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Error Creating ListView Control in Design View

I have an aspx page in a web (vb.net) application with a sqldatasource and I am trying to drop a listview into the designer view of Visual Studio 2010, but I am getting the following error in the designer view:

Error Creating Control - ListView1
Failed to create designer 'System.Web.UI.WebControls.ListView, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'

Some related oddities:

My web.config doesn't list version 4.0.0.0 anywhere.
When I hover over the ListView in the Toolbox, it claims to be Version 3.5.0.0.
All the other data controls (DetailsView, GridView, etc.) appear to work in designer view

I've tried resetting the toolbox items, but that didn't work.  If you've seen this before and resolved it, please let me know what you did, I am quite baffled!  I can provide screenshots and code if needed.  Thanks in advance!
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MeteorComm
Asked:
MeteorComm
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1 Solution
 
SameerJagdaleCommented:
this is version conflct, the listview control is not able to find the matching designer for 4.0, check for the correct namespaces, or if you are not doing anything specific to 4.0, try changing the framework version to 3.5.
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MeteorCommAuthor Commented:
The framework version is and always has been 3.5, there is no reference to 4.0 anywhere (except for the error message), which is why it is so baffling.
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MeteorCommAuthor Commented:
.Net Framework 4 is installed on the machine, because it comes pre-packaged with VS 2010, but the application is a 3.5 app. But somehow, parts of it are still trying to reference 4.0 though.

I'm getting a runtime error on the page directive in Visual Studio saying that my masterpage cannot be processed because the code directory has not yet been built.  And when I debug, even though it comes up fine in the browser, I get the following error:

Error      2      The type 'System.Web.Compilation.WCFBuildProvider' is ambiguous: it could come from assembly 'C:\Windows\assembly\GAC_MSIL\System.Web.Extensions\3.5.0.0__31bf3856ad364e35\System.Web.Extensions.dll' or from assembly 'C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.Extensions\v4.0_4.0.0.0__31bf3856ad364e35\System.Web.Extensions.dll'. Please specify the assembly explicitly in the type name.      App_WebReferences/      

How do I remove the ambiguity and ensure that everything points to 3.5?
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SameerJagdaleCommented:
try dereferencing -
remove System.Web.Extensions refrence from your project and add it from -
 C:\Windows\assembly\GAC_MSIL\System.Web.Extensions\3.5.0.0__31bf3856ad364e35 folder.
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MeteorCommAuthor Commented:
I never had time to try out your suggestion unfortunately, but I'm moving to a new system now anyway and am hoping that will solve it.  
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MeteorCommAuthor Commented:
So I ran into this issue again on my new system and tried your suggestion...I removed the visible system.web.extensions (which listed version 3.5) from the References list but was unable to add it again because it says that one already exists. Where is it?!? It's not in web.config and it's not listed in References. If I try attempting to add a ListView, design view still complains about version 4.0.
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