Can't access instance of custom inherited webcontrol from code-behind page in Web Application project_问答_开发者

I've inherited from an ASP.NET WebControl to add some additional functionality. I tested my control in a blank ASP.NET Web Site (created through File > New Web Site...) and it worked fine. But I can't get it to work when I add it to a Web Application Project (created throgh File > New Project... > Visual Basic > Web > ASP.NET Web Application. I personally prefer Web Sites for ease of development, but the project leadership I'm working with now wants to keep it a Web Application (they like how it pre-compiles all page code-behind into a single DLL in the Bin folder.)

To isolate the issue I tried to reproduce it using the simple MSDN Walkthrough: Developing and Using a Custom Server Control. To my surprise, I had the same issue there: If you build it into an ASP.NET Web Application Project, and then try to access the instance of the control from the page's code-behind, you get the compilation error BC30456: 'WelcomeLabel1' is not a member of 'WebApplication1._Default'. E.g., in Page Load, write:

Me.WelcomeLabel1.Text = "foo"

However, if you simply remove that line of code behind, it compiles properly and the browser shows the page with the label formatted correctly. To me this is a bug in the compiler/framework, because if the control gets injected properly into the page, it should always be available in the code-behind, right?

Any ideas? I suppose I could compile my control into an assembly as suggested further into the walkthrough, but that seems like overkill.

Interestingly, I can use FindControl to get a reference to the WelcomeLabel, but I am unable to cast it to the appropriate type. Doing so causes the error "Unable to cast object of type 'Samples.AspNet.VB.Controls.WelcomeLabel' to type 'WebApplication2.Samples.AspNet.VB.Controls.WelcomeLabel'." I've tried every combination of namespaces and imports that I could think of, even adding declarations to the designer.vb file manually, and I can't get this to work, unless of course I switch to a Web Site project, which I can't do.

UPDATE 1.1:

To clarify, what I was trying to achieve was:

Web Application Project
Inherited web control class file in the same assembly (i.e. not referenced from a separate compiled DLL)
Control added to page through design-time markup
Control instance referenced from page code-behind

EDIT: After re-reading Bryan's comments, I finally understood. It's very simple actually: all you have to do is add Assembly="WebApplication1" to the <%@Register%> directive in the markup, and add the "WebApplication1." prefix to the Namespace directive. So:

<%@ Register Assembly="WebApplication1" TagPrefix="aspSample" Namespace="WebApplication1.Samples.AspNet.VB.Controls" %>

And now you should have no problem instantiating the control through markup and accessing the instance from code-behind, whether you place the cont开发者_开发问答rol's class file in App_Code or anywhere else.

Jordan Rieger

You've got something wonky. A couple things to check...

One, make sure your code does not have a CodeFile directive. It should be compiled into the same assembly as your page classes. Make sure the Inherits directive is correct. It sounds very much like you are modifying a class that isn't the one you think it is.

Two, make sure the designer file is using the correct type declaration for your control. If it says "WebControl" or something beside your type, which I suspect, that is a problem. You may need to tell Visual Studio where the code for this control is by setting the application root in Project Properties. That is how Visual Studio knows how to resolve the "~" path at design time.

If that doesn't work... post some code.

UPDATE:
OK, the reason you are getting the original error is that the control is not declared in the designer file. If it's not in the designer file, then it doesn't exist and hence you get this error: "WelcomeLabel1' is not a member of 'WebApplication1._Default'. One reason this can happen is that you are not registering the control, or not registering it properly... so Visual Studio has no idea what to do with it. It's like declaring some random element that has no definition:

<bp:WhatIsThis ID="someID" runat="server">

Visual Studio will not update your .designer file here because it has no clue what this object represents. I'm guessing the same thing is happening here.

UPDATE 2: You must include the .cs file in your project. This step is probably missing from the walk-through because it targets a Web Site, not a WAP. That's why your namespace is empty.

Sometimes opening the designer gets it to add correctly to the designer.cs file that contains the control references. THat's what is probably missing. Not sure why, works for me fine...

Is there a build error or a compile error, or is the @Page directive showing an error?

HTH.