Could not load file or assembly System.Web.Mvc or one of its dependencies

Question

currently working on asp.net mvc5 (old mvc3 project). It builds just fine, but when I start the project but when I run the project I am facing the following error.

Answer 1

If you carefully read the fusion log you posted, it gives you the information you need. I'll strip it down to the important lines.

LOG: Attempting download of new URL file:///C:/Users/joaki/Source/Repos/2015-TimeAdministration/Source/V2/SalesWeb2/TimeReportV2/bin/System.Web.Mvc.DLL.

The app is looking for System.Web.Mvc.dll. It's resolved it to the one under your web app's bin folder: C:\Users\joaki\Source\Repos\2015-TimeAdministration\Source\V2\SalesWeb2\TimeReportV2\bin\System.Web.Mvc.DLL

Next, it checks to see if there is any assembly binding redirect for that DLL in your web.config, and finds one:

LOG: Using application configuration file: C:\Users\joaki\Source\Repos\2015-TimeAdministration\Source\V2\SalesWeb2\TimeReportV2\web.config
LOG: Redirect found in application configuration file: 4.0.0.1 redirected to 5.2.3.0.
LOG: Post-policy reference: System.Web.Mvc, Version=5.2.3.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35

If you look in your web.config, there will be something like this:

It then checks that the System.Web.Mvc.dll it's found in your app's bin directory is the same version:

WRN: Comparing the assembly name resulted in the mismatch: Major Version
ERR: Failed to complete setup of assembly (hr = 0x80131040). Probing terminated.

It complains that the Major Version mismatches. This is the first number in the 4-number version. If you go back in the log, that's because the System.Web.Mvc.dll in your bin folder is actually version 4.0.0.1, not the desired 5.2.3.0:

LOG: Redirect found in application configuration file: 4.0.0.1 redirected to 5.2.3.0.

You need to check your solution to make sure all projects that reference System.Web.Mvc are looking for the same, latest version. If you look at the Warnings in the Error List, you'll likely find warnings about version mismatches for this assembly.

Updating References

One solution is to right click on the Solution in Visual Studio, choose "Manage NuGet Packages for Solution..." and try to update the projects to use the same version of System.Web.Mvc.

It will also help if you clear out your bin folders beforehand.

You can also use the Package Manager Console instead to update the package across your solution:

Update-Package Microsoft.AspNet.Mvc -version 5.2.3.0