How come azure website requires Owin?

Question

I have created a simple WebApi project that doesn't reference MVC and everything and doesn't require Owin or anything that I don't yet understand. It runs OK on IIS Express, but once I deploy to cloud it says

"Could not load file or assembly 'Microsoft.Owin' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)"

There is no reference to Owin anywhere.

OK. I added Owin package, but NuGet has only 2.0.2.0 package at the moment, but for some reason it requires 3.0.0.0.

[FileLoadException: Could not load file or assembly 'Microsoft.Owin' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)]

[FileLoadException: Could not load file or assembly 'Microsoft.Owin, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)] System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks) +0 System.Reflection.RuntimeAssembly.nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks) +34 System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, Evidence assemblySecurity, RuntimeAssembly reqAssembly, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks) +152 System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean forIntrospection) +77 System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection) +16 System.Reflection.Assembly.Load(String assemblyString) +28 System.Web.Configuration.CompilationSection.LoadAssemblyHelper(String assemblyName, Boolean starDirective) +38

[ConfigurationErrorsException: Could not load file or assembly 'Microsoft.Owin, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)] System.Web.Configuration.CompilationSection.LoadAssemblyHelper(String assemblyName, Boolean starDirective) +736 System.Web.Configuration.CompilationSection.LoadAllAssembliesFromAppDomainBinDirectory() +217 System.Web.Configuration.CompilationSection.LoadAssembly(AssemblyInfo ai) +130 System.Web.Compilation.BuildManager.GetReferencedAssemblies(CompilationSection compConfig) +170 System.Web.Compilation.BuildManager.GetPreStartInitMethodsFromReferencedAssemblies() +91 System.Web.Compilation.BuildManager.CallPreStartInitMethods(String preStartInitListPath, Boolean& isRefAssemblyLoaded) +284 System.Web.Compilation.BuildManager.ExecutePreAppStart() +153 System.Web.Hosting.HostingEnvironment.Initialize(ApplicationManager appManager, IApplicationHost appHost, IConfigMapPathFactory configMapPathFactory, HostingEnvironmentParameters hostingParameters, PolicyLevel policyLevel, Exception appDomainCreationException) +521

[HttpException (0x80004005): Could not load file or assembly 'Microsoft.Owin, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)] System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +9930568 System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +101 System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context) +254

I finally fixed that by

1. Adding Owin package
2. Explicitly Owin binding to version 2.0.1.0
3. Removing Owin from Startup.

But that is something I just don't understand. (Started happening after I updated Visual Studio 2013 to Update 4)

Answer 1

OWIN is the standard interface between .NET web servers and web applications used by Microsoft since they went "fully" open-source with the ASP.NET Web Stack. I think that the OWIN Package became mandatory for ASP.NET WEB.API projects since Visual Studio 2013 Update 4.

You can find more info on OWIN here: http://coding.abel.nu/2014/05/whats-this-owin-stuff-about/