usercontrol中的所有控件都为null

Question

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.

Answer 1

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.

Answer 2

I had this problem when I was adding a user control in the code behind the wrong way. You have to use the Page.LoadControl method to initialize the control you can't just use new.

        //WRONG
        UserControls.BingoCardPage bcp = new UserControls.BingoCardPage();
        form1.Controls.Add(bcp);
        //RIGHT
        UserControls.BingoCardPage bcp = (UserControls.BingoCardPage)Page.LoadControl("~/UserControls/BingoCardPage.ascx");
        form1.Controls.Add(bcp);

Answer 3

The issue here is usually due the the load mechanics of user controls, they load after the page typically. So as a result the controls have not yet been initialized on your usercontrol (causing the null ref) during the containing page_load method. One way to work around this is to just create and set a property on the usercontrol and have the usercontrol wire-up/populate its own UI in its Page_Load method.

Something like this:

//Page
protected void Page_Load(object sender, EventArgs e)
{
    test.Text = "Hello World!";
}

//User Control
public string Text {get; set;}

protected void Page_Load(object sender, EventArgs e)
{
    lblTest.Text = Text;
}

Answer 4

Please try to put code in Page_prerender event of page. It will work for you.