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>