Failed to Load file or assembly "system. Web. Extensions"

Note: An error occurred while processing the configuration file required to provide services to the request. Check the following error details and modify the configuration file as appropriate.
Analyzer error message: failed to Load file or Program Set "system. Web. Extensions, version = 1.0 . 61025.0 , Culture = neutral, publickeytoken = 31bf3856ad364e35 "or one of its dependencies. The system cannot find the specified file.

The following error message is displayed when you call a website program created by ASP. NET:
Error 1 Failed to Load file or assembly "system. Web. Extensions, version = 1.0 . 61025.0 , Culture = neutral, publickeytoken = 31bf3856ad364e35 "or one of its dependencies. The system cannot find the specified file. D:/inetpub/wwwroot/xmlbbs/Web. config 19 
Performance:
 1 , "System. Web. Extensions, version = 1.0 . 61025.0 , Culture = neutral, publickeytoken = 31bf3856ad364e35 "or one of its dependencies. The system cannot find the specified file.
 2 Background systems, even after adding a reference, no matter whether the reference is a system or a user, their attributes and methods cannot be identified;
Solution 1:

 1 If your program runs on a server that you can control, you can install Asp.net Ajax. This is system. web. extensions and other DLL are deployed as GAC; (usually AJAX can be installed)
 2 If your program runs on a VM (You do not have permission to install the Program), you need to set system. Web. Extensions. dll and System. Web. Extensions. Design. dll
(Usually in C:/program files/Microsoft ASP. NET/ASP. NET 2.0 Ajax extensions/v1. 0 . 61025 directory) copy the two DLL files to the bin directory of your web application. This is application-level deployment.
Check Code :
<Assemblies>
<Add Assembly = "  System. Web. Extensions, version = 1.0.61025.0, culture = neutral, publickeytoken = 31bf3856ad364e35  " />
</Assemblies>
<Controls>
<Add tagprefix = "  Atlas  "  Namespace = "  System. Web. UI  " Assembly = "  System. Web. Extensions, version = 1.0.61025.0, culture = neutral, publickeytoken = 31bf3856ad364e35  " />
</Controls>

Solution 2:

 1 . Open Web. config and find the following nodes:
<Add Assembly = " System. Web. Extensions, version = 3.5.0.0, culture = neutral, publickeytoken = 31bf3856ad364e35  " />
<Add Assembly = "  System. Web. Extensions, version = 1.0.61025.0, culture = neutral, publickeytoken = 31bf3856ad364e35  " > </Add>
<Add Assembly = "  System. Core, version = 3.5.0.0, culture = neutral, publickeytoken = b77a5c561934e089  " />
<Add Assembly = " System. Data. LINQ, version = 3.5.0.0, culture = neutral, publickeytoken = b77a5c561934e089  " />
<Add Assembly = "  System. Web. Extensions, version = 3.5.0.0, culture = neutral, publickeytoken = 31bf3856ad364e35  " />
<Add Assembly = "  System. Core, version = 3.5.0.0, culture = neutral, publickeytoken = b77a5c561934e089  " />
<Add Assembly = " System. Data. LINQ, version = 3.5.0.0, culture = neutral, publickeytoken = b77a5c561934e089  " />
 2 . Comment out or delete these results and re-compile them.
This is because the Ajax compiled by vs2005 is 1.0, while vs2008 can simultaneously compile ajax1.0 and other versions, so there will be conflicts.

Solution 3:
Publickeytoken = 31bf3856ad364e35"
Or one of its dependencies. The system cannot find the specified file.
Install Asp.net Ajax, and then set X:/program files/Microsoft ASP. NET/ASP. NET 2.0 Ajax extensions/v1. 0.61025 
Add the system. Web. Extensions. Design. dll file under the directory to the bin directory of the project. copy the file to the bin directory of the website and run it again,
After installing Asp.net Ajax, there will be three DLL files
System. Web. Extensions. Design. dll
Ajaxextensionstoolbox. dll
System. Web. Extensions. dll
Generally, you should add these three files to the bin directory of the project. If you want to insure them, copy them all.