Question

我正在尝试使用MySQL在SQL Server上实现工厂模式，SQL Server面临奇怪的错误

对象引用未设置为对象的实例

在SQL命令对象

上

internal class SqlServerDB : IDatabase
{
    private SqlConnection _Connection = null;
    private SqlCommand _Command = null;

    public IDbCommand Command
    {
            get
            {
                if (_Command == null)
                {
                    _Command.Connection = (SqlConnection)Connection;

                    //_Command = new SqlCommand();
                }
                return _Command;
            }
    }

    public IDbConnection Connection
    {
            get
            {
                if (_Connection == null)
                {
                    string connectionString = ConfigurationManager.ConnectionStrings["testSQL"].ConnectionString;
                    _Connection = new SqlConnection(connectionString);
                }
                return _Connection;
            }
    }
}

数据库工厂部分：

public static class DatabaseFactory
{
            public static IDatabase CreateDatabase(DBType type)
            {
                switch (type)
                {
                    case DBType.SqlServer:
                        return new SqlServerDB();

                    case DBType.MySql:
                        return new MySQLDB();
                }

                return null;
            }
}

主要方法

static void Main(string[] args)
{
    IDatabase database;
    DBType databaseType = DBType.SqlServer;

    database = DatabaseFactory.CreateDatabase(databaseType);
    IDbConnection connection = database.Connection;

    IDbCommand command = database.Command;
    command.CommandType = CommandType.Text;
    command.CommandText = "select * from User";

    connection.Open();
}

和Enum选择数据库。

Answer 1

第一个if上有错误，

if (_Command == null)
{
    _Command.Connection = (SqlConnection)Connection;
    //_Command = new SqlCommand();
}

可能更像是：

if (_Command == null)
{
    _Command = new SqlCommand();
    _Command.Connection = (SqlConnection)Connection;        
}

Answer 2

您不必使用自己的switch语句。

首先，确保您的connectionString包含这样的providerName：

< ?xml version='1.0' encoding='utf-8'?>
<configuration>
    <connectionstrings>
        <add name="Name"  providerName="System.Data.ProviderName" connectionString="Valid Connection String;"></add>
    </connectionstrings>
</configuration>

然后您可以使用以下方式创建连接：

var connectionString = ConfigurationManager.ConnectionStrings["MyConName"];
var providerName = connectionString.ProviderName;
var factory = DbProviderFactories.GetFactory(providerName);
var connection = factory.CreateConnection();
connection.ConnectionString = connectionString.ConnectionString;
connection.Open();

（添加空检查，以便您可以告诉您的用户/开发人员配置错误）

只要您坚持常见的SQL查询，就可以支持大多数SQL数据库。

想要了解有关正确ADO.NET处理的更多信息？ http://blog.gauffin.org/2013/01/ado-net-the-right-way/

工厂模式数据库连接

2 个答案: