Observer Design Pattern using Delegates/events

Today I want to discuss with you how to implement Observer design pattern using delegate and event in asp.net /c#.

Observer Pattern

The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods. It is mainly used to implement distributed event handling systems. The Observer pattern is also a key part in the familiar Model View Controller (MVC) architectural pattern.(wikipedia).

UML class diagram of Observer pattern(wikipedia)

The Observer Pattern describes the dependence relationship between one object (observable) to many objects (observers). It's also called Model/View, Dependents, or Distributor/Listener pattern.This article introduces an implementation when we can use the Observer pattern to keep all observer objects updated after things have changed.

Case project

In this case, patient is an ObservableObject that needs to send out the notification when the hospital updates his/her physical status. The hospital will determine who will be notified when the patient status is updated.

Prevent Duplicate record insertion on page refresh in asp.net

Problem:
Today I have faced a new problem(new for me), when I press save button it works fine and then refresh(F5 or Refresh button on browser) the page but this time it is execute the button click event again. For this duplicate data posting on the server.

I try to find out the problem, nothing wrong in my code. Now turn to google and figure out the problem with solution. I have got several solutions. Now I would like to share this solution which solved my problem ......

Solution:

Assume that in Test.aspx page you have a button and a TextBox:

<asp:Button ID="btnSubmit" runat="server" Text="Submit" Width="110px" OnClick="btnSubmit_Click" />
<asp:TextBox ID="TxtName" runat="server"></asp:TextBox>

In Test.aspx.cs..:

public partial class Test : System.Web.UI.Page
{
protected override void OnInit(EventArgs e)
{
btnSubmit.Click += new EventHandler(btnSubmit_Click);base.OnInit(e);
}
protected void Page_Load(object sender, EventArgs e)
{

}
private void Save_Click(object sender, EventArgs e)
{
if (CheckIfDuplicateRequest())

{
return;

}
//Your code to insert the value of the SomeText textbox into the database.
//Set the value of one of the key fields in your web form in session
Session["Refresh"] = TxtName.Text;

}

private Boolean CheckIfDuplicateRequest()
{
if (Session["Refresh"] == null)
{
//Probably submitting the page for the first time
return false;
}
String previousValue = Session["Refresh"].ToString();
if (previousValue != TxtName.Text)
{
//Submitting the page with a different set of values
return false;
}
//Duplicate request.
return true;
}

}

you can get more help from these post:
http://aspalliance.com/687
http://forums.asp.net/p/1296011/3702515.aspx
http://forums.asp.net/p/1190997/2045619.aspx