Set Visible property with server tag <%= %> in Framework 3.5

Question

I have been working in a .NET Framework 4 project using server tags like <%=whatever %> to set the visibility of runat="server" controls, like the following:

  <div id="MyId" runat="server" visible="<%=MyVisiblePropertyOnCodeBehind %>" >     Content   </div> 

This works on framework 4, but now trying to use this on a Framework 3.5 project it doesn't seems to work. Is this a Framework 4 only feature? Is there a coolest (and .aspx side) alternative to setting the visibility from codebehind? I'm using the ugly:

    MiId.Visible = MyVisiblePropertyOnCodeBehind 

Thanks in advance,

Tom

[EDITED] SOLUTION:

Thanks for your comments that makes me understand my problem and the solution!

It was my fault in more than one thing.

In the VS2010 project we were using <%# instead of <%=

Also, I didn’t notice that in the VS2010 project we were using pages inherited not from “Page”, but from a CustomPage class, that was making the binding automatically, without me noticing it, and that makes me think that was a Framework 4.0 only feature.

As you told here, if you have the following markup:

  <div id="MyId" runat="server" visible="<%# MyVisiblePropertyOnCodeBehind %>" >     Content   </div> 

you can make it work, adding the following to the codebehind:

    public bool  MyVisiblePropertyOnCodeBehind = true;     protected void Page_Load(object sender, EventArgs e) {         DataBind();         // Or if you want only for one control, MyId.DataBind();                  } 

As I read, this DataBind() can reduce performance of the application. Do you have idea of how much? Could this be understood as a “professional” technique to be used on big projects, or do you think it should be avoided?

I love the way it makes markup readable and easy to understand in a single view, but I wouldn’t like to be guilty of slow code because that.

Answer 1

The code you posted is not valid syntax for server tags in the ASP.NET 2.0 or ASP.NET 4.0 runtimes. In either version, trying to set the visible property using <%= ... %> in a server tag should result in a parser error:

Parser Error Message: Cannot create an object of type 'System.Boolean' from its string representation '<%=MyVisiblePropertyOnCodeBehind%>' for the 'Visible' property.

You have two options other than just setting the Visible property in the codebehind or a <script runat="server"> tag. The first is to use a databinding on the Visible property. You'll need to call the DataBind() method on either MyId or one of its parent controls for the value to be bound.

<div id="MyId" runat="server" visible="<%# MyVisiblePropertyOnCodeBehind %>" >     Content </div> 

The other option is to write the code as follows:

<% if(MyVisiblePropertyOnCodeBehind) { %> <div id="MyId" runat="server">     Content </div> <% } %> 

The disadvantage of this approach is that you won't be able to programmatically add controls to the page or control that contains the code blocks. If you try to you should get an error:

The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>)

All that being said, I think just setting the property the way you are doing it now is the way to go.

Answer 2

As for ASP.NET aspx page's inline expression. <% %> can only be used at aspx page or user control's top document level, but can not be embeded in server control's tag attribute (such as <asp:Button... Text =<% %> ..>). As you've found you can create custom expression builder in ASP.NET 2.0 to add your inline expression.

BTW, another means for supplying values to server control properties in aspx inline tempalte is using <%# %> databinding expression. This is built-in supported. The only different from other inline expression is that method on the target control or its Container control.

Steven Cheng

Microsoft MSDN Online Support Lead

Full post here: http://www.aspnet-answers.com/microsoft/ASP-NET/29389067/dynamically-set-a-control-property.aspx

And workaround here: ASP.net Inline Expression Issue