I have the following code:
<tr id="test" runat="server">
<td align="<%=myalignment%>">
I set myalignment = "center", however, when the page is rendered, it doesn't grab the variable. It just shows exactly how it is above. I wanted to programatically change certain TDs to "center" or "left" and I thought this would work. Any ideas?
If tr doesn't have runat="server", then it works fine...
Controls that run server-side can't have properties set declaratively and resolved dynamically via the <%= %> (Response.Write) syntax.
Expressions can be used, or in a pinch, you can use <%# %> databinding syntax, but then you need to call DataBind() on the control or page, and that can have nasty side effects.
As an alternative, you should be able to iterate over cells or reference specific cells in the codefile. You might have to add the runat=server attribute to each cell as well -- not sure about that.
Related
Agree my question is duplicate of this one and accepted answer works for me too. Let me clarify why.
When I have <%= in head it gives error.
When I have <%= in body it works.
When I have <%# in head it works.
I am just curious to know the reason for all three scenarios.
Additionally I created test project to emulate the issue but in that case all three situation works.
My page is too big and I am unable to decide what code to paste.
<%= %> is in fact doing Response.Write, which is literally writing symbols to the response. To the final markup that is.
Now notice that your head tag has this attribute runat="server". That makes it a server control. That is, this is not a final markup, and but rather a control that will output some markup to response during the control rendering stage. You cannot call Response.Write on this control, because it is not a final markup yet.
For the same reason it would work/not work in the body of the page. If you put it somewhere in plain markup it would work no problem:
<div><%= "Blah" %></div> <%-- works! --%>
But as soon as it appears inside anything with runat="server" you'll get an error
<div runat="server><%= "Blah" %></div> <%-- error! --%>
<asp:Panel runat="server"><%= "Blah" %></asp:Panel> <%-- error! --%>
Now <%# %> is a different beast. This is a data binding markup, something that is being evaluated when the server side control is being data bound. Thus is makes no sense (and is invalid) inside plain markup, and can be used whenever your control is bound to some data. Using it with header is not very common, use cases with GridView or Repeater are the most typical ones that come to mind.
When working with markup if I want to include some content conditionally, I use a placeholder in a normal way:
<asp:Placeholder Visible=<%# IsExpired %>
<span>Prolong your subscription</span>
</asp:PlaceHolder>
Also I can use if-directive:
<% if(IsExpired) {%>
<span>Prolong your subscription</span>
<% }%>
I prefer using the first one just because it does not make my markup messy. And what's the best way to conditionally include content? From the performance point of view, are they similar?
native HTML tags are always faster than rendering server controls as there is no time spent for rendering them
I think there is hardly anything to do with performance whether way you choose here. But actually you may use the following code:
<asp:Label runat="server" Visible=<%# IsExpired %>
Prolong your subscription</asp:Label>
instead the other two. This could make it look more straight-forward.
I'd never use C# code in Web Forms view. In addition I will avoid setting the Visible property in the markup and I will set it in the code behind on some event.
phWhatever.Visible = IsExpired;
Quite often you can avoid creating the IsExpired property.
Of course what #Johnny suggested is correct. If you need to hide what is effectively only one control you hide the control directly.
What are the differences in these tags?
<%
<%#
<%=
<%$
More importantly, how do I display a page property using declarative syntax in an ASP.NET control? I'm trying to do this in an ASP.NET control. The task is to set the text of a label but I do not want to do this pro grammatically in the event I want to change the output control. I get an error about server side controls can't contain this syntax. I'm not sure that I need a databound control for what I want to do but that is another option.
Partial answer coming up.
Update
There is a new tag I've seen in ASP.NET 4.5? site
<%:
Partial answer
quoted from Mike Banavige
<% %> An embedded code block is
server code that executes during the
page's render phase. The code in the
block can execute programming
statements and call functions in the
current page class.
http://msdn2.microsoft.com/en-gb/library/ms178135(vs.80).aspx
<%= %> most useful for displaying
single pieces of information.
http://msdn2.microsoft.com/en-us/library/6dwsdcf5(VS.71).aspx
<%# %> Data Binding Expression Syntax.
http://msdn2.microsoft.com/en-us/library/bda9bbfx.aspx
<%$ %> ASP.NET Expression.
http://msdn2.microsoft.com/en-us/library/d5bd1tad.aspx
<%# %> Directive Syntax.
http://msdn2.microsoft.com/en-us/library/xz702w3e(VS.80).aspx
<%-- --%> Server-Side Comments.
http://msdn2.microsoft.com/en-US/library/4acf8afk.aspx
Update:
Okay this appears to work
<asp:Label ID="MyLabel" runat="server" Text='<%# MyProperty%>'></asp:Label>
If I use the eval syntax then I get an error about databound control or I use the <% then I get a server side controls error. Any more color appreciated.. not sure I really understand what is going on.
Perhaps it has something to do with the render phase.
Few more observations:
I can use <%= without databinding and get the property value but can not use it in a server side control without getting error.
If I use <%# in server side control but I'm required to do a Page.Databind.
Interestingly, I can use either <%= or <%# when I want to render text that is not inside a control. Although the latter requires databinding.
The new <%: syntax is explained, also called code expression syntax
With ASP.NET 4 we are introducing a new code expression syntax (<%:
%>) that renders output like <%= %> blocks do – but which also
automatically HTML encodes it before doing so.
http://weblogs.asp.net/scottgu/new-lt-gt-syntax-for-html-encoding-output-in-asp-net-4-and-asp-net-mvc-2
No, server-side controls can't. For example, I have a string property named SkinPath that give me the full App_Themes path to the current theme. I use it in the following way:
<img src='<%= SkinPath %>/Images/myImage.png' />
However, the following doesn't work:
<asp:Image ID='image' runat='server' ImageUrl='<%= SkinPath %>/Images/myImage.png' />
Instead, it renders the src literally in the result <img>.
A simple question, I think:
I want to put a tag into an ASP.NET app I've been asked to maintain, so I'm coming at this from a newbie point of view just tinkering around the edges without knowing a lot.
I wrote an old ASP application back in 1998, so I am just running on memory...
How do I write some output to the webpage?
I know I can use an
<asp:label id="blah">
but then I need to define a Label blah; in my code behind and then assign it.
I believe that I can put in-place:
<% Response.Write("sometext"); %>
and that will write sometext in the location within the page. (Am I correct?)
Lastly, I remember there was a syntax to the effect of
<%= "some string" %>
but I can't find the documentation on it, to say either it is deprecated, unadvised, or the rationale for such a decision.
I have tried googling for "ASP.NET grammar" but I can't even find a good description that "<%=" even exists, though it is mentioned in a few blogs.
For something simple, like inject the global version number, or the current date, then I can't see anything particularly wrong with in-place composition - it would save me defining 15 labels and having to initialise them all - though perhaps the asp:label approach could reference one global instance of a label?
Just asking for opinions on good practices :)
<%= string %> is perfectly valid ASP.NET syntax. The reason you will often find references to problems with using that is people use <%= (equivalent to Response.Write) when they should use <%# (for databinding) or vice-versa.
For example, we use it very extensively in our content managed site, where we pull in values from a global settings repository:
<%= SiteContext.Current.GetSetting("SiteTitle") %>
MSDN:
MSDN entry on <%= (this is under the JScript.NET section but still applies)
MSDN entry on <%#
Some others suggest <%= is not a "best practice" or a very good approach, but I strongly disagree with that sentiment. For an MVC-ish type site (especially a site that is template- or view-driven in some way), the most direct approach is frequently more effective than using server controls.
Just be mindful that when you use an <asp:Label /> it renders the .Text inside the <span> tag whereas an <asp:Literal /> adds no extraneous HTML to the string passed to it.
For example, if you were building a content management system and wanted to display user-driven HTML, a Label control would not correctly display the output from a WYSIWYG type rich textbox whereas a Literal control is the appropriate choice.
The <%= %> is the late-bound equivalent of the Literal's .Text property. The only difference here is when the value is placed in the page (aside from obvious syntax and separation of concerns paradigm) during the course of the page lifecycle.
Since the .Text property is on a control inherited from WebControl, it can be set/read/manipulated during any of the events following the control's Load event (wherever/whenever you load the control inside the page), but the <%= %> text cannot be directly read/used/manipulated by the code-behind without referencing some other control to get to it (like a containing div's InnerHtml property).
There are lots of options. You could use a single label, and string concatenate all the data you want displayed in that location.
You could create a user control with the layout you want and assign values that way.
You could inject it directly with response.write or the <%= %> syntax
You could create an HtmlGenericControl in your code behind (it's a div), add some text to it, and inject it into the pages controls collection.
Whatever you pick, try and go with the existing style of the coded page.
Look up the term "render blocks" for the <% %> syntax.
How about using
<asp:Literal id="z" text="goofy" runat="server" />?
Labels are usually used with forms.
You can also take full control of the rendering of your pages and controls and compose whatever you need to. You control the HTML, the order of rendering your controls, etc...
Go with the <asp:label /> (or a literal control if you want to customize some html in the content). Seriously. I'ts not that hard: when you put label in your markup visual studio will create it in the code-behind for you, so there's no extra work involved.
You could use the <%= "some string" %> syntax in web forms, but there can be issues when mixing that with the asp controls and there's a good reason new frameworks moved away from mixing logic like that in with your markup.
I have a Repeater which displays a list of items (in a grid-like/table view). When a user clicks an item, I display an UpdatePanel under this item with additional information relevant to the item (similar to Accordion control). I know how to populate the elements of the UpdatePanel in code-behind (I pass the ID of the selected element in the Repeater control as CommandArgument, get additional info for this ID, and set up the text fields of the active UpdatePanel controls). But I'm wondering if I could set up binding directly in the ASPX (instead of code-behind). When I used the <%= %> syntax to assign text fields of the UpdatePanel control the values of the page properties, e.g. <%= Comment %>, it sort of worked, but it changed the fields of all UpdatePanels in the repeater. Is there any way to bind the active UpdatePanel to the current values and leave already bound UpdatePanels unchanged?
Are you looking to display a container that displays additional information? Is there other activity in the "box" that requires it be an updatepanel?
<asp:repeater>
<itemtemplate>
<%# Eval("Name") %> <%# Eval("LastName") %><br />
<span onclick="$get('<%# Eval("Id") %>')">View Age</span>
<div id="<%# Eval("Id")%>" style="display:none;">
Age: <%# Eval("Age") %>
</div>
<itemtemplate>
</asp:repeater>
Ithink that's right, some syntax may be off a bit (typing without intellisense). Would that work?
I used ID as a unique identifier for the div id and the onclick command. You could also use jquery, asp:controls or whatever else you wanted.
Easiest way is to nest a FormView inside the update panel. Then the only thing you need to do in the code behind is get the additional info, assign it to the FormView.DataSource, and call FormView.DataBind(). Everything in the FormView will use the <%# Eval("SomeColumn") %> syntax. You'll probably need to use a FindControl() to get a reference to the FormView. I'd type up the code for you but I'll save you some headaches down the road and say DON'T DO THIS.
The update panel is about the most inefficient way to do any ajax stuff. The only way to get it all to wire up correctly with this repeater and server side code is to either have a gigantic viewstate or to rebind the repeater in your page load. You are turning a request that could be 300ms into something that will take over a second...or longer! Get familiar with a good ajax framework and don't be afraid to write real html. At the very least, use a webservice that loads a usercontrol with your markup.
I know the update panel is easy, and it's built in. It might even be adequate for what you are doing, but you must resist. You'll be glad you did.