IsPostBack in ASP.NET

What is IsPostBack in ASP.NET?

IsPostBack is a Page level property, that can be used to determine whether the page is being loaded in response to a client postback, or if it is being loaded and accessed for the first time. 

In real time there are many situations where IsPostBack property is used. For example, consider the webform used to register employee details. A sample form that we will use for this example is shown below. The form has First Name, Last Name and City fields. 

 If you want to follow along with me, copy and paste the following HTML in a web form.

<table style="font-family: Arial">
<tr>
<td colspan = "2"><b>Employee Details Form</b></td>
</tr>
<tr>
<td>First Name: </td>
<td> <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox> </td>
</tr>
<tr>
<td>Last Name: </td>
<td> <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox> </td>
</tr>
<tr>
<td>City:</td>
<td>
<asp:DropDownList ID="ddlCity" runat="server">
</asp:DropDownList>
</td>
</tr>
<tr>
<td></td>
<td>
<asp:Button ID="Button1" runat="server" onclick="Button1_Click"
Text="Register Employee" />
</td>
</tr>
</table>

Copy and Paste the following code in the code behind file of the web form.
protected void Page_Load(object sender, EventArgs e)
{
LoadCityDropDownList();
}
public void LoadCityDropDownList()
{
ListItem li1 = new ListItem("London");
ddlCity.Items.Add(li1);

ListItem li2 = new ListItem("Sydney");
ddlCity.Items.Add(li2);

ListItem li3 = new ListItem("Kabul");
ddlCity.Items.Add(li3);
}
protected void Button1_Click(object sender, EventArgs e)
{
}