问题
I had a version number problem with system.web.http.owin.
So I removed it from my projects package references, with the idea that I would re-added it to see if it fixed the problem. However it has disappeared completely. I.e. in visual studio if I go:
solution explorer > myProject > references > RMC > add reference >
and then search for system.web.http.owin it doesn't appear.
Any ideas what has gone wrong or what package I need to install to make it re-appear.
My original error message...
Error 7 Assembly 'System.Web.Http.Owin, Version=5.1.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' uses 'System.Web.Http, Version=5.1.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' which has a higher version than referenced assembly 'System.Web.Http, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' e:\user\project1\src\packages\Microsoft.AspNet.WebApi.Owin.5.1.1\lib\net45\System.Web.Http.Owin.dll Smots.Core
回答1:
The NuGet package Microsoft.AspNet.WebApi.Owin contains that DLL.
You can install it by right-clicking on the References folder of your project and selecting "Manage NuGet Packages..." and searching for "Microsoft.AspNet.WebApi.Owin". Or you can use the Package Manager Console and type the following:
Install-Package Microsoft.AspNet.WebApi.Owin
If you use the Package Manager Console within Visual Studio, be sure you have the intended project selected in the drop down labeled "Default project:" That is easy to overlook.
来源：https://stackoverflow.com/questions/21690845/how-do-i-get-system-web-http-owin