Why is the content inside the invisible asp:PlaceHolder rendered?

Question

Why is the content inside the placeholder rendered? This code results in: "Object reference not set to an instance of an object." For the MainGuard object!

How should one handle this situation?

<asp:PlaceHolder runat="server" Visible="<%# Model.MainGuard != null %>">
    <asp:Image runat="server" ImageUrl="<%# Model.MainGuard.Image.RenderImage() %>" Height="50" />
    <%# Model.MainGuard.Name %>
</asp:PlaceHolder>

score 1 · Accepted Answer · answered Jul 24 '13 at 18:07

It's not rendered -- but it still has to be parsed by the runtime, hence you still get the exception. Your only recourse is to check for null each time:

<asp:Image runat="server"
    ImageUrl="<%# Model.MainGuard == null ? "" : Model.MainGuard.Image.RenderImage() %>" />
<%# Model.MainGuard == null ? "" : Model.MainGuard.Name %>

You might consider using an extension method to allow for cleaner syntax:

public static string StringOrEmpty(this MyClass self, Func<MyClass, string> selector)
{
    if (self == null) return "";

    return selector(self);
}

Then you can write:

<asp:Image runat="server"
    ImageUrl="<%# Model.MainGuard.StringOrEmpty(mainGuard => mainGuard.Image.RenderImage()) %>" />
<%# Model.MainGuard.StringOrEmpty(mainGuard => mainGuard.Name) %>

Okay - I just find it odd that there isn't an even cleaner way.. But thanks - the extension method makes it acceptable.. — Morten Holmgaard, Jul 24 '13 at 18:13
I changed it to a generic: `public static string StringOrEmpty(this T self, Func selector) where T : new()` — Morten Holmgaard, Jul 24 '13 at 18:18

Why is the content inside the invisible asp:PlaceHolder rendered?

1 Answers1