Develop Reference

WebForms TextBox Lost Focus Event

I'm trying to add an event handler to a TextBox control that is called when the text box loses focus. Is this possible?
I know WinForms has a LostFocus event. I'm looking for something similar in ASP.NET WebForms.

You can Use OnBlur
<asp:TextBox id="TextBox1" runat="server" onblur="Javascript:alert('1234');" />
Or
EDIT:
Server side
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
TextBox1.Attributes.Add("onblur", "alert('hi User')");
}
}

I think you are going to have to use javascript for this and the event you will need to search and look up would be onBlur() I believe. If you need an example, I might be able to scrounge one up for you. But here are some links..
W3Schools
Stack
Java Page
Hope these help.

This would be the blur event, and it's a client-side event on the resulting input element instead of a server-side event on the TextBox control.
For example, let's say you have a TextBox with the ID of "TextBox1", and let's assume jQuery is included in your project template (because it probably is), then in your client-side code you might have something like this:
$('#<%= TextBox1.ClientID %>').blur(function () {
// code to respond to the event
});
This would execute any code in that function whenever the input for that TextBox control loses focus. Notice the use of a small block of server-side code to output the server-side control's client-side ID.

How do I validate a textbox when it no longer has focus?

I would like to validate my textbox when the textbox is deselected.
It should validate the textbox value in the same way as Google's Create Account page.
If the textbox value passes validation it should proceed further otherwise it should show message like: This value is not valid.

Events of Validation And LostFocus can be you here.
Here is some code for validate textbox value
http://msdn.microsoft.com/en-us/library/system.windows.forms.control.lostfocus.aspx

if its a desktop app/WinForms u can use what Suspitsyn Kirill said, and for a web app it would be quite close.
The textbox has event handlers for on focus etc. using the code behind you can assign what the events will do.
Dont remember the C# ones exactly but it should be
private void textBox1_LostFocus(object sender, System.EventArgs e)
{
textBox1_Validating()
}
add the lostfocus on the element to direct to this method.
You could also do it with javascript same concept.

How to show MessageBox in Asp.Net based on conditions given in the Server side code?

I have found so many questions in Stack Overflow based on "Displaying Message Box in Asp.Net".
But is it possible to check the condition in the button click event and to display MessageBox if the Condition is true? How?

I think Yes, you can check your condition in the PAGE LOAD event and set the visible propertie of the message box at this time..
If you want the condition can be set in a session variable and the message box can be written in the master page. Like this you just have to set the condition to true from everywhere when you decide that the message have to be visible.
After Question edition :
Yes you can, just catch the button_click event do verification stuff...
Aspx
<asp:Button ID="btnValid" runat="server" Text="Valid" OnClick="btn_click" />
Code Behind (C#)
public void btn_click(Object sender, EventArgs e)
{
//Verification Stuff and set the messageBox visibility
}

Can I fire a Text Changed Event for an asp.net Text Box before it loses focus?

I have an asp.net TextBox in which I want to check if the text entered into the TextBox is > 0. It works once I tab out or click out of the TextBox, but if I keep focus on the TextBox, it won't fire the Text Changed Event, so I have the following scenario, I want to enable something if and only if the TextBox.Text.Length = 0. Now, if I put my caret in the TextBox and delete all the characters and then leave the caret in the TextBox so it still has focus and take my mouse and click a button, it will not do what it was supposed to do because it never fired the Text Changed Event. How would something like this be handled?

friend, keyup, keydown and keypress are your friends

The best idea is to write some client-side javascript to do what you want. The TextChanged event handler requires a postback to the server, and posting back to the server before a text box loses focus is impossible. Unless that is what you intend, I would suggest the former.

you can also use the setInterval javascript method to check for a change in the value of the textbox on a timed basis. just remember, you need to use the form name followed by control name and value to reference the control.
setInterval(MethodName, 100);
function MethodName()
{
if(formname.controlid.value.length > 0)
{
//do something here
{
}

Determine when Postback occurs because of a textbox being changed

I've got a usercontrol that has a <asp:TextBox> with autopostback set to true. When the user types text in and moves off the textbox, asp.net fires a postback event. However, it isn't handled specifically by the textbox, but is instead simply a postback - the page_load fires and then life goes on.
What I need is a way to know that this textbox fired the event. I then want to fire my own custom event so that my main page can subscribe to it and handle the event when the textbox changes.
At first I thought I could capture the sender - I was assuming that the sender would be the textbox, but it appears to be the page itself.
Anyone have any thoughts?

I think you're missing something, so I want to explain it step by step :
your textbox should be something like that :
<asp:TextBox runat="server" ID="TextBox1" AutoPostBack="true"
OnTextChanged="TextBox1_TextChanged"></asp:TextBox>
and in your codebehind you shhould have an event like that :
protected void TextBox1_TextChanged(object sender, EventArgs e)
{
string str = TextBox1.Text;
}
NOTE : if you want to attach your event in code-behind you can do this by :
TextBox1.TextChanged +=new EventHandler(TextBox1_TextChanged);
but if you talking about custom controls, you should implement IPostBackDataHandler interface to raise events.

Is this i dynamically created usercontrol or textbox? If so you have to recreate that control in the page_load event and the event should be fired on the textbox as normal.

The page load sender will always be ASP.default_aspx because the page is what is calling that event.
You can hook up to the even OnTextChanged where that sender would be the textbox that has changed or caused the post back.
Keep in mind the page load will always fire before any of the events on the controls.

This is potentially something that could be overlooked, but do you actually have an event coded for the textbox's TextChanged event? If you have it set to autopostback but never actually created an event for it, you're only going to get your page load and other related events.

As alluded to in other answers you need to handle the 'OnTextChanged' event.
<asp:TextBox ID="TextBox1" runat="server" OnTextChanged="TextBox1_TextChanged"></asp:TextBox>
private void TextBox1_TextChanged(object sender, System.EventArgs e)
{
TextBox txt = (TextBox)sender;
string id = txt.ID;
switch(id)
{
case "TextBox1":
break;
}
}