eval in if statement?

 <% if(Eval("SaveDate") != DBNull.Value){ %>
     do magic                           
 <%} %>

gives me error: Databinding methods such as Eval(), XPath(), and Bind() can only be used in the context of a databound control.

I could write : <%# Eval(“SaveDate”) != DBNull.Value ? do magic
But I need to do lots of html magic in if statement.

I know I should add # in order to use Eval, but not sure about correct syntax.

Answers:

Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.

Method 1

One solution is to wrap the content in a runat=”server” tag with a Visible value, e.g.,

<div runat="server" Visible='<%# Eval("SaveDate") != DBNull.Value %>'>
   do magic
</div>

div can be any HTML tag, but <asp:Panel> and <asp:PlaceHolder> could also be used. Note that “do magic” is still databound, so it’s not a perfect solution if it contains expensive code or code that could generate an error if Eval("SaveDate") == DBNull.Value.

Note that Visible="false" will omit the tag and all its contents from the generated HTML, this means that it is very different from style="display:none" or style="visible:hidden", so don’t worry about that.

But, if your “do magic” is reasonably complex, another rather simple solution (a bit of a hack) is: use a Repeater (or FormView) with its DataSource set to an array of one item (visible) or no items (hidden):

<asp:Repeater runat="server" DataSource='<%# ElementIfTrue(Eval("SaveDate") != DBNull.Value) %>'
    <ItemTemplate>
        do magic
    </ItemTemplate>
</asp:Repeater>

protected IEnumerable ElementIfTrue(bool condition) 
{
    if (condition)
        return new object[] { Page.GetDataItem() };
    else
        return new object[0];
}

The actual contents of the datasource array is either empty (hidden) or the element you were already binding to. This makes sure you can still call <%# Eval(...) %> inside the ItemTemplate.

With this approach, your “do magic” is a template which will only be executed if DataSource has one or more items. Which is taken care of by ElementIfTrue. It’s a bit of a mind bender, but it can save you every once in a while.

As a side note: packing your “do magic” in a user control can also keep the complexity down. You don’t really need to change a thing in your HTML/ASP.NET tag mix (<%# Eval("...") %> still works even inside a user control).

Method 2

I usually add a protected function returning a string to the code-behind to generate the content:

On the page:

<%# Eval("SaveDate") != DBNull.Value ? GenerateContent() : string.Empty %>

In my class:
protected string GenerateContent()
{
    return "Hello, World!"
}


All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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