Importing Data From Excel with C#

The following code block accepts an Excel file path and returns an ordinary DataTable object, which can be manipulated easily by the code that calls this method. The first row of the Excel document becomes the columns in the DataTable object and each row thereafter are DataRow objects.

ImportingFromExcel

If you like, take a look at both techniques for working with Excel data and see what one speaks to you.

(Source: http://blogs.popart.com/2008/07/importing-data-from-excel-with-c/ )

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Delegates and Events in C# / .NET

Delegates
A delegate in C# is similar to a function pointer in C or C++. Using a delegate allows the programmer to encapsulate a reference to a method inside a delegate object. The delegate object can then be passed to code which can call the referenced method, without having to know at compile time which method will be invoked.

Call a Function directly – No Delegate

In most cases, when we call a function, we specify the function to be called directly. If the class MyClass has a function named Process, we’d normally call it like this (SimpleSample.cs):

using System;
namespace evacion.NoDelegate{

public class MyClass {
public void Process() {
Console.WriteLine(“Process() begin”);
Console.WriteLine(“Process() end”);
}
}

public class Test {
static void Main(string[] args) {
MyClass myClass = new MyClass();
myClass.Process();
}
}
}

That works well in most situations. Sometimes, however, we don’t want to call a function directly – we’d like to be able to pass it to somebody else so that they can call it. This is especially useful in an event-driven system such as a graphical user interface, when I want some code to be executed when the user clicks on a button, or when I want to log some information but can’t specify how it is logged.
The very basic Delegate
An interesting and useful property of a delegate is that it does not know or care about the class of the object that it references. Any object will do; all that matters is that the method’s argument types and return type match the delegate’s. This makes delegates perfectly suited for “anonymous” invocation.
The signature of a single cast delegate is shown below:

delegate result-type identifier ([parameters]);
where:
result-type: The result type, which matches the return type of the function.
identifier: The delegate name.
parameters: The Parameters, that the function takes.
Examples:
public delegate void SimpleDelegate ()
This declaration defines a delegate named SimpleDelegate, which will encapsulate any method that takes
no parameters and returns no value.
public delegate int ButtonClickHandler (object obj1, object obj2)
This declaration defines a delegate named ButtonClickHandler, which will encapsulate any method that takes
two objects as parameters and returns an int.
A delegate will allow us to specify what the function we’ll be calling looks like without having to specify which function to call. The declaration for a delegate looks just like the declaration for a function, except that in this case, we’re declaring the signature of functions that this delegate can reference.

There are three steps in defining and using delegates:

Declaration
Instantiation
Invocation
A very basic example (SimpleDelegate1.cs):

using System;
namespace evacion.BasicDelegate{
// Declaration
public delegate void SimpleDelegate();

class TestDelegate {
public static void MyFunc(){
Console.WriteLine(“I was called by delegate …”);
}
public static void Main(){
// Instantiation
SimpleDelegate simpleDelegate = new SimpleDelegate(MyFunc);
// Invocation
simpleDelegate();
}
}
}

Compile an test:
# csc SimpleDelegate1.cs
# SimpleDelegate1.exe
I was called by delegate …

Calling Static Functions
For our next, more advanced example (SimpleDelegate2.cs), declares a delegate that takes a single string parameter and has no return type:

using System;
namespace evacion.SimpleDelegate{
// Delegate Specification
public class MyClass {
// Declare a delegate that takes a single string parameter
// and has no return type.

public delegate void LogHandler(string message);

// The use of the delegate is just like calling a function directly,
// though we need to add a check to see if the delegate is null
// (that is, not pointing to a function) before calling the function.

public void Process(LogHandler logHandler){
if (logHandler != null){
logHandler(“Process() begin”);
}
if (logHandler != null){
logHandler (“Process() end”);
}
}
}
// Test Application to use the defined Delegate
public class TestApplication {
// Static Function: To which is used in the Delegate. To call the Process()
// function, we need to declare a logging function: Logger() that matches
// the signature of the delegate.

static void Logger(string s){
Console.WriteLine(s);
}
static void Main(string[] args){
MyClass myClass = new MyClass();
// Crate an instance of the delegate, pointing to the logging function.
// This delegate will then be passed to the Process() function.

MyClass.LogHandler myLogger = new MyClass.LogHandler(Logger);
myClass.Process(myLogger);
}
}
}

Compile an test:
# csc SimpleDelegate2.cs
# SimpleDelegate2.exe
Process() begin
Process() end

Calling Member Functions
In the simple example above, the Logger( ) function merely writes the string out. A different function might want to log the information to a file, but to do this, the function needs to know what file to write the information to (SimpleDelegate3.cs)

using System;
using System.IO;
namespace evacion.SimpleDelegate{
// Delegate Specification
public class MyClass{
// Declare a delegate that takes a single string parameter
// and has no return type.

public delegate void LogHandler(string message);
// The use of the delegate is just like calling a function directly,
// though we need to add a check to see if the delegate is null
// (that is, not pointing to a function) before calling the function.

public void Process(LogHandler logHandler) {
if (logHandler != null) {
logHandler(“Process() begin”);
}
if (logHandler != null) {
logHandler (“Process() end”);
}
}
}
// The FileLogger class merely encapsulates the file I/O
public class FileLogger {
FileStream fileStream;
StreamWriter streamWriter;
// Constructor
public FileLogger(string filename) {
fileStream = new FileStream(filename, FileMode.Create);
streamWriter = new StreamWriter(fileStream);
}
// Member Function which is used in the Delegate
public void Logger(string s) {
streamWriter.WriteLine(s);
}
public void Close() {
streamWriter.Close();
fileStream.Close();
}
}
// Main() is modified so that the delegate points to the Logger()
// function on the fl instance of a FileLogger. When this delegate
// is invoked from Process(), the member function is called and
// the string is logged to the appropriate file.
public class TestApplication {
static void Main(string[] args) {
FileLogger fl = new FileLogger(“process.log”);
MyClass myClass = new MyClass();
// Crate an instance of the delegate, pointing to the Logger()
// function on the fl instance of a FileLogger.

MyClass.LogHandler myLogger = new MyClass.LogHandler(fl.Logger);
myClass.Process(myLogger);
fl.Close();
}
}
}

The cool part here is that we didn’t have to change the Process() function; the code to all the delegate is the same regardless of whether it refers to a static or member function.

Compile an test:
# csc SimpleDelegate3.cs
# SimpleDelegate3.exe
# cat process.log
Process() begin
Process() end

Multicasting

Being able to point to member functions is nice, but there are more tricks you can do with delegates. In C#, delegates are multicast, which means that they can point to more than one function at a time (that is, they’re based off the System.MulticastDelegate type). A multicast delegate maintains a list of functions that will all be called when the delegate is invoked. We can add back in the logging function from the first example, and call both delegates. Here’s what the code looks like:

using System;
using System.IO;
namespace evacion.SimpleDelegate{
// Delegate Specification
public class MyClass{
// Declare a delegate that takes a single string parameter
// and has no return type.

public delegate void LogHandler(string message);
// The use of the delegate is just like calling a function directly,
// though we need to add a check to see if the delegate is null
// (that is, not pointing to a function) before calling the function.

public void Process(LogHandler logHandler){
if (logHandler != null){
logHandler(“Process() begin”);
}
if (logHandler != null) {
logHandler (“Process() end”);
}
}
}
// The FileLogger class merely encapsulates the file I/O
public class FileLogger{
FileStream fileStream;
StreamWriter streamWriter;
// Constructor
public FileLogger(string filename) {
fileStream = new FileStream(filename, FileMode.Create);
streamWriter = new StreamWriter(fileStream);
}
// Member Function which is used in the Delegate
public void Logger(string s){
streamWriter.WriteLine(s);
}

public void Close() {
streamWriter.Close();
fileStream.Close();
}
}
// Test Application which calls both Delegates
public class TestApplication {
// Static Function which is used in the Delegate
static void Logger(string s) {
Console.WriteLine(s);
}
static void Main(string[] args) {
FileLogger fl = new FileLogger(“process.log”);
MyClass myClass = new MyClass();
// Crate an instance of the delegates, pointing to the static
// Logger() function defined in the TestApplication class and
// then to member function on the fl instance of a FileLogger.

MyClass.LogHandler myLogger = null;
myLogger += new MyClass.LogHandler(Logger);
myLogger += new MyClass.LogHandler(fl.Logger);
myClass.Process(myLogger);
fl.Close();
}
}
}

Compile an test:
# csc SimpleDelegate4.cs
# SimpleDelegate4.exe
Process() begin
Process() end
# cat process.log
Process() begin
Process() end

Events
The Event model in C# finds its roots in the event programming model that is popular in asynchronous programming. The basic foundation behind this programming model is the idea of “publisher and subscribers.” In this model, you have publishers who will do some logic and publish an “event.” Publishers will then send out their event only to subscribers who have subscribed to receive the specific event.
In C#, any object can publish a set of events to which other applications can subscribe. When the publishing class raises an event, all the subscribed applications are notified. The following figure shows this mechanism.

events

Conventions

The following important conventions are used with events:

  • Event Handlers in the .NET Framework return void and take two parameters.
  • The first paramter is the source of the event; that is the publishing object.
  • The second parameter is an object derived from EventArgs.
  • Events are properties of the class publishing the event.
  • The keyword event controls how the event property is accessed by the subscribing classes.

Simple Event
Let’s modify our logging example from above to use an event rather than a delegate:

using System;
using System.IO;
namespace evacion.SimpleEvent{
/* ========= Publisher of the Event ============== */
public class MyClass {
// Define a delegate named LogHandler, which will encapsulate
// any method that takes a string as the parameter and returns no value

public delegate void LogHandler(string message);
// Define an Event based on the above Delegate
public event LogHandler Log;
// Instead of having the Process() function take a delegate
// as a parameter, we’ve declared a Log event. Call the Event,
// using the OnXXXX Method, where XXXX is the name of the Event.

public void Process() {
OnLog(“Process() begin”);
OnLog(“Process() end”);
}
// By Default, create an OnXXXX Method, to call the Event
protected void OnLog(string message) {
if (Log != null) {
Log(message);
}
}
}
// The FileLogger class merely encapsulates the file I/O
public class FileLogger {
FileStream fileStream;
StreamWriter streamWriter;
// Constructor
public FileLogger(string filename) {
fileStream = new FileStream(filename, FileMode.Create);
streamWriter = new StreamWriter(fileStream);
}
// Member Function which is used in the Delegate
public void Logger(string s) {
streamWriter.WriteLine(s);
}
public void Close() {
streamWriter.Close();
fileStream.Close();
}
}
/* ========= Subscriber of the Event ============== */
// It’s now easier and cleaner to merely add instances
// of the delegate to the event, instead of having to
// manage things ourselves

public class TestApplication {
static void Logger(string s) {
Console.WriteLine(s);
}
static void Main(string[] args) {
FileLogger fl = new FileLogger(“process.log”);
MyClass myClass = new MyClass();
// Subscribe the Functions Logger and fl.Logger
myClass.Log += new MyClass.LogHandler(Logger);
myClass.Log += new MyClass.LogHandler(fl.Logger);
// The Event will now be triggered in the Process() Method
myClass.Process();
fl.Close();
}
}
}

Compile an test:
# csc SimpleEvent.cs
# SimpleEvent.exe
Process() begin
Process() end
# cat process.log
Process() begin
Process() end

The Second Change Event Example
Suppose you want to create a Clock class that uses events to notify potential subscribers whenever the local time changes value by one second. Here is the complete, documented example:

using System;
using System.Threading;
namespace evacion.SecondChangeEvent{
/* ======================= Event Publisher =============================== */
// Our subject — it is this class that other classes
// will observe. This class publishes one event:
// SecondChange. The observers subscribe to that event.

public class Clock {
// Private Fields holding the hour, minute and second
private int _hour;
private int _minute;
private int _second;
// The delegate named SecondChangeHandler, which will encapsulate
// any method that takes a clock object and a TimeInfoEventArgs
// object as the parameter and returns no value. It’s the
// delegate the subscribers must implement.

public delegate void SecondChangeHandler (
object clock,
TimeInfoEventArgs timeInformation
);

// The event we publish
public event SecondChangeHandler SecondChange;
// The method which fires the Event
protected void OnSecondChange(
object clock,
TimeInfoEventArgs timeInformation
) {
// Check if there are any Subscribers
if (SecondChange != null) {
// Call the Event
SecondChange(clock,timeInformation);
}
}

// Set the clock running, it will raise an
// event for each new second

public void Run() {
for(;;) {
// Sleep 1 Second
Thread.Sleep(1000);
// Get the current time
System.DateTime dt = System.DateTime.Now;
// If the second has changed
// notify the subscribers

if (dt.Second != _second) {
// Create the TimeInfoEventArgs object
// to pass to the subscribers

TimeInfoEventArgs timeInformation =
new TimeInfoEventArgs(
dt.Hour,dt.Minute,dt.Second);

// If anyone has subscribed, notify them
OnSecondChange (this,timeInformation);
}
// update the state
_second = dt.Second;
_minute = dt.Minute;
_hour = dt.Hour;
}
}
}
// The class to hold the information about the event
// in this case it will hold only information
// available in the clock class, but could hold
// additional state information

public class TimeInfoEventArgs : EventArgs {
public TimeInfoEventArgs(int hour, int minute, int second) {
this.hour = hour;
this.minute = minute;
this.second = second;
}
public readonly int hour;
public readonly int minute;
public readonly int second;
}
/* ======================= Event Subscribers =============================== */
// An observer. DisplayClock subscribes to the
// clock’s events. The job of DisplayClock is
// to display the current time

public class DisplayClock {
// Given a clock, subscribe to
// its SecondChangeHandler event

public void Subscribe(Clock theClock) {
theClock.SecondChange +=
new Clock.SecondChangeHandler(TimeHasChanged);

}
// The method that implements the
// delegated functionality

public void TimeHasChanged(
object theClock, TimeInfoEventArgs ti)
{
Console.WriteLine(“Current Time: {0}:{1}:{2}”,
ti.hour.ToString(),
ti.minute.ToString(),
ti.second.ToString());
}
}
// A second subscriber whose job is to write to a file
public class LogClock {
public void Subscribe(Clock theClock) {
theClock.SecondChange +=
new Clock.SecondChangeHandler(WriteLogEntry);
}
// This method should write to a file
// we write to the console to see the effect
// this object keeps no state

public void WriteLogEntry(
object theClock, TimeInfoEventArgs ti) {
Console.WriteLine(“Logging to file: {0}:{1}:{2}”,
ti.hour.ToString(),
ti.minute.ToString(),
ti.second.ToString());
}
}
/* ======================= Test Application =============================== */
// Test Application which implements the
// Clock Notifier – Subscriber Sample

public class Test {
public static void Main() {
// Create a new clock
Clock theClock = new Clock();
// Create the display and tell it to
// subscribe to the clock just created

DisplayClock dc = new DisplayClock();
dc.Subscribe(theClock);
// Create a Log object and tell it
// to subscribe to the clock

LogClock lc = new LogClock();
lc.Subscribe(theClock);
// Get the clock started
theClock.Run();
}
}
}

Conclusion
The Clock class from the last sample could simply print the time rather tahn raising an event, so why bother with the introduction of using delegates? The advantage of the publisg / subscribe idiom is that any number of classes can be notified when an event is raised. The subscribing classes do not need to know how the Clock works, and the Clock does not need to know what they are going to do in response to the event. Similarly a button can publish an Onclick event, and any number of unrelated objects can subscribe to that event, receiving notification when the button is clicked.
The publisher and the subscribers are decoupled by the delegate. This is highly desirable as it makes for more flexible and robust code. The clock can chnage how it detects time without breaking any of the subscribing classes. The subscribing classes can change how they respond to time changes without breaking the Clock. The two classes spin indepentdently of one another, which makes for code that is easier to maintain.

(Source: http://www.akadia.com/services/dotnet_delegates_and_events.html )

Cross-Page Postbacks in ASP.NET 2.0

By default, an ASP.NET web form posts back to itself whenever a postback operation occurs. That behavior wasn’t terribly common in web pages before ASP.NET appeared, and isn’t always what you want to have happen: What if you want to post a web form to another web page in the application? ASP.NET made that fairly difficult in ASP.NET 1.0, but ASP.NET 2.0 makes developers lives easier in this regard by adding a new feature called “cross-page postbacks” that allows a web page to post its data back to a different web page. In cross-page postbacks, the page that initiates the postback is called the source page and the page to which the client posts is called the target page. Conveniently, the target page can still retrieve any control values posted by the source web page in a cross-page postback operation. In other words, from a development point of view, you can process the posted data in much the same way you would process any normal ASP.NET postback. The new feature means ASP.NET 2.0 developers now have three techniques for transferring processing from one web page to another in an application: Response.Redirect, Server.Transfer, and the new cross-page postback features.

The Response.Redirect Method

The Response.Redirect method is by far the simplest method of redirecting processing from a source page to a different destination or target page in the same or even across different applications. When the web server receives a request for redirection, it sends a response header to the client that causes the client to send a new request back to the server. In other words, a redirect causes two request/response cycles: one for the original and one for the new redirected request.
ASP.NET makes redirects easy. The following code snippet illustrates how to use the Response.Redirect method:
protected void Redirect_Click(object sender, EventArgs e) {
Response.Redirect(“menu.aspx”);
}
Note that the redirect request is a GET request, meaning that you can’t post data from the original page via the Response.Redirect command. You can work around that to pass data from the source to the target by using query strings in the Response.Redirect method call as shown below:

protected void Redirect_Click(object sender, EventArgs e) {
Response.Redirect(“menu.aspx?userName=” + UserName.Text));
}

The preceding example passes a query string as a parameter to the Response.Redirect method along with the target URL. The target can now use code such as the following to retrieve the source’s data as shown below:

protected void Page_Load(object sender, EventArgs e) {
string userName = Request[“userName”];
//Other code
}

The Server.Transfer Method
Instead of relying on the client to make a request to a new page, Server.Transfer is a server-side redirection technique that simply changes the code’s “focus” on the web server to a new page. Server.Transfer is far more efficient than Response.Redirect when the target page resides on the same web server, because it avoids the extra roundtrip and uses only server resources for the redirection. Note that it has a side effect—the URL shown in the web browser does not change either—meaning clients may think they posted their data to one page while actually posting it to a different page. In most cases, that’s not a problem, but it can make debugging more difficult.
Server.Transfer also preserves the HttpContext of the initiating page; therefore the target page can access the control values of the source page. Specifically, you can use the FormsCollection property to retrieve the source control values from the target page when using the Server.Transfer technique. First, make sure you use the overloaded Server.Transfer method, which accepts two parameters—the target URL, and a Boolean preserveForm value which tells the server to preserve the query string and any control values from the source page. Set the preserveForm parameter to true in the source web page as shown below:
Server.Transfer(“Menu.aspx”,true);
Then, from the target page, to retrieve the value of a textbox control called txtUserName for example, use the following code:
object obj = Request.Form[“txtUserName”];

Response.Redirect vs. Server.Transfer
Because Response.Redirect requires an extra round trip, you should avoid it for high-volume web sites due to associated performance and scalability issues. However, this is the only technique that you can use to redirect from one web server to another. In contrast, Server.Transfer is more efficient, but can transfer execution only to a different web page on the same server. In essence, you’d use Server.Transfer to eliminate unnecessary roundtrips when both the source and the target web page reside on the same server (they can be in different ASP.NET applications on that server), and use Response.Redirect when you need to redirect to a different server.

Cross-page Postbacks
In ASP.NET 2.0, the cross-page postback feature lets you configure page controls that implement the new IButtonControl interface to post to a different target page from the originating web form. Like Response.Redirect, cross-page postbacks are a client-based transfer mechanism—but like Server.Transfer, the target web page can (usually) access the control values of the source page. To set up a cross-page postback, you specify the target URL in the PostBackUrl property of the source web page.
Implementing Cross-page Postbacks
This section discusses how to implement cross-page postbacks in an ASP.NET 2.0 application. If you’re not already thoroughly familiar with ASP.NET postback operations, you can find out more here.
To begin with, assume that you have two web pages—a source and a target. The source web page initiates the cross-page postback operation using a Button control. You first have to set the PostBackUrl property for the button to the target web page’s URL. Incidentally, all web controls that implement the System.Web.UI.WebControls.IButtonControl interface have the cross-page postback feature enabled by default. The following code snippet shows how to set the property.

When you set the PostBackUrl property, the ASP.NET framework binds the corresponding control to a new JavaScript function named WebForm_DoPostBackWithOptions, generating HTML similar to the following:

With that HTML in place, when a user clicks the button, the browser will submit to the target URL (Target.aspx) rather than to the source URL.

Retrieving Source Control Values from the Target Page
ASP.NET 2.0 provides a new property called PreviousPage that returns a reference to the type of the source page whenever a cross-page post back occurs in your web application. Note that this property contains null (it’s not initialized) when the source and the target web pages are in different applications. Also note that when the target page accesses the PreviousPage property to retrieve the control values of the source page, the ASP.NET runtime loads and executes the source page. That fires the ProcessChildRequest event handler. Moreover, it also fires the Page_Init, Page_Load and any other source page button click events.
So that you can avoid doing all that work accidentally, it’s best to check the IsCrossPostBack Boolean property to determine whether a cross-page postback has actually occurred; the value is true if the target page was requested by a cross-page postback has actually occurred. The value is false if control reached the target web page another way (such as a normal request, a Response.Redirect, or a Server.Transfer). Here’s an example that illustrates how to use this property:

if ( PreviousPage.IsCrossPagePostBack) {
//Some code
}
The PreviousPage property works for both Server.Transfer and cross-page postbacks in ASP.NET 2.0. In ASP.NET 2.0, you can use the PreviousPageProperty after a Server.Transfer operation to retrieve the control values of the source page from within the target page. Here’s an example:

protected void Redirect_Click(object sender, EventArgs e) {
Server.Transfer(“menu.aspx”);
}

The receiving web page can now retrieve the control values of the sender web page as shown in the code snippet below:

protected void Page_Load(object sender, EventArgs e) {
if (PreviousPage != null) {
TextBox txtBox = (TextBox)
PreviousPage.FindControl(“txtUserName”);
if (textBox != null)
string userName = textBox.Text;
//Other code
}
}
Notice that the preceding code has to cast the txtUserName control to a TextBox type to access the value. There’s a better way, as you’ll see.

A Better Approach: Using PreviousPageType
The PreviousPageType property provides strongly typed access to a source web page in a cross-page postback operation, letting you retrieve control values from the source page without any typecasting overhead. The code snippets below illustrate how you can use this property.
In the source page you might write:

Note that clicking the button redirects the user to a “Menu.aspx” target page. The target page can retrieve the username and password values using this code:

protected void Page_Load(object sender, System.EventArgs e){
String userName = PreviousPage.txtUserName.Text;
String password = PreviousPage.txtPassword.Text;
//Other code
}

In the preceding code, the PreviousPageType property returns a strongly typed reference to the source web page, eliminating the need to cast.

ViewState Preserved
For cross-page postbacks, ASP.NET 2.0 embeds a hidden input field named __POSTBACK that contains the ViewState information of the source web page—provided the source page contains a server control with a non-null PostBackUrl property value. The target page can use that __POSTBACK information to retrieve the ViewState information of the source web page as shown below:

if(PreviousPage!=null && PreviousPage.IsCrossPagePostBack &&
PreviousPage.IsValid) {
TextBox txtBox = PreviousPage.FindControl(“txtUserName”);
Response.Write(txtBox.Text);
}

The preceding code checks to ensure that the PreviousPage property is not null. Incidentally, the PreviousPage property contains null if the target page is not in the same application. The IsCrossPagePostBack property will be true only if processing reached the target page due to a cross-page postback operation.
The Cross Page Postback feature, one of the most powerful features in ASP.NET 2.0, allows a web page to postback to another web page and still enable retrieval of the values of the server controls of the source web page from within the target web page seamlessly.

(Source: http://www.devx.com/dotnet/Article/33835/1763/page/1 )