R – ASP.NET custom control rendering before <%= %> code executes to populate property

asp.netcustom-server-controls

I have a custom control that exposes a property. When I set it using a fixed value, everything works correctly. But if I try to set its value using the <%= %> tags, it goes a little whacky:

<cc:CustomControl ID="CustomControl" runat="server" Property1='<%= MyProperty %>' />
<%= MyProperty %>

When this gets rendered, the <%= MyProperty %> tag underneat the custom control is rendered as I expect (with the value of MyProperty). However, when I step into the Render function of the CustomControl, the value for Property1 is literally the string "<%= MyProperty %>" instead of the actual underlying value of MyProperty.

Best Answer

You control is initialized from the markup during OnInit. So if that syntax worked, it wouldn't have the effect you wanted anyway, since MyProperty would be evaluated during OnInit and not at render time (like it is with the second usage).

You want to use the data binding syntax instead:

<cc:CustomControl ID="CustomControl" runat="server" Property1='<%# MyProperty %>' />

Just make sure to call DataBind() on the container (Page, UserControl, etc).

Alternatively, you can set the property in your code behind:

CustomControl.Property1 = MyProperty;

Related Solutions

Asp – Linkbutton not rendering correctly inside a custom web control

To me, looks like a bug in ASP.Net that has to do with the LinkButton order in its parent's Controls collection.

This snippet:

    protected override void OnLoad(EventArgs e)
    {
        // show some data
        _l = new Label();
        _l.Visible = false;
        _l.Text = "Current Web's Url: " + SPContext.Current.Web.Url;
        Controls.Add(_l);

        // add a button with some processing
        _lb = new LinkButton();
        _lb.Text = "Click here";
        _lb.Click += new EventHandler(_lb_Click);
        Controls.Add(_lb);
    }

Generates this HTML:

<span id="ctl00_PlaceHolderMain_ads1"><h1>This is a custom control</h1><span>Current Web's Url: http://arielki03:9999/sites/ads</span><a href="javascript:__doPostBack('ctl00$PlaceHolderMain$ctl01','')">Click here</a></span>

But this snippet (just inverted the order in which I'm adding the controls above, so LinkButton is first):

    protected override void OnLoad(EventArgs e)
    {
        // add a button with some processing
        _lb = new LinkButton();
        _lb.Text = "Click here";
        _lb.Click += new EventHandler(_lb_Click);
        Controls.Add(_lb);

        // show some data
        _l = new Label();
        _l.Visible = false;
        _l.Text = "Current Web's Url: " + SPContext.Current.Web.Url;
        Controls.Add(_l);
    }

Doesn't show neither control!!!

The problem seems to be with LinkButton being the first in the parent's Controls collection. Workaround: add an empty Label before it, it gonna work :-)

R – My asp.net control loses its property values

Your control's Page_Load method uses the SelectedDateTime property rather than the underlying _value field.

The Get method for the SelectedDateTime property rebuilds _value from the contents of txtDate (and txtHour etc...). So regardless of what you set _value to, the SelectedDateTime property will return what was in the text boxes, which I presume is nothing because the page is only just loading.

I suggest you change

If SelectedDateTime().CompareTo(Nothing) > 0 Then

If _value.CompareTo(Nothing) > 0 Then

Best Answer

Related Solutions

Asp – Linkbutton not rendering correctly inside a custom web control

R – My asp.net control loses its property values

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