SELECT LAST ITEM IN LISTBOX

 Use the following code but select the element before the last

 LIST_ITEM.TopIndex = LIST_ITEM.Items.Count - 1

Get value of ReadOnly TextBox in ASP.Net and retain its value across PostBack

Here Sandeep Kumar Rauniyar has explained how to get the value of ReadOnly TextBox on PostBack and also how to retain its value across PostBacks.

In this article I will explain how to get the value of ReadOnly TextBox on PostBack and also how to retain its value across PostBacks.

 A common example is when working with Calendar Control, when date is selected the value is set client side and if the TextBox is set as ReadOnly the selected date value is lost and the updated value is not available in the TextBox Text property on PostBack.

Issue

Whenever for the ASP.Net Textbox if the ReadOnly property of the Textbox is set to true, the selected data is not available in the Text property of the Textbox on PostBack as shown in the screenshot below.

 

Reason

 

The reason behind this issue is that whenever value of an ASP.Net Textbox is set to ReadOnly and the value of the textbox is set client side using client side script like JavaScript, in such cases the value of the Textbox is not available in the Text property of the ASP.Net Textbox. The ASP.Net AJAX Control Toolkit CalendarExtender makes use of JavaScript to set the selected date in the Textbox.

 

Solution

 

The solution to the issue is making use of Request.Form collection. As this collection has values of all fields that are posted back to the server and also it has the values that are set using client side scripts like JavaScript.

 

Thus we need to do a small change in the way we are fetching the value server side.

 

C#

 

protected void Submit(object sender, EventArgs e)

{

    string date = Request.Form[txtDate.UniqueID];

}

 

VB.Net

 

Protected Sub Submit(ByVal sender As Object, ByVal e As System.EventArgs)

    Dim strDate As String = Request.Form(txtDate.UniqueID)

End Sub

 

As you can see above I am fetching the value of the Textbox from the Request.Form collection using the UniqueID property of the Textbox which is nothing but Client Side name of the ASP.Net TextBox control

The screenshot below describes that the selected date value is now available server side when fetched using the Request.Form collection

 

Set MaxLength property to Multiline Textbox

It is so easy to limit the number of characters allowed in a normal textbox by using MaxLength property, But same doesn’t work if the TextMode property of textbox is set to Multiline.

By default TextMode = "SingleLine" for a normal textbox, and it gets rendered as an input type textbox and when we set its TextMode property to MultiLine, it gets rendered as a textarea.



Note: MaxLength property works only for input type and not for textarea. So to handle this, either you can use JavaScript, jQuery or any other scripts. But in this article I used C# to handle this.

So let's take an example, and take one normal textbox and one multiline textbox and see differences before and after writing codebehind.

.aspx:

1. <asp:TextBox ID="txtMultiLine" runat="server" TextMode="MultiLine" MaxLength="5"></asp:TextBox>    
2.         <br />    
3.  <asp:TextBox ID="txtNormal" runat="server" MaxLength="5"></asp:TextBox>    

CodeBehind:

1. using System;    
2. using System.Web.UI;    
3.     
4. namespace Maxlength    
5. {    
6.     public partial class MacLength : System.Web.UI.Page    
7.     {    
8.         protected void Page_Load(object sender, EventArgs e)    
9.         {    
10.             if (!Page.IsPostBack)    
11.             {    
12.                 txtMultiLine.Attributes.Add("maxlength", txtMultiLine.MaxLength.ToString());    
13.             }    
14.         }    
15.     }    
16. }  

I hope you enjoyed it. Please provide your valuable suggestions and feedback.

Types of Constructor in C# with Example

Constructor is a special method of a class which will invoke automatically whenever instance or object of class is created. Constructors are responsible for object initialization and memory allocation of its class. If we create any class without constructor, the compiler will automatically create one default constructor for that class. There is always at least one constructor in every class.

Here you need to remember that a class can have any number of constructors and constructors don’t have any return type, not even void and within a class we can create only one static constructor.

Generally constructor name should be same as class name. If we want to create constructor in a class we need to create a constructor method name same as class name check below sample method for constructor

class SampleA { public SampleA() { Console.WriteLine("Sample A Test Method"); } }

Types of Constructors

Basically constructors are 5 types those are

      1.    Default Constructor

      2.    Parameterized Constructor

      3.    Copy Constructor

      4.    Static Constructor

      5.    Private Constructor

Default Constructor

A constructor without having any parameters called default constructor. In this constructor every instance of the class will be initialized without any parameter values like as shown below

using System; namespace ConsoleApplication3 { class Sample { public string param1, param2; public Sample()     // Default Constructor { param1 = "Welcome"; param2 = "Aspdotnet-Suresh"; } } class Program { static void Main(string[] args) { Sample obj=new Sample();   // Once object of class created automatically constructor will be called Console.WriteLine(obj.param1); Console.WriteLine(obj.param2); Console.ReadLine(); } } }

When we run above program it will show output like as shown below

Output

Welcome Aspdotnet-Suresh

Parameterized Constructors

A constructor with at least one parameter is called as parameterized constructor. In parameterized constructor we can initialize each instance of the class to different values like as shown below

using System; namespace ConsoleApplication3 { class Sample { public string param1, param2; public Sample(string x, string y)     // Declaring Parameterized constructor with Parameters { param1 = x; param2 = y; } } class Program { static void Main(string[] args) { Sample obj=new Sample("Welcome","Aspdotnet-Suresh");   // Parameterized Constructor Called Console.WriteLine(obj.param1 +" to "+ obj.param2); Console.ReadLine(); } } }

When we run above program it will show output like as shown below

Output

Welcome to Aspdotnet-Suresh

Constructor Overloading

In c# we can overload constructor by creating another constructor with same method name and different parameters like as shown below

using System; namespace ConsoleApplication3 { class Sample { public string param1, param2; public Sample()     // Default Constructor { param1 = "Hi"; param2 = "I am Default Constructor"; } public Sample(string x, string y)     // Declaring Parameterized constructor with Parameters { param1 = x; param2 = y; } } class Program { static void Main(string[] args) { Sample obj = new Sample();   // Default Constructor will Called Sample obj1=new Sample("Welcome","Aspdotnet-Suresh");   // Parameterized Constructor will Called Console.WriteLine(obj.param1 + ", "+obj.param2); Console.WriteLine(obj1.param1 +" to " + obj1.param2); Console.ReadLine(); } }

When we run above program it will show output like as shown below

Output

Hi, I am Default Constructor Welcome to Aspdotnet-Suresh

Copy Constructor

A parameterized constructor that contains a parameter of same class type is called as copy constructor. Main purpose of copy constructor is to initialize new instance to the values of an existing instance. Check below example for this

using System; namespace ConsoleApplication3 { class Sample { public string param1, param2; public Sample(string x, string y) { param1 = x; param2 = y; } public Sample(Sample obj)     // Copy Constructor { param1 = obj.param1; param2 = obj.param2; } } class Program { static void Main(string[] args) { Sample obj = new Sample("Welcome", "Aspdotnet-Suresh");  // Create instance to class Sample Sample obj1=new Sample(obj); // Here obj details will copied to obj1 Console.WriteLine(obj1.param1 +" to " + obj1.param2); Console.ReadLine(); } } }

When we run above program it will show output like as shown below

Output

Welcome to Aspdotnet-Suresh

Static Constructor

When we declared constructor as static it will be invoked only once for any number of instances of the class and it’s during the creation of first instance of the class or the first reference to a static member in the class. Static constructor is used to initialize static fields of the class and to write the code that needs to be executed only once.

using System; namespace ConsoleApplication3 { class Sample { public string param1, param2; static Sample() { Console.WriteLine("Static Constructor"); } public Sample() { param1 = "Sample"; param2 = "Instance Constructor"; } } class Program { static void Main(string[] args) { // Here Both Static and instance constructors are invoked for first instance Sample obj=new Sample(); Console.WriteLine(obj.param1 + " " + obj.param2); // Here only instance constructor will be invoked Sample obj1 = new Sample(); Console.WriteLine(obj1.param1 +" " + obj1.param2); Console.ReadLine(); } } }

When we run above program we will get output like as shown below

Output

Static Constructor Sample Instance Constructor Sample Instance Constructor

Importance points of static constructor

-      Static constructor will not accept any parameters because it is automatically called by CLR.

-      Static constructor will not have any access modifiers.

-      Static constructor will execute automatically whenever we create first instance of class

-      Only one static constructor will allowed.

Private Constructor

Private constructor is a special instance constructor used in a class that contains static member only. If a class has one or more private constructor and no public constructor then other classes is not allowed to create instance of this class this mean we can neither create the object of the class nor it can be inherit by other class. The main purpose of creating private constructor is used to restrict the class from being instantiated when it contains every member as static.

using System; namespace ConsoleApplication3 { public class Sample { public string param1, param2; public Sample(string a,string b) { param1 = a; param2 = b; } private Sample()  // Private Constructor Declaration { Console.WriteLine("Private Constructor with no prameters"); } } class Program { static void Main(string[] args) { // Here we don't have chance to create instace for private constructor Sample obj = new Sample("Welcome","to Aspdotnet-Suresh"); Console.WriteLine(obj.param1 +" " + obj.param2); Console.ReadLine(); } } }

Output

Welcome to Aspdotnet-Suresh

In above method we can create object of class with parameters will work fine. If create object of class without parameters it will not allow us create.

// it will works fine Sample obj = new Sample("Welcome","to Aspdotnet-Suresh"); // it will not work because of inaccessability Sample obj=new Sample();

Important points of private constructor

-      One use of private construct is when we have only static member.

-      Once we provide a constructor that is either private or public or any, the compiler will not allow us to add public constructor without parameters to the class.

-      If we want to create object of class even if we have private constructors then we need to have public constructor along with private constructor