Tip: Solving error “Could not load type” in Visual Studio

Posted on 17 July 2009

If you copy or import code from one Visual Studio 2008 project to another, for instance the aspx, ascx or master files, then you may receive the above error and it may take ages before you find out that the solution is extraordinarily simple:

change CodeBehind in CodeFile

The following example shows that for a Web UserControl, but it works the same for a Page, a Master Page and a WebPart:

<!-- change this (ASP.NET Web Site version) -->
<%@ Control Language="C#" AutoEventWireup="true"
     CodeFile="WebUserControl.ascx.cs"
     Inherits="Controls_WebUserControl" %>

<!-- into this (ASP.NET Web Application version), or vice versa -->
<%@ Control Language="C#" AutoEventWireup="true"
     CodeBehind="WebUserControl.ascx.cs"
     Inherits="Controls_WebUserControl" %>

That’s all you need to do to get rid of that annoying error!

The reason? If you have an “ASP.NET Web Application”, Microsoft needs CodeBehind in the Page, Control or Master directive, if you have an “ASP.NET Web Site”, the compiler expects CodeFile instead. I have no idea why this difference exists, it is very annoying and you can stare forever at your code before you find it, but this is the solution for this scenario.

– Abel –

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Tags: csharp, designer, dotnet, error, vb, Visual Studio

19 responses to Tip: Solving error “Could not load type” in Visual Studio

Gayatri says:

January 12, 2012 at 07:07

Thanks a ton.. It saved my time

Tim says:

January 11, 2012 at 05:54

Thank you very much, this resolved my problem and saved me much headache!

D. Li says:

October 25, 2011 at 21:16

Thanks so much, converting between a web application project and a web project has so many obscure gotchas!

John W says:

October 21, 2011 at 06:17

“it is very annoying and you can stare forever at your code before you find it”

Well, I stared forever but never saw it–thanks SO MUCH!

HG says:

September 14, 2011 at 22:12

Ohh, Thanks, good info

Benedict Howard says:

May 1, 2011 at 19:02

As soon as I came over to Tip: Solving error “Could not load type” in Visual Studio | Under My Hat I can only see portion of it, is this my net internet browser or the web website? Should I reboot? Cheers Benedict Howard

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kamal says:

March 7, 2011 at 13:30

I’ve been struggling with this error!

you saved me!

Thanks a lot!!!

Igor says:

February 6, 2011 at 01:52

Your article very helped me and saved time.

Thanks!

Arch says:

January 11, 2011 at 05:49

Wow!!! You made my day…. I was thinking of creating new aspx files copy paste the code ebehind…but that would be a nightmare…..thanks a lot

Bob Sherwood says:

July 9, 2010 at 16:55

Wow… thanks!

Nandan D says:

March 26, 2010 at 19:45

Thanks.Was driving me crazy!!

joebloe says:

January 15, 2010 at 20:19

Worked perfectly. What exactly is the difference between a web application and website?

I was working in a ecommerce site that I guess was compiled as a web application, because it used ‘codebehind’. I was editing the site as a ‘web site’ and added a control file. Nothing worked until I changed to ‘codefile’.

Just trying to understand the why.

Abel Braaksma says:

January 24, 2010 at 12:05

There are many differences between a website and a web application. The most apparent being that a website does not have a project file but is just a bunch of files that together comprise your website (you can even open a live website over FTP or HTTP). It’s very much “what you see is what you get”, no hidden feats. A file in a website project is a file in the actual website.

A web project, on the contrary, has a project file, contains files that are included / excluded and has compile settings on the project. It can be compared to a classic WinForms project.

More information in this blog post about Website vs Web Application.

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