使用多态来实现数据库之间的切换

1、一般数据库之间的转换时这样实现的 

         static void Main(string[] args)
        {
            //必须引入System.Data程序集
            string connectionString = ConfigurationManager.ConnectionStrings["SqlConfigurationManager"].ConnectionString;
            string providerName = ConfigurationManager.ConnectionStrings["SqlConfigurationManager"].ProviderName;
            IDbConnection con;
            if (providerName == "Access")
            {
                con = new OleDbConnection(connectionString);
            }
            else if (providerName == "Sql")
            {
                con = new SqlConnection(connectionString);
            }
            else
            {
                throw new Exception("不存在！");
            }
            using (con)
            {
                con.Open();
                using (IDbCommand cmd = con.CreateCommand())
                {
                    cmd.CommandText = "insert into Person (age) values(@age) ";//Access占位符通常使用问号？，当多个占位符参数时，要注意个数，而如果使用的是@作为占位符的话，就必须要注意Command添加参数时的顺序，不然报错，而？问号不要在意这个顺序，所以推荐使用问号？作为占位符
                    IDbDataParameter parameter = cmd.CreateParameter();
                    parameter.ParameterName = "@age";
                    parameter.DbType = DbType.Int32;
                    parameter.Value = "110";

                    cmd.Parameters.Add(parameter);
                    cmd.ExecuteNonQuery();
                }
            }
            Console.WriteLine("插入成功！");
            Console.ReadKey();
}

 App.config配置如下

<configuration>
  <connectionStrings>
    <add name="SqlConfigurationManager" connectionString="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=E:\db1.mdb"   providerName="Access"/>
    <!--<add name="SqlConfigurationManager" connectionString="Server=.;Database=Person;Integrated Security=True;Pooling=False"
          providerName="MsSqlServer"/>-->
  </connectionStrings>
</configuration>

 2、通过DbProviderFactories实现

 一般providerName是有规范的命名的
在C:\Windows\Microsoft.NET\Framework\v2.0.50727\CONFIG\machine.config里面的system.data节点下定义了

<system.data>
   <DbProviderFactories>
     <add name="Odbc Data Provider" invariant="System.Data.Odbc" description=".Net Framework Data Provider for Odbc" type="System.Data.Odbc.OdbcFactory, System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
     <add name="OleDb Data Provider" invariant="System.Data.OleDb" description=".Net Framework Data Provider for OleDb" type="System.Data.OleDb.OleDbFactory, System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
     <add name="OracleClient Data Provider" invariant="System.Data.OracleClient" description=".Net Framework Data Provider for Oracle" type="System.Data.OracleClient.OracleClientFactory, System.Data.OracleClient, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
     <add name="SqlClient Data Provider" invariant="System.Data.SqlClient" description=".Net Framework Data Provider for SqlServer" type="System.Data.SqlClient.SqlClientFactory, System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
     <add name="Microsoft SQL Server Compact Data Provider" invariant="System.Data.SqlServerCe.3.5" description=".NET Framework Data Provider for Microsoft SQL Server Compact" type="System.Data.SqlServerCe.SqlCeProviderFactory, System.Data.SqlServerCe, Version=3.5.0.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91"/>
     <add name="Microsoft SQL Server Compact Data Provider 4.0" invariant="System.Data.SqlServerCe.4.0" description=".NET Framework Data Provider for Microsoft SQL Server Compact" type="System.Data.SqlServerCe.SqlCeProviderFactory, System.Data.SqlServerCe, Version=4.0.0.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91"/>
   </DbProviderFactories>
</system.data>

如果providerName为System.Data.Odbc，那么可以用于所有数据库配置
如果为System.Data.OleDb，可用于Access数据库配置
如果为System.Data.OracleClient，可用于Oracle数据库配置
如果为System.Data.SqlClient，可用于MsSqlServer数据库配置

        static void Main(string[] args)
        {
            string connectionString = ConfigurationManager.ConnectionStrings["SqlConfigurationManager"].ConnectionString;
            string providerName = ConfigurationManager.ConnectionStrings["SqlConfigurationManager"].ProviderName;
            //必须引入System.Data程序集
            DbProviderFactory factory = DbProviderFactories.GetFactory(providerName);

            using (DbConnection con = factory.CreateConnection())
            {
                con.ConnectionString = connectionString;
                con.Open();
                using (DbCommand cmd = factory.CreateCommand())
                {
                    cmd.CommandText = "insert into Person (age) values(@age)";
                    cmd.Connection = con;
                    DbParameter parameter = cmd.CreateParameter();
                    parameter.ParameterName = "@age";//Access不用此占位符（？）也能顺利插入
                    parameter.DbType = DbType.Int32;
                    parameter.Value = "1010";
                    cmd.Parameters.Add(parameter);

                    cmd.ExecuteNonQuery();
                }
            }
            Console.WriteLine("插入成功！");
            Console.ReadKey();
        }

注意ProviderName的配置

<configuration>
  <connectionStrings>
    <add name="SqlConfigurationManager" connectionString="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=E:\db1.mdb"
         providerName="System.Data.OleDb"/>
    <!--<add name="SqlConfigurationManager" connectionString="Server=.;Database=Person;Integrated Security=True;Pooling=False"
          providerName="System.Data.SqlClient"/>-->
  </connectionStrings>
</configuration>