Fix error: "Could not load file or assembly 'Microsoft.Web.Extensions..."

One day, you are coding... And run it! But an error show!

Could not load file or assembly 'Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified.

You don\'t worry! And try some solutions

1. Review web.config

- Install the latest Asp.NET Ajax - Open my web.config in my production server and replace this line

<add assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"></add> 

by

<add assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"></add> 

PublicKeyToken is in capitals....

2. Check your bin folder
Copy the System.Web.Extensions.dll into the app's bin directory if you have.

 3. Install ASP.NET AJAX 1.0
Check you had install it? If not, you download it at ASP.NET AJAX 1.0 

4. If you using Ajaxcontrol toolkit
Download and install it at Ajaxcontrol toolkit

Try one solution or try all until you resolve an error!