Develop Reference

Response.Write not working in ASP.NET

I have a calendar named poDateCalendar, I wanted to make it so whenever the user selects the calendar to be any earlier than today's date, there will be a response as follows:
protected void poDateCalendar_SelectionChanged(object sender, EventArgs e)
{
if (poDateCalendar.SelectedDate < System.DateTime.Today)
{
Response.Write("<script>alert('Warning: you are selecting a PO date earlier than today's date')</script>");
}
poDateCalendar.Visible = false;
poDateBtn.Text = poDateCalendar.SelectedDate.ToShortDateString();
}
It worked the first time I tried it, but somehow now it doesn't work anymore, is there anything I did wrong?
I also tried doing breakpoints, it passed through the if statement and did run the response.write. However, there's just nothing that is displayed after.

Just replace your code with the following line
Response.Write("<script> alert(\"Warning: you are selecting a PO date earlier than today's date\");</script>");
In your case your code is failing because you have a single quote at "today's" which is making your code to fail. So I am using escape sequence at start and end of alert message.

Check the raw HTML sent to the browser. Likely that script was rendered before the opening html tag. There is no page or DOM yet, and so even if the browser ran the javascript at all it wouldn't know what to do with it.
This happens because of where you are in the ASP.Net Page Life Cycle at the time that code runs. A page class in ASP.Net works by first building up all of the controls and properties to have the correct data. Then, once that is all finished, there is a rendering phase where those controls are transformed into HTML for your browser. Before this rendering phase, anything you send to Response.Write() is going to a response stream that hasn't even sent headers to the browser yet.
To get around this, instead of writing to the Response stream direclty, use the Page.RegisterStartupScript() method.

I don't quite get the point - Allow users to do what they're not supposed to do, and then shout when they do so. + Message box / alerts are the old school ways of doing things. That's a bad user experience, IMHO.
I have a better suggestion here for you. You could instead disable to previous dates so that they can't select in first place. You can use DayRender event for that.
<asp:Calendar ID="poDateCalendar" runat="server" OnDayRender="poDateCalendar_DayRender" />
In code-behind:
protected void poDateCalendar_DayRender(object sender, DayRenderEventArgs e)
{
if (e.Day.Date < DateTime.Today)
{
e.Day.IsSelectable = false;
// Additionally grey out dates if you want
}
}

Difference between !IsPostBack and refresh in Asp.Net

I have written some codes in !IsPostBack block. This code is getting executed when the page loads for the first time. That is fine. But the problem is, when I refresh the page by hitting the f5 key this executes again which I don't want to do. I have searched many articles and found difference between PostBack and refresh. I know about this. But my question is difference between !IsPostBack and Refresh. Can we write some code which executes only when the page loads for the 1st time not when we refresh the page. By the way I have written my !IsPostBack block inside Page_Init() method and I am using c# for codebehind. Thanks.

Refersh and IsPostback are somewhat unrelated:
Refresh in browser generally mean "re-run last action that resulted in this page". Usually it causes GET request, but it as well can cause POST if page was shown as result of postback. Side note: you often can find sites warning you not to refresh page during non-reversible operations like "charge my credit card" as it may trigger duplicate post.
IsPostBack simply states that request come to server as POST, not GET.
Combining that you can get Refresh that triggers either branch of if (IsPostBack) check. In most cases so server will receive GET request and hence execute !IsPostBack branch.
If you really need to detect if page was rendered once already - setting cookie or writing information into Session would be reasonable solution.

Please change your code behind code as given below.
Boolean IsPageRefresh;
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
ViewState["postids"] = System.Guid.NewGuid().ToString();
Session["postid"] = ViewState["postids"].ToString();
}
else
{
if (ViewState["postids"].ToString() != Session["postid"].ToString())
{
IsPageRefresh = true;
}
Session["postid"] = System.Guid.NewGuid().ToString();
ViewState["postids"] = Session["postid"].ToString();
}
}

ASP.NET UpdatePanel: Make browser "back" button return to *last* version of the page

I have page A and page B. You can do the following things in page A:
Do stuff on page A (e.g., choose an item in a list box), which causes an UpdatePanel in page A to be redrawn with additional information.
Move on to page B. This is done with a Button and Response.Redirect.
Now the problem is as follows:
The user does stuff on page A. Page A is now different from its initial state.
The user moves to page B.
The user hits the back button of the browser.
What happens: Page A in its initial state is shown.
What I would like to happen: Page A in its final state is shown (i.e., with the "correct" item selected in the list box).
I know about ScriptManager.AddHistoryPoint! As far as I can see, it does not solve my problem:
I could call AddHistoryPoint every time something is done on page A. This is bad, because it litters the browser history with lots of entries. (A new entry every time a different list box item is selected.) But that's exactly what I want to avoid by using an UpdatePanel! Of course, if there were a ReplaceLastHistoryPoint method, that would be perfect, but I did not find one...
I tried to call AddHistoryPoint right before Response.Redirect, to save only the last state of page A, but, alas, that doesn't work (no history point is saved). This is not surprising, considering how Response.Redirect works.
Is there some solution I have missed? I'm using .NET 3.5SP1, in case it matters.

This is a bit of an old question, but I'll go ahead and provide the mechanism I use for this. The basic idea is that instead of allowing AddHistoryPoint to manage your name-value pairs, just allow it to manage a key to your NameValueCollection that you keep somewhere else, like in your Session cache. Then as subsequent Ajax requests come in, you never make another call to AddHistoryPoint; instead, you just replace your NameValueCollection with the state of the current request.
The only other bit is to keep track of whether you're on your first Ajax call and need to make that first call to AddHistoryPoint or not.
My code looks something like this:
protected void Page_LoadComplete(object sender, EventArgs e) {
if (!ScriptManager.GetCurrent(this).IsNavigating && (IsCallback || IsInAsyncPostback())) {
var state=new NameValueCollection();
//OnCallbackHistory(state); // this gets state for all interested parties
if (state.Count != 0) {
string key=ViewState["HistoryStateKey"] as string; // empty on first AJAX call
if (string.IsNullOrEmpty(key) || ScriptManager.GetCurrent(this).EnableHistory) {
key=CallbackHistoryKeyRoot+Interlocked.Increment(ref callbackHashKey).ToString();
ViewState["HistoryStateKey"]=key;
ScriptManager.GetCurrent(this).AddHistoryPoint("", key);
}
Session[key]=state;
}
}
}

Instead of calling AddHistoryPoint on the server you could call addHistoryPoint on the client using the Sys.Application class, http://msdn.microsoft.com/en-us/library/vstudio/cc488025(v=vs.90).aspx.
So you'd add a client side click listener to the button which would addHistoryPoint on the client before the button does the post back and redirect.

Submit a form from code behind

I'm having trouble implementing a functionality on my c#/asp.net app.
I have a form with a RadioButtonList and a submit button.
The RadioButtonList is generated on Page_Load() from a list of objects I retrieve from the database.
I would like to automatically submit the form if there is only 1 object in the list.
I have access to my Form object, to the submit button etc... but I can't seem to find a solution (in the end I'm looking for a kind of form.Submit() ) method.
Does anyone have an idea of how I could do this ?
Thanks in advance !
EDIT >> Here is the code :
.aspx : http://pastebin.com/0E6T7dqH
.aspx.cs : http://pastebin.com/54payZJP
EDIT2 >>>
As it seems there is no way to do what I wanted to do at first, I ended up using a session variable with a response.redirect()
Source :
http://dotnetslackers.com/Community/blogs/haissam/archive/2007/11/26/ways-to-pass-data-between-webforms.aspx

Post happens in the client side. As in Page_Load you are currently executing in the server side, just call the code you want to execute on post.
Edit: For actually going to another aspx
public void Page_Load(object sender, EventArgs e) {
if(!IsPostback && OnlyOneItem) {
Server.Transfer("TheOtherPage.aspx");
}
}
Server.Transfer will maintain the entire request, so your post data will be available.
http://msdn.microsoft.com/en-us/library/system.web.httpserverutility.transfer.aspx

Try something like this
In your Page_Load
if(!IsPostBack)
{
if(check for only one object)
{
//your submit code
}
}

I actually had to do something similar, once. Here is a way you can do it.
Asp.Net buttons have a property called PostBackUrl, and it does exactly what you would expect - it controls where the form will post if you click the button.
You can also use the RegisterStartupScript function to render javascript on the page.
Now, with these two pieces, you can achieve your goal.
if(!IsPostBack)
{
if(results == 1)
{
button.PostBackUrl = "next page's url"
//Register script to click the button using RegisterStartupScript
}
}
Now, having shown you this, I will warn you it may not make for the best user experience. When I did it, it was for a very specific case that had no other solution. The page will actually post back to the user, and they will see the page for a moment before the javascript to click the button takes effect. Additionally, when you set a button's PostBackUrl, that means that when it is clicked, your entire form will be posted to the page specified. The code behind for the current page will not fire at all, so if you have any validation, it won't run.
There's nothing wrong with letting the user click the button to submit the form even if they only have one choice. In my experience, users like to feel like they are in control on the system; they don't like it when pages just do things without their input.
Also, there is not really anything wrong with putting the information the next page needs into the session or even a database table, and using Response.Redirect. It's a fairly common practice and works reliably in most scenarios.

Call an event handler on another page

Simple question here, but I've got a nagging feeling that there's a more interesting solution than the one I've chosen:
Page Two consists of a dropdown, and the change event is handled to execute some query.
protected void ddlSavedQueries_SelectedIndexChanged(object sender, EventArgs e)
{
/* stuff happens */
}
Page One is a home page, where I'm providing another version of that dropdown. I'd like the change event in this case to redirect control to Page Two, and then execute the event handler.
My cheap solution is just a Redirect with a querystring value that is handled on page load. Am I missing a more interesting approach?

If you don't want to ugly things up with a querystring value, I suppose you could put something in Session and pick it up on Page_Load of the second page (and then clear it out of Session). Not exactly an awesome improvement though.

Does the same page always get displayed when you change that dropdown? If so, consider using client side javascript to redirect to the correct page, then fire any logic on the subsequent page in the page_load event. Example using jQuery:
$(function() {
$("select.classyouneedtodefine").change(function() {
document.location.href = "somepage.aspx?value=" + $(this).val();
});
});
haven't tested the above...just shooting from the hip