Abstract Factory（抽象工厂模式）

System.Data.Common.DbProviderFactory
public abstract class DbProviderFactory
{
　　public virtual DbConnection CreateConnection()
　　{
　　　　return null;
　　}
　　public virtual DbCommand CreateCommand()
　　{
　　　　 return null;
　　}
}

System.Data.SqlClient.SqlClientFactory
public sealed class SqlClientFactory : DbProviderFactory, IServiceProvider
{
　　public override DbConnection CreateConnection()
　　{
　　　　return new SqlConnection();
　　}
　　public override DbCommand CreateCommand()
　　{
　　　　return new SqlCommand();
　　}
}

System.Data.Odbc.OdbcFactory
public sealed class OdbcFactory : DbProviderFactory
{
　　public override DbConnection CreateConnection()
　　{
　　　　return new OdbcConnection();
　　}
　　public override DbCommand CreateCommand()
　　{
　　　　return new OdbcCommand();
　　}
}

System.Data.Common.DbConnection
public abstract class DbConnection : Component, IDbConnection
{
}

System.Data.SqlClient.SqlConnection
public sealed partial class SqlConnection : DbConnection, ICloneable
{
}

System.Data.Odbc.OdbcConnection
public sealed partial class OdbcConnection : DbConnection, ICloneable
{
}

System.Data.Common.DbCommand
public abstract class DbCommand : Component, IDbCommand
{
}

System.Data.SqlClient.SqlCommand
public sealed class SqlCommand : DbCommand, ICloneable
{
}

System.Data.Odbc.OdbcCommand
public sealed class OdbcCommand : DbCommand, ICloneable
{
}

public class Client
{
public static void Main()
{
　　　　string connectionString =
System.Configuration.ConfigurationManager.AppSettings["DB_CONNECTION_STRING"];
　　　　string providerName =
System.Configuration.ConfigurationManager.AppSettings["DB_PROVIDER_NAME"];
　　　　DbProviderFactory factory = DbProviderFactories.GetFactory(providerName);
　　　　DbConnection connection = factory.CreateConnection();
　　　　connection.ConnectionString = connectionString;
　　　　using (connection)
　　　　{
　　　　　　DbCommand command = factory.CreateCommand();
　　　　　　command.CommandText =
"INSERT INTO t_users(user_username, user_password) VALUES('admin' ,'admin))";
　　　　　　command.Connection = connection;
　　　　　　command.ExecuteNonQuery();
　　　　}
　　}
}