Generic Data Access in Microsoft.NET: a Compelling Example of Inheritance, Interfaces, and the Factory Method Design Pattern OOPSLA 2004 : Design Patterns and Objects First Workshop Joe Hummel, PhD Dept of Math/CS Lake Forest College

2 OOPSLA 2004 Introductions… Joe Hummel, PhD –Chicago, USA – –web: I wear 2 hats: –Academic: Associate Prof. of CS at Lake Forest College ( PhD from UC-Irvine (Optimizing Compilers, 1998) –Industry: professional trainer (1-5 day workshops, webcasts, conferences) specializing in Microsoft Windows development

3 OOPSLA 2004 Part 1 The problem…

4 OOPSLA 2004 Database access in.NET is vendor-specific A different set of classes for each technology: –ODBC –OLEDB –SQL Server –Oracle –DB2 –… Saving grace: –Each set of classes is based on a common design: Connection object to open connection with DB Command object for executing SQL DataReader object for reading records etc. DB

5 OOPSLA 2004 Naïve result Developers create a distinct class for each DB type: MSAccessDB SQLServerDB OracleDB MySQLDB

6 OOPSLA 2004 Goal Apply good OOD to obtain reusable, maintainable result: GenericDataAccess MSAccessDBSQLServerDBOracleDB MySQLDB

7 OOPSLA 2004 Killer example? Real Important Inheritance Interfaces Factory Method design pattern

8 OOPSLA 2004 Part 2 From naïve to gOOd solution…

9 OOPSLA 2004 Outline 4-step process: 1.identify generic vs. vendor-specific data access code 2.apply Factory Method design pattern 3.use interfaces to define common design across Microsoft’s classes 4.use inheritance to: –ensure our generic & concrete data access classes integrate properly –enable single, polymorphic access to different types of databases GenericDataAccess db; db = new ConcreteDataAccessClass (…); db.Insert(…); db.Update(…); GenericDataAccess db; db = new ConcreteDataAccessClass (…); db.Insert(…); db.Update(…);

10 OOPSLA 2004 Step 1 Identify generic vs. vendor-specific… System.Data.SqlClient.SqlConnection conn; System.Data.SqlClient.SqlCommand cmd; System.Data.SqlClient.SqlDataReader reader; conn = new System.Data.SqlClient.SqlConnection(“connection info…”); cmd = new System.Data.SqlClient.SqlCommand(“Select * …”, conn); conn.Open(); // open connection to DB reader = cmd.ExecuteReader(); // execute SQL and return object for accessing DB… while ( reader.Read() ) // for each record… {. } conn.Close(); System.Data.SqlClient.SqlConnection conn; System.Data.SqlClient.SqlCommand cmd; System.Data.SqlClient.SqlDataReader reader; conn = new System.Data.SqlClient.SqlConnection(“connection info…”); cmd = new System.Data.SqlClient.SqlCommand(“Select * …”, conn); conn.Open(); // open connection to DB reader = cmd.ExecuteReader(); // execute SQL and return object for accessing DB… while ( reader.Read() ) // for each record… {. } conn.Close(); SQL Server

11 OOPSLA 2004 Step 2 Apply Factory Method design pattern… ??? conn; ??? cmd; ??? reader; conn = CreateConnection(“connection info…”); cmd = CreateCommand(“Select * …”, conn); conn.Open(); // open connection to DB reader = cmd.ExecuteReader(); // execute SQL and return object for accessing DB… while ( reader.Read() ) // for each record… {. } conn.Close(); ??? conn; ??? cmd; ??? reader; conn = CreateConnection(“connection info…”); cmd = CreateCommand(“Select * …”, conn); conn.Open(); // open connection to DB reader = cmd.ExecuteReader(); // execute SQL and return object for accessing DB… while ( reader.Read() ) // for each record… {. } conn.Close(); private ??? CreateConnection(string connectInfo) { return new System.Data.SqlClient.SqlConnection(connectInfo); }. private ??? CreateConnection(string connectInfo) { return new System.Data.SqlClient.SqlConnection(connectInfo); }.

12 OOPSLA 2004 Step 3 Use interfaces to define common design across DB classes… System.Data.IDbConnection conn; System.Data.IDbCommand cmd; System.Data.IDataReader reader; conn = CreateConnection(“connection info…”); cmd = CreateCommand(“Select * …”, conn); conn.Open(); // open connection to DB reader = cmd.ExecuteReader(); // execute SQL and return object for accessing DB… while ( reader.Read() ) // for each record… {. } conn.Close(); System.Data.IDbConnection conn; System.Data.IDbCommand cmd; System.Data.IDataReader reader; conn = CreateConnection(“connection info…”); cmd = CreateCommand(“Select * …”, conn); conn.Open(); // open connection to DB reader = cmd.ExecuteReader(); // execute SQL and return object for accessing DB… while ( reader.Read() ) // for each record… {. } conn.Close(); private IDbConnection CreateConnection(string connectInfo) { return new System.Data.SqlClient.SqlConnection(connectInfo); }. private IDbConnection CreateConnection(string connectInfo) { return new System.Data.SqlClient.SqlConnection(connectInfo); }.

13 OOPSLA 2004 Step 4 Use inheritance & abstract base class to enforce design… public abstract class GenericDataAccess { public void Insert(…) { … } public void Update(…) { … } protected abstract IDbConnection CreateConnection(string info); protected abstract IDbCommand CreateCommand(string SQL, IDbConnection conn); } public abstract class GenericDataAccess { public void Insert(…) { … } public void Update(…) { … } protected abstract IDbConnection CreateConnection(string info); protected abstract IDbCommand CreateCommand(string SQL, IDbConnection conn); } public class SQLServerDB : GenericDataAccess { protected IDbConnection CreateConnection(string info) { return new System.Data.SqlClient.SqlConnection(info); }. public class SQLServerDB : GenericDataAccess { protected IDbConnection CreateConnection(string info) { return new System.Data.SqlClient.SqlConnection(info); }.

14 OOPSLA 2004 End result? Reusable, maintainable data access hierarchy! Single, polymorphic database access! GenericDataAccess db1, db2, db3; db1 = new SQLServerDB (…); db2 = new MSAccessDB (…); db3 = new OracleDB (…); db1.Insert(…); db2.Update(…); db3.Delete(…); GenericDataAccess db1, db2, db3; db1 = new SQLServerDB (…); db2 = new MSAccessDB (…); db3 = new OracleDB (…); db1.Insert(…); db2.Update(…); db3.Delete(…); GenericDataAccess MSAccessDBSQLServerDBOracleDB

15 OOPSLA 2004 Part 3 Extensions…

16 OOPSLA 2004 Possible extensions Specialized exception handling: –different databases respond differently to error conditions –define generic exception handler in base class –derived classes can override & specialize if desired –Template Method design pattern Data access code in.NET really consists of 2 levels: –low-level code that performs database access: execute Action query (insert, update, delete) server-side Select query client-side Select query –high-level code for domain-specific stuff: validation, build SQL, etc. –redesign data access hierarchy to take this into account…

17 OOPSLA 2004 Other possible extensions (These are from the workshop discussion…) Abstract Factory pattern: –derived classes are really producing objects from a family of classes –should recognize this via Abstract Factory pattern Configuration pattern: –make system more dynamic by reading class names from a file –apply Configuration pattern

18 OOPSLA 2004 Application in CS1 / CS2? Perhaps a similar problem involving files instead of DBs… –we give students a set of classes for reading / writing text files –they apply 4 steps outlined earlier.txt TxtReader.html HtmlReader.xml XmlReader

19 OOPSLA 2004 Other ideas for CS1 / CS2 I’ve used the following successfully in CS1 / CS2 sequence: –compilers use AST as an internal representation –AST (abstract syntax tree) involves: non-trivial inheritance hierarchy Visitor design pattern –I had students build a recursive-descent parser I provided lexer they designed AST & built parser ASTNode Stmt Expr WhileAssignIfBinaryLiteral

20 OOPSLA 2004 That's it! Questions? Discussion? Thank you!