尝试自学 C# 以及如何使用它与数据库进行交互。该程序打开与 SQL 服务器的连接，从源 table 检索记录，稍微验证数据，然后将每个源记录的两条记录写入目标 table。最初所有这些代码都是线性的并且在 Main 方法中，但我希望能够重新使用 Open 和 Close 指令，并将 Open 指令用于其他 servers/databases.

当我创建 OpenSQLConnection 和 CloseSQLConnection 方法时，我遇到了很多麻烦，因为实例是我不熟悉的东西。我收到类似

的错误

Cannot implicitly convert type string to System.Data.SqlClient.SqlConnection" and reading various threads pointed me to the using function (Cannot implicitly..., Error while converting string...).

一旦我实现了 using 函数，错误和警告就消失了，但是当我运行代码时，连接不起作用，而且我认为它根本没有打开。这是输出的示例部分（很长）。

at System.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection owningObject, TaskCompletionSource1 retry, DbConnectionOptions userOptions, DbConnectionInternal& connection)<br> at System.Data.ProviderBase.DbConnectionFactory.TryGetConnection(DbConnection owningConnection, TaskCompletionSource1 retry, DbConnectionOptions userOptions, DbConnectionInternal oldConnection, DbConnectionInternal& connection)
at System.Data.ProviderBase.DbConnectionInternal.TryOpenConnectionInternal(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource1 retry, DbConnectionOptions userOptions)<br> at System.Data.ProviderBase.DbConnectionClosed.TryOpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource1 retry, DbConnectionOptions userOptions)
at System.Data.SqlClient.SqlConnection.TryOpenInner(TaskCompletionSource1 retry)<br> at System.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource1 retry)
at System.Data.SqlClient.SqlConnection.Open()
at dbsql.MainClass.OpenSQLConnection(String sqlServer, String sqlDatabase) in c:\Users\tromik\Documents\Projects\dbsql\Program.cs:line 153
ClientConnectionId:8eb63155-4ec6-4795-9f68-98b01f650ffc
Error Number:4060,State:1,Class:11

SQL 资料：

-- Create source table
CREATE TABLE [dbo].[expenses_source]
(
    [ID] [int] NULL,
    [Name] [nvarchar](255) NULL,
    [Date] [datetime] NULL,
    [Category] [nvarchar](255) NULL,
    [Amount] [float] NULL
) ON [PRIMARY]

-- Create target table
CREATE TABLE [dbo].[expenses_stage]
(
    [ID] [int] NULL,
    [Name] [nvarchar](255) NULL,
    [Date] [datetime] NULL,
    [Category] [nvarchar](255) NULL,
    [Amount] [float] NULL
) ON [PRIMARY]

-- Insert source data
INSERT [dbo].[expenses_source] ([ID], [Name], [Date], [Category], [Amount]) 
VALUES (1, 'Maryland Duford  ', CAST(0x0000A33D00000000 AS DateTime), 'Gas', 70);

INSERT [dbo].[expenses_source] ([ID], [Name], [Date], [Category], [Amount]) 
VALUES (2, 'Suzanne Sechrist  ', CAST(0x0000A35F00000000 AS DateTime), 'Food', 30);

INSERT [dbo].[expenses_source] ([ID], [Name], [Date], [Category], [Amount]) 
VALUES (3, 'Reggie Nally  ', CAST(0x0000A35200000000 AS DateTime), 'Food', 22);

INSERT [dbo].[expenses_source] ([ID], [Name], [Date], [Category], [Amount]) 
VALUES (6, 'Stefani Rochell  ', CAST(0x0000A2A600000000 AS DateTime), 'Hotel', 110);

INSERT [dbo].[expenses_source] ([ID], [Name], [Date], [Category], [Amount]) 
VALUES (10, 'Chong Hecht', CAST(0x0000A3AE00000000 AS DateTime), 'Office', 100);

INSERT [dbo].[expenses_source] ([ID], [Name], [Date], [Category], [Amount])
VALUES (8, 'Tiffaney Tooker  ', CAST(0x0000A40C00000000 AS DateTime), 'Food', 500);

INSERT [dbo].[expenses_source] ([ID], [Name], [Date], [Category], [Amount]) 
VALUES (4, 'Joseph Takemoto  ', NULL, 'Hotel', 100);

INSERT [dbo].[expenses_source] ([ID], [Name], [Date], [Category], [Amount]) 
VALUES (5, 'Bryon Bettis  ', NULL, 'Food', 17);

INSERT [dbo].[expenses_source] ([ID], [Name], [Date], [Category], [Amount]) 
VALUES (9, 'Elfreda Byun  ', NULL, 'Gas', 50);

INSERT [dbo].[expenses_source] ([ID], [Name], [Date], [Category], [Amount]) 
VALUES (11, 'Rosio Boggs ', CAST(0x0000A2F100000000 AS DateTime), NULL, 20);

INSERT [dbo].[expenses_source] ([ID], [Name], [Date], [Category], [Amount]) 
VALUES (7, 'Gregg Smithey  ', CAST(0x0000A29500000000 AS DateTime), 'Food', 8);

如果更多的输出会有所帮助，我很乐意效劳。提前致谢！

Answer 1

您在 using 块中创建了连接对象。 using 块将在您退出其作用域时关闭连接，因为 SqlConnection 实现了 IDisposable。

System.Data.SqlClient.SqlConnection: 用方法打开和关闭SqlConnection，在Main方法中读取和写入

System.Data.SqlClient.SqlConnection: Open and Close SqlConnection with methods, Read and Write in Main method

c#

sql

sql-server

using

sqlconnection