Use c# expression to define server tag property value

Question

Is it possible to set the property of a server tag from a c# expression, i.e. something like

<asp:TextBox Width='<%= [some c# expression] %>'/>

?

I though this would be pretty straightforward, but I can't get such an expression to run.

Thanks for any help

Ryan

Oded · Accepted Answer · 2010-08-04 11:58:09Z

6

Yes, this is possible. You need to make sure the control runs server side (runat="server"), but depends on exactly what you are trying to evaluate in the expression.

So long as the expression returns a string, it should be fine.

<asp:TextBox id="txt" runat="server" Width='<%= (10 * 10).ToString() %>px'/>

This will result in a width='100' in the browser.

Update:

The above is completely wrong. You cannot put server side code render blocks (<%%> and <%=%>) in a server side control markup in this manner (since it already is a run server side).

In order to dynamically control the value, this needs to be done either in codebehind or within separate render blocks:

<%
  txt.Width = (10 * 10).ToString() + "px";
%>
<asp:TextBox id="txt" runat="server" />

See this and this for reference.

edited Aug 4, 2010 at 11:58

answered Aug 4, 2010 at 9:16

Oded

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7 Comments

Giuseppe Accaputo Over a year ago

I'd also like to add that <%= (10 * 10).ToString() %> is a Response.Write((10 * 10).ToString());, just for better comprehension

Ryan Over a year ago

This is odd. I tried your example: <asp:TextBox ID="TextBoxAmt" runat="server" Text='<%# DataBinder.Eval(Container.DataItem, "percent", "{0:###.##}") %>' Width='<%= (10 * 10).ToString() %>'/> But I get the error:Cannot create an object of type 'System.Web.UI.WebControls.Unit' from its string representation '<%= (10 * 10).ToString() %>' for the 'Width' property. (The TextBox is in a databound grid) I also tried a simple expression <%="40"%> and <%="40px"%>. Any idea what I'm doing wrong? Thank you.

Oded Over a year ago

@Ryan - Width is a Unit, so needs a number + a unit. My example should really be Width='<%= (10 * 10).ToString() %>px'

Ryan Over a year ago

Hi @Oded, it still complained about not being a valid representation. I have boiled this down to the simplest form: <asp:Label runat="server" ID="tst" Text='<%="hi"%>' /> - this label doesn't display anything! (If I replace the expression with a literal, it displays). Thanks for your ongoing help.

Oded Over a year ago

@Ryan - just use <asp:Label runat="server" ID="tst"><%= "hi"%></asp:Label>

|

Arthis · Accepted Answer · 2010-08-04 10:36:46Z

1

Beware of the type you bind to .

for instance

<asp:TextBox runat=server ID=C_TB_MyTB 
        Text=<%# MyText %> 
        Width=<%# MyWidth %>
    ></asp:TextBox>

Binding this textbox with

protected string MyWidth="300";
protected string MyText = "Bla bla bla...";

won't work , whereas :

protected int MyWidth=300;
protected string MyText = "Bla bla bla...";

will do...

good code to you,

answered Aug 4, 2010 at 10:36

Arthis

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