7. The FCL: files, databases, & data structures

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Objectives

“The Framework Class Library (FCL) contains thousands of classes, from graphics to I/O to networking to XML processing. The goal of the FCL? To present an abstract, portable view of the underlying operating system…”

• File I/O • Database access• Data structures

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Part 1

• File I/O…

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I/O library

• Input/output library in System.IO namespace• Compiled into mscorlib.dll assembly• Support provided for:

– file and directory management– text files– binary files

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Character I/O

• Classes provided to do character IO– most methods defined in base classes

• Two possible data locations– disk– string

TextReader

StreamReader StringReader

TextWriter

StreamWriter StringWriter

read from disk read from stringwrite to disk write to StringBuilder

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StreamWriter

• StreamWriter usage:– open file with one of many constructors– write with overloaded Write / WriteLine methods– close

automaticallyconverted to stringusing ToString

StreamWriter sw = new StreamWriter("Chores.txt");int n = 3;

sw.WriteLine("Go to pet store");sw.Write("Feed all ");sw.Write(n);sw.WriteLine(" cats");

sw.Close();

open

write

close

char, bool, string,short, int, long,float, double, etc.

text file

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StreamReader

• StreamReader usage:– open file with one of many constructors– read characters or strings with Read / ReadLine methods– close

StreamReader sr = new StreamReader("Chores.txt");string s;

while ((s = sr.ReadLine()) != null) Console.WriteLine(s);

sr.Close();

open

read

close

can read onlychar or string

char, string text file

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Files and directories

• Lots of utility classes for working with files & directories

– Directory: for manipulating directories and drives

– File: for manipulating files

– Path: for manipulating path strings

using System.IO;

string[] drives = Directory.GetLogicalDrives();

foreach (string s in drives){ if (Directory.Exists(s)) ...}

all drives

disk in drive?

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Part 2

• Database access…

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Database library

• Database access provided by System.Data.* namespaces• Compiled into System.Data.dll assembly • Known collectively as ADO.NET

– native support for SQL Server and Oracle– support for other databases via older OleDB technology– requires a knowledge of SQL

• Core namespaces:– general: System.Data, System.Data.Common– SQL Server: System.Data.SqlClient– Oracle: System.Data.OracleClient– OleDB: System.Data.OleDb

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Relational technology

• ADO.NET is designed to access relational databases• Example:

– Sales database with customers, orders, and products

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Overview of database access

• Three steps:

1. open connection to database

2. execute SQL to update DB / retrieve records

3. close connection

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Step 1: open connection

• Connections are opened based on connection string info– here we open a connection to a MS Access 2000 database– "Sales.mdb" must exist in same dir as .EXE (e.g. bin\Debug)

using System.Data;using System.Data.OleDb;

string sConnection;sConnection = "Provider=Microsoft.Jet.OLEDB.4.0;" + "Data Source=Sales.mdb";

IDbConnection dbConn;dbConn = new OleDbConnection(sConnection);dbConn.Open();

MessageBox.Show( dbConn.State.ToString() );

connection

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Building connection strings

• Connection strings are vendor-specific, not well-documented• Where to turn for help?

– www.connectionstrings.com– www.able-consulting.com/ADO_conn.htm

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Step 2: retrieve records

• Retrieve records via SQL Select query– read-only access by database field names

string sql, name;sql = "Select * From Customers Order By LastName Asc, FirstName Asc;";

IDbCommand dbCmd;dbCmd = new OleDbCommand();dbCmd.CommandText = sql;dbCmd.Connection = dbConn;

IDataReader dbReader;dbReader = dbCmd.ExecuteReader();

while (dbReader.Read()) { // retrieve records 1-by-1...

name = dbReader["LastName"] + ", " + dbReader["FirstName"]; this.listBox1.Items.Add(name);

}

data readerrecordrecordrecord

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Step 3: close connection

• Be sure to close connection…– to flush pending updates– so others can access DB (connections are limited resources)

dbConn.Close();

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Guaranteed close?

• Ensure DB is closed via try-catch-finally:

IDbConnection dbConn = null;

try { dbConn.Open(); . . .}

catch(Exception ex) {

System.Diagnostics.EventLog.WriteEntry("MyApp", ex.Message); System.Diagnostics.EventLog.WriteEntry("MyApp", ex.StackTrace); throw ex;}finally { if ((dbConn != null) && (dbConn.State != ConnectionState.Closed)) dbConn.Close();}

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Updating a database

• To update database, execute an SQL Action query• Example:

– delete customer by their id number

int result, cid = ?;

string sql;sql = String.Format("Delete From Customers Where CID={0};", cid);

IDbCommand dbCmd;dbCmd = new OleDbCommand();dbCmd.CommandText = sql;dbCmd.Connection = dbConn;

dbConn.Open(); result = dbCmd.ExecuteNonQuery();dbConn.Close();

if (result != 1) throw new Exception("Action failed to delete 1 customer?!");

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Example of action queries

• Insert, update and delete:

Insert Into Customers(CID, FirstName, LastName, CreditLimit, Balance) Values(118, 'Jia', 'Zhang', 10000.0, 0.0);

Update Customers Set CreditLimit = 40000000000.0, Balance = 0.0 Where LastName = 'Gates' and FirstName = 'Bill';

Delete From Customers Where CID = 666;

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DataSets

• DataSets are an in-memory, read-write data structure– easily filled with data from a database– easily displayed in a GUI app

DataSet

Product Price Quantity

Ants $ 0.49 5000

Birds $ 4.49 500

Cats $29.95 100

Dogs $79.95 20

DBCommand ConnectionDataAdapter

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Example

• Retrieve product info and display in a DataGrid:

sql = "Select * From Products;" . . .

DataSet ds;IDataAdapter adapter;

ds = new DataSet();adapter = new OleDbDataAdapter((OleDbCommand) dbCmd);

dbConn.Open(); adapter.Fill(ds);dbConn.Close();

this.dataGrid1.SetDataBinding(ds, "Table");

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Flushing changes back to database

• Reconnect, and apply adapter's Update() method:

sql = "Select * From Products;" . . .

DataSet ds;IDataAdapter adapter;

ds = (DataSet) this.dataGrid1.DataSource;adapter = new OleDbDataAdapter((OleDbCommand) dbCmd);

OleDbCommandBuilder cmdBuilder;cmdBuilder = new OleDbCommandBuilder((OleDbDataAdapter) adapter);

dbConn.Open(); adapter.Update(ds); // this fails if updates conflictdbConn.Close();

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Part 3

• Data structures…

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Collections library

• Data structures in .NET are generally known as Collections• Located in the namespace System.Collections• Compiled into mscorlib.dll assembly• Defined in terms of object for generic use

• Core classes:– Array– ArrayList– Hashtable– Stack– Queue

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Collection interfaces

• Collections implement various interfaces to ensure uniformity– classes that implement the same interface offer same services– makes library easier to learn and use– allows generic code to be written against interface

• Core interfaces:– ICollection– IEnumerable– IEnumerator– IList– IComparer– IComparable

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ArrayList

• ArrayList provides storage for sequence of elements– duplicate values ok– data stored internally as an array, automatically resized– primarily manipulated via Ilist

public class ArrayList : IList, IEnumerable, ...{ // IList services ...

// additional services int Capacity { get... set... } void TrimToSize()

int BinarySearch(object value) int IndexOf (object value, int startIndex) int LastIndexOf (object value, int startIndex) ...}

control of memoryin underlying array

searching

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IList interface

• IList defines sequence of elements– can be accessed by index

public interface IList : ICollection { int Add (object value); void Insert(int index, object value);

void Remove (object value); void RemoveAt(int index); void Clear ();

bool Contains(object value); int IndexOf (object value);

object this[int index] { get; set; }

bool IsReadOnly { get; } bool IsFixedSize { get; }}

add new elements

remove

containment testing

read/write existing element

structural properties

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Example

• Creating and using an ArrayList:

using System.Collections;

ArrayList a = new ArrayList();

a.Add("mom"); // added to end...a.Add("dad");a.Add("sister");

Console.WriteLine(a[2]); // direct access

if (a.Contains("dad")) // search ...

foreach (string s in a) // iterate Console.WriteLine(s);

handles iterationinterfaces, casting

element 0element 1element 2

true

"sister"

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Hashtable

• Hashtable provides collection of key/value pairs– keys must be unique, values hold the data– stores object reference for both key and value– GetHashCode method of key used to determine placement

Hashtable ages = new Hashtable();

ages["Ann"] = 27;ages["Bob"] = 32;ages.Add("Tom", 15);

ages["Ann"] = 28;

int a = (int) ages["Ann"];

create

add

update

retrieve

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Hashtable traversal

• Can traverse Hashtable contents– each element is DictionaryEntry struct– data exposed in Key and Value properties

Hashtable ages = new Hashtable();

ages["Ann"] = 27;ages["Bob"] = 32;ages["Tom"] = 15;

foreach (DictionaryEntry entry in ages){ string name = (string) entry.Key; int age = (int) entry.Value; ...}

enumerate entries

get key and value

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Summary

• The FCL is huge• The FCL is quite powerful• The FCL is essentially a portable OS

– fully-implemented on Windows 98 and above– partially-implemented on FreeBSD, Mac OS X, and Linux

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References

• Books:– J. Richter, "Applied Microsoft .NET Framework Programming"– T. Thai and H. Lam, ".NET Framework Essentials" – W. Vaughn and P. Blackburn, "ADO.NET Examples and Best

Practices for C# Programmers"– B. Beauchemin, "Essential ADO.NET"

• Web sites:– http://msdn.microsoft.com/net– Oracle for .NET: http://otn.oracle.com/tech/windows/odpnet/– Rotor (SSCLI): http://msdn.microsoft.com/net/sscli– Mono: http://www.go-mono.com/

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Lab?

• Work on lab #4, "Interfaces and FCL"…