Upgrading an ASP.NET MVC 3 Project to ASP.NET MVC 4

ASP.NET MVC 4 can be installed side by side with ASP.NET MVC 3 on the same computer, which gives you flexibility in choosing when to upgrade an ASP.NET MVC 3 application to ASP.NET MVC 4.

The simplest way to upgrade is to create a new ASP.NET MVC 4 project and copy all the views, controllers, code, and content files from the existing MVC 3 project to the new project and then to update the assembly references in the new project to match the old project. If you have made changes to the Web.config file in the MVC 3 project, you must also merge those changes into the Web.config file in the MVC 4 project.

To manually upgrade an existing ASP.NET MVC 3 application to version 4, do the following:

1. In all Web.config files in the project (there is one in the root of the project, one in the Views folder, and one in the Views folder for each area in your project), replace every instance of the following text:

System.Web.Mvc, Version=3.0.0.0
System.Web.WebPages, Version=1.0.0.0
System.Web.Helpers, Version=1.0.0.0
System.Web.WebPages.Razor, Version=1.0.0.0

with the following corresponding text:

System.Web.Mvc, Version=4.0.0.0
System.Web.WebPages, Version=2.0.0.0
System.Web.Helpers, Version=2.0.0.0,
System.Web.WebPages.Razor, Version=2.0.0.0,

2. In the root Web.config file, update the webPages:Version element to "2.0.0.0" and add a newPreserveLoginUrl key that has the value "true":

   key="webpages:Version" value="2.0.0.0" />
   key="PreserveLoginUrl" value="true" />

3. In Solution Explorer, delete the reference to System.Web.Mvc (which points to the version 3 DLL). Then add a reference to System.Web.Mvc (v4.0.0.0). In particular, make the following changes to update the assembly references. Here are the details:
   1. In Solution Explorer, delete the references to the following assemblies:
      • System.Web.Mvc (v3.0.0.0)
      • System.Web.WebPages (v1.0.0.0)
      • System.Web.Razor (v1.0.0.0)
      • System.Web.WebPages.Deployment (v1.0.0.0)
      • System.Web.WebPages.Razor (v1.0.0.0)
   2. Add a references to the following assemblies:
      • System.Web.Mvc (v4.0.0.0)
      • System.Web.WebPages (v2.0.0.0)
      • System.Web.Razor (v2.0.0.0)
      • System.Web.WebPages.Deployment (v2.0.0.0)
      • System.Web.WebPages.Razor (v2.0.0.0)
4. In Solution Explorer, right-click the project name and then select Unload Project. Then right-click the name again and select Edit ProjectName.csproj.
5. Locate the ProjectTypeGuids element and replace {E53F8FEA-EAE0-44A6-8774-FFD645390401} with {E3E379DF-F4C6-4180-9B81-6769533ABE47}.
6. Save the changes, close the project (.csproj) file you were editing, right-click the project, and then select Reload Project.
7. If the project references any third-party libraries that are compiled using previous versions of ASP.NET MVC, open the root Web.config file and add the following three bindingRedirect elements under theconfiguration section:

   
    xmlns="urn:schemas-microsoft-com:asm.v1">
    name="System.Web.Helpers" publicKeyToken="31bf3856ad364e35" /> oldVersion="1.0.0.0" newVersion="2.0.0.0"/>
          
    name="System.Web.Mvc" publicKeyToken="31bf3856ad364e35" />
    oldVersion="1.0.0.0-3.0.0.0" newVersion="4.0.0.0"/>
        
   name="System.Web.WebPages" publicKeyToken="31bf3856ad364e35" />
    oldVersion="1.0.0.0" newVersion="2.0.0.0"/>
        
   

PS:  This text is from  http://www.asp.net/learn/whitepapers/mvc4-release-notes