Database factory class design

Question

Hi i have this class to instantiate DAL classes:

public class Factory
{
    public static T GetInstance<T>() where T : new()
    { 
        return new T();
    }
}

I want to make my application capable of using multiple databases. I was planning on setting the database in my web.config and then pass in that setting possibly to the factory class where it will return the correct DAL class. I think my methodology is ok im just a bit stuck on how to implement it whilst keeping it generic.

Maybe something like this:

public class Factory
{
    private static readonly string dbType = ConfigurationSettings.Appsettings["SqlServer"];
    public static T GetInstance<T>() where T : new()
    { 
        switch(dbType)
        {
             case "SqlServer":
                return new T(); //Not sure what to put here.
             break;
             case: "MySql":
                return new T(); 
             break;
             default: "No datasource";
        }
    }
}

If anyone could help or point me in the right direction that would be great.

Thanks in advance.

Answer 1

Don't use generics on your GetInstance() method. Have all your data access classes implement an interface instead, and have that interface be the return value from the function.

public class Factory
{
    private static readonly string dbType = ConfigurationSettings.Appsettings["SqlServer"];
    public static IDataAccess GetInstance()
    { 
        switch(dbType)
        {
             case "SqlServer":
                return new SqlServerDataAccess(); //SqlServerDataAccess should implement IDataAccess
             break;
             case: "MySql":
                return new MySqlDataAccess(); //MySqlDataAccess should implement IDataAccess
             break;
             default: "No datasource";
        }
    }
}

Separate interfaces from implementation!

Answer 2

You should look at the System.Data.Common NameSpace. This namespace uses structures like DBConnection, DBReader, and so on, and itself uses a factory method to great the required DbProvider.

So instead of going down your current path, I would suggest letting the current .net data framework do the lifting for you. here's a quick example.

DbProviderFactory m_factory = DbProviderFactories.GetFactory("System.Data.SqlClient");
    DbConnection m_connection = m_factory.CreateConnection();
    m_connection.ConnectionString = _connstrbldr.ConnectionString;
    m_connection.Open();
    using (DBCommand cmd = m_connection.CreateCommand())
    {
        cmd.CommandType = CommandType.Text;
        cmd.Commandtext = "";
        cmd.ExecuteNonQuery();
    }

The GetFactory call can accept any Provider that installed on the machine, Oracle, MySql, Sql, etc. You can also Get all providers that are installed on a machine by making a call to static DataTable GetFactoryClasses() this returns a datatable object.

The idea behind this would be to avoid provider specific implementations and rely on a generic implementation that would accommodate all your needs.

Writing Provider Independent Code in ADO.NET

I hope that you find this helpful.