I am reading a book and it says : "if you will create your own data access layer by using ADO.NET for access into you database, you will be minimally affected whether the data schema exists or not. If however you are using an O/RM, your flexibility will be limited by the tool you use".
What is the major difference between ADO.NET and any other ORM?
ADO.NET provides consistent access to data sources such as SQL Server and XML, and to data sources exposed through OLE DB and ODBC. Data-sharing consumer applications can use ADO.NET to connect to these data sources and retrieve, handle, and update the data that they contain.
ADO.NET separates data access from data manipulation into discrete components that can be used separately or in tandem. ADO.NET includes .NET Framework data providers for connecting to a database, executing commands, and retrieving results. Those results are either processed directly, placed in an ADO.NET DataSet object in order to be exposed to the user in an ad hoc manner, combined with data from multiple sources, or passed between tiers. The DataSet object can also be used independently of a .NET Framework data provider to manage data local to the application or sourced from XML.
ADO.NET is layer that allows you to connect to DB and modify it using SQL connections, commands, parameters. ADO.NET MSDN
Object-relational mapping (ORM, O/RM, and O/R mapping tool) in computer science is a programming technique for converting data between incompatible type systems in object-oriented programming languages. This creates, in effect, a "virtual object database" that can be used from within the programming language. There are both free and commercial packages available that perform object-relational mapping, although some programmers opt to construct their own ORM tools.
Entity Framework an
NHiberante are ORMs. It means that you do not operate by SQL connections, commands, parameters - ORM tool do it for you and it allow to map your database structure in OOP manner: you can add, read, update, delete records in your DB using objects in C#. You need only map your object to DB correctly.
Entity Framework is built on ADO.NET and it uses ADO.NET inside. SQL statements are generated by ORM. ORM
Generally, access to DB without ORM is faster, but you should provide more lines of code. If you want to operate your DB in OOP manner and write more readable code you should choose ORM. It depends on your purposes what to choose.
There are Micro ORMs (Dapper, BLToolkit) which allows you to write SQL queries and map parameters to object properties. Micro ORMs, in general, have better perfomance than Full ORMs, but ADO.NET is still faster.
Also, there are some questions and answers on StackOverflow: EF vs ADO.NET