C# – All controls are null within usercontrol

asp.netcnetuser-controls

I have a UserControl which uses a UserControl, among other controls.

In the ascx file I have the following code:

<%@ Register TagPrefix="tag" Namespace="some.name.space" Assembly="some.assembly" %>
<tag:control ID="test" runat="server" />

In my Page_Load method, I try to set a property on test like so: 

test.Text = "Hello World!";

This actually sets the Text property of a literal control in my user control test.

This throws an exception:

Object reference not set to an instance of an object

When it tries to set the 

lblTest.Text = value;

The object that is null is lblTest.

Am I not adding the user control correctly? Should I – or do I have to – specify the Src property when registering a Tag? If so, I'd have to register every usercontrol I use?

This also results in no controls loading in usercontrol and all controls are null within usercontrol.

Best Answer

If the user control is in your current project, then you need to include the src in the register statement:

<%@ Register TagPrefix="uc1" TagName="NavTop" Src="controls/NavTop.ascx" %>

However, if you use this user control in more than one page, then you can also register it in web.config:

<system.web>
  <pages>
    <controls>
      <add tagPrefix="uc1" tagName="NavTop" src="~/controls/NavTop.ascx" />
    </controls>
  </pages>
</system.web>

One other thing to be aware of: there are times when the visual studio designer does not "see" your changes to controls on the page if you only make the changes in source view. If you change a control name, for example, you could end up with a control with the new name in the ascx but a reference to a control with the old name in the designer file. At runtime, this will result in the designer file property being null.

After having been burnt by this a number of times, if I make any changes in source view, I either check to see that the designer file has been updated correctly or I switch to design view, make a minor change, then save the page/user control.

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