Usage of doPostBack in a Real Environment

Jaish Mathews

7y229.4k11100Article

C#ASP.NET JavaScript PostBackjaishdoPostBack

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Shalin Bhavsar11y ago
HI,I am working on "Android app",I called web paged from server into "Webview". When my URL load at moment I am finding page an Element using "javascript" and Display/Hide them.But when AutoPostback event fired,My URL not call in Webview,How to handle it?if any one know then plz tell me.
Roman15y agoedited
Hi, There can be some problems when calling __doPostBack from inside a page which is on a master page or from within a ascx control. You need to use element.UniqueID instead of element.ClientID. See more details on my article: http://extremedev.blogspot.com/2011/03/aspnet-dopostback-not-working.html ----------------------------------------------------------- Development Solutions: http://extremedev.blogspot.com
visaka19y ago
I am validating a phoneno in textbox.If the phone no is invalid i will display a messagebox specifying error.On clicking ok on the message box i need to focus the cursor in the phoneno field.I have done this.But a problem occurs that when the user enters an invalid number(Previously tried numbers) by pressing the down arrow key and clicks the mouse on next textbox that invalid number will be accepted without the message box display.If i enter the same number again it will be accepted?What is the solution? my code is as follows (javascript) function FormValidate() { if(document.WO_form.txt_Phoneno.value.search(/\d{3}\-\d{3}\-\d{4}/)==-1) { alert( "Invalid phone number.\r\nPlease enter a phone number with the format xxx-xxx-xxxx."); document.getElementById( "txt_Phoneno").focus(); document.getElementById( "txt_Phoneno").value=""; /* clearing previously entered error value*/ } } i am calling it on textchanged event.
- Jaish Mathews19y agoedited
  Hi Visaka You didn't mentioned that which type of TextBox control r u using web, html runat or mere html. I am assuming that you are using a asp.net webcontrol. If that is the case, the better logic for validation is, check the control validations at the time of submission ie at the time the button clicked. If the validation passed, submission will occur otherwise not. you can easily call the client side validation methods during a button click using "Attributes" property . Ie the following code will call a javascript method named "ValidateFields" during button click. I assumed that ur button id is "Button1" //Add this on the Page_Load event section of code behind. Button1.Attributes.add("onclick","return ValidateFields();"); //Below is the javascript method function ValidateFields() { var returnValue = true; if(<checking one validation for a failure>) { returnValue = false; } if(<checking another validation for a failure>) { returnValue = false; } return returnValue; } NB:If you want to check one control validation during entry itself rather than submission, then better is using "blur" event of the text box. Because that will give you a Lost_Focus event effect. JaishMathews
visaka20y ago
I am trying to send mails using C#.Presently i would like to use localhost as SMTP server.I have done it as follows MailMessage mail = new MailMessage(); mail.To = "[email protected]"; mail.From = "[email protected]"; mail.BodyFormat = MailFormat.Text; mail.Subject = "welcome message"; mail.Body = "Hearty welcome to our land"; SmtpMail.SmtpServer = "localhost"; //I tried with IP address of my system// SmtpMail.Send(mail); In addition to this i made some configurations to IIS server (SMTP virtual server) reffering sites :I changed connection and relay to the IP of my machine.It is not working.Also i tried adding the IP as 127.0.0.1 but in vain.I am not getting any error messages. But the mail i sent is found in the folder inetpub/mailroot/Queue.It is not available to the recepient.Do i have to take into account the mailaddresses of sender and receiver.Can i use any valid mailid's(yahoo.gmail......)?Can u suggest an idea to solve it?
- Jaish Mathews20y ago
  Hi, First of all u want to check that, after a restart of ur machine, all mails in ur QUE folder is sending or the mails are still be in that folder after the resatrt. If mails are sending after restart, contact ur network admin, as it's only because of small security/config issues.If ur mails are not senidng even after restart, u want to reconfigure ur mail server, as SMTP can't send the mail with the existing configuration and may want to test with real email addresses like yahoo/gmail. Why are u using CDO objects [CDONTS(before XP) or CDOSYS(after XP)] for sending mails? those components may in ur system32 page.Just refrer to that dll.You can fetch enormous samples to use CDO, as it's popular even at ASP era. JaishMathews
visaka20y ago
In ur reply u have mentioned: server side event for a webcontrol to work, make the AutoPostBack="true" But in the same code i have used a button as follows: <asp:Button ID="cmdship" runat="server" Text="SHIP" OnClick="Button1_Click"/> In the Button1_Click event I have put values of a form to database(in C#).Here i am not using AutoPostBack="true" in the web control.But it works well.Why is it so?
- visaka20y ago
  Sir, Now the concept of postback became clear for me.Thanks 4 ur help.
- Jaish Mathews20y ago
  Hi, you want to find the purpose of postback.In GUI let devide controls into 2 based on their purposes. 1st type controls meant for storing and displaying information.ie texboxes,comboboxes etc..Some times there is no need for this controls to interact with webserver and only need to perform within browser. So we may On/OFF AutoPostBack according to those needs. 2nd type controls meant for firing an event and perform some actions. <Asp:Button is such a control meant for perform an event on server side (not client side) always. So when we click on a webcontrol button, that button will automatically fire a postback(no need to set it explicitly) to the page itself where it resides. if u want a button with action only resides on client side, use a HTML control button. Web control buttons are bydefault autopost. JaishMathews
visaka20y ago
sir, I have two files named controls.aspx and controls.aspx.cs.In controls.aspx i have included a code as below <asp:DropDownList ID="Dropdownlist" runat="server" Width="51px" OnSelectedIndexChanged="DropDownList3_SelectedIndexChanged" > <asp:ListItem>-----</asp:ListItem> <asp:ListItem Value="No">No</asp:ListItem> <asp:ListItem Value="Yes">Yes</asp:ListItem> </asp:DropDownList></div></td> On the second file i have included a code to execute while selecting a particular item from dropdown list(The included code is to fill some textboxes(as web controls) with some hardcoded values while clicking "Yes").The code is as below protected void DropDownList3_SelectedIndexChanged(object sender, EventArgs e) { if(Dropdownlist.SelectedItem.Value.Equals("Yes")) { cust_state.Text = "Orissa"; //cust_state is the attribute id of textbox cust_country.Text = "India"; } } But this is not working.No effect while selecting any value from combobox.I have heard of some postback problems.But it is not so clear to me.Can u tell the problem of code and the need of postback in this context.
- Jaish Mathews20y ago
  Hi Visaka, You need to do very little for ur code to work, as just redefine ur combobox as below "<asp:DropDownList AutoPostBack="true" ID="Dropdownlist" runat="server" Width="51px" OnSelectedIndexChanged="DropDownList3_SelectedIndexChanged" >" Ie just add AutoPostBack="true" attribute. If u r not aware of postback, then want to know that a visit of webpage from client page to webserver. You handled a server side event like "OnSelectedIndexChanged="DropDownList3_SelectedIndexChanged". So it's obvious that to handle this code, a trip to webserver must occur. To activate this webserver trip and perform the sever side actions, you need to set the "AutoPostBack" property of a web control to "True", as by default it's "False". So the point s are 1.Any server side event for a webcontrol to work, make the AutoPostBack="true" 2.When u are doing with client side event(using Javascript), no need of this formality. Hoping this will help you. JaishMathews

Write a JavaScript function as follows
1. <script language="javascript">
2. function enablePostBack()
3. {
4. //T1 is the first argument(name of our control) I mentioned earlier and give the
5. // value of second argument as "" that's all
6. __doPostBack("T1","");
7. }
8. </script>
Call the above function on the blur event of T1 like onblur = "enablePostBack();"
Just paste a linkbutton control on the page(Purpose of this will discuss later)

Now run the program and you will see our tool is working well. The only thing we extra done is manually fired a postback using "__doPostBack";. So I think you got the situations in which we want to use "__doPostBack".

4. Please take some precautions.

Before using "__doPostBack" you want to confirm about certain things. After all "__doPostBack" fires a "postback". Just do the things as below:

Just remove the pasted linkbutton from the above mentioned program and run it. You will try to enter a value in T1 and press tab, then a script error will generate with message as "object expected". Yes expected objected not got. That's the problem and the object is "__doPostBack". ULTIMATELY A POSTBACK WILL ONLY FIRE, IF THERE IS A CONTROL IN THE PAGE WITH A PROPERTY "AUTOPOSTBACK=TRUE". So just paste a web "TextBox" control and make it's "autopostback" property to "True". Then ur program will run without any problem. There is another method also to avoid the above mentioned script error. Just paste a web "linkbutton" control on the page and the program will be perfectly OK. Now you got why I paste a "linkbutton" in our program. But pasting controls to avoid this error is not professional. Do you want to handle it in a professional manner? Just remove the "linkbutton" from the page and add the following to the HTML part.

Add 2 hidden controls as below
1. <input type =hidden name ="__EVENTTARGET" value ="">
2. <input type =hidden name ="__EVENTARGUMENT" value ="">
Include a JavaScript function as below
1. function __doPostBack(eventTarget, eventArgument)
2. {
3. document.Form1.__EVENTTARGET.value = eventTarget;
4. document.Form1.__EVENTARGUMENT.value = eventArgument;
5. document.Form1.submit();
6. }
Now you have only 2 controls (T1&T2) in your page and not extra controls or linkbutton for activating the "postback". Run the program and you will see it works. So using the above javascript function and hidden controls along with "__doPostBack" is ofcourse the professional way of activating the client side postback.