Using ADO.NET is a great way to connect to data provider that exists outside the .NET framework to connect to a database. There are many data providers out there, MySQL,PostgreSQL, FlySpeed etc. which are in commercial use but are not necessarily that popular.

It can be difficult sometimes create a DAL for a custom database architecture. Fortunately we have ADO.NET along with DbProviderFactory class which allows any custom DB provider to connect to the .NET CLR and allow developers to write custom execution queries against the database.

When using these custom DB providers you need to update your application configuration file so that the .NET runtime has knowledge of the DbProviderFactory that you intend on using, if you check your machine.config (C:\WINDOWS\Microsoft.NET\Framework\v4.0.30319\Config) for .NET v4.0 you'll should see the following entries:

<section name="system.data" type="System.Data.Common.DbProviderFactoriesConfigurationHandler, System.Data, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
	<DbProviderFactories>
		<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" />   
		<add name="MySQL Data Provider" invariant="MySql.Data.MySqlClient" description=".Net Framework Data Provider for MySQL" type="MySql.Data.MySqlClient.MySqlClientFactory, MySql.Data, Version=6.3.6.0, Culture=neutral, PublicKeyToken=c5687fc88969c44d" />     
	</DbProviderFactories> 

These are the default factories that cone with the .NET framework, however if you want to introduce your own custom factory you can just add an entry. Depending on whether you want your factory to be available across applications or not you could add the entry to your local application/web configuration file.