System.Data.SqlClient.SqlConnection：用方法打开和关闭SqlConnection，在Main方法中读写答案

【问题标题】：System.Data.SqlClient.SqlConnection: Open and Close SqlConnection with methods, Read and Write in Main methodSystem.Data.SqlClient.SqlConnection：用方法打开和关闭SqlConnection，在Main方法中读写
【发布时间】：2015-03-18 02:11:40
【问题描述】：

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 $200, 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 Server 的连接，从源表中检索记录，稍微验证数据，然后将两条记录写入每个源记录的目标表。最初所有这些代码都是线性的并且在 Main 方法中，但我希望能够重用 Open 和 Close 指令，并将 Open 指令用于其他服务器/数据库。

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。我遇到了类似

的错误

无法将类型字符串隐式转换为 System.Data.SqlClient.SqlConnection”并读取各种线程将我指向使用函数（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，我认为它实际上根本没有打开。这是输出的示例部分（很长）。

在 System.Data.ProviderBase.DbConnectionPool.TryGetConnection（DbConnection owningObject，TaskCompletionSource1 retry, DbConnectionOptions userOptions, DbConnectionInternal& connection) at System.Data.ProviderBase.DbConnectionFactory.TryGetConnection(DbConnection owningConnection, TaskCompletionSource1 重试，DbConnectionOptions userOptions，DbConnectionInternal oldConnection，DbConnectionInternal& 连接）
在 System.Data.ProviderBase.DbConnectionInternal.TryOpenConnectionInternal（DbConnection externalConnection，DbConnectionFactory connectionFactory，TaskCompletionSource1 retry, DbConnectionOptions userOptions) at System.Data.ProviderBase.DbConnectionClosed.TryOpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource1 重试，DbConnectionOptions userOptions）
在 System.Data.SqlClient.SqlConnection.TryOpenInner(TaskCompletionSource1 retry) at System.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource1 重试)
在 System.Data.SqlClient.SqlConnection.Open()
在 dbsql.MainClass.OpenSQLConnection(String sqlServer, String sqlDatabase) 在 c:\Users\tromik\Documents\Projects\dbsql\Program.cs:line 153
ClientConnectionId:8eb63155-4ec6-4795-9f68-98b01f650ffc
错误号：4060，状态：1，类：11

SQL 东西：

-- 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);

如果更多输出有帮助，我很乐意效劳。提前致谢！

【问题讨论】：

标签： c# sql sql-server using sqlconnection

【解决方案1】：

您在 using 块中创建了连接对象。 using 块将在您退出其范围时关闭连接，因为 SqlConnection 实现了 IDisposable。

【讨论】：

感谢您的快速回复，帕特里克。您是否建议根本不实现 using 块，或者只是将对象创建移到 using 块之外？我将如何做到这一点？
@tromik：using() { ... } 块是首选的处理方式 - 但您应该在执行查询时非常“本地化”。因此，理想情况下，您执行的每个查询应该(1) 创建SqlConnection（在using 块中），(2) 创建SqlCommand（再次 - 在using 块），(3) 打开连接，执行查询，关闭连接。这是普遍接受的最佳实践 - 不要在需要之前打开连接方式，不要在执行查询后关闭它
感谢您的解释，marc_s。我试试看。
是否可以在单独的方法中配置/设置连接（这样我可以将源与转换/加载分开定义），然后在 main 方法中打开、使用和关闭连接?
@tromik 你可以看看 EntityFramework 是如何做到的。 msdn.microsoft.com/en-us/data/jj729737.aspx。本质上，它们将 DbConnection 生命周期包装在更高级别的对象中。连接的生命周期就是 DbContext 的生命周期。然后，您在 using 范围内创建更高级别的对象，并在更高级别的对象上调用方法。您还可以查看存储库和工作单元模式。