Common Issue: Missing HttpResponseMessage
The WebApi sometimes needs to return an HttpResponseMessage, for example to trigger redirects.
In some cases you simply cannot get the code to run, with an error similar to this
2sxc Api Controller Finder: Error while selecting / compiling a controller for the request. Pls check the event-log and the code. See the inner exception for more details.", "ExceptionMessage": "[Some-Path]\Portals\0\2sxc\[Some-App]\api\InstallController.cs(23): error CS0012: The type 'System.Net.Http.HttpRequestMessage' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Net.Http, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.", "ExceptionType": "System.Web.HttpCompileException", "StackTrace": " at System.Web.Compilation.AssemblyBuilder.Compile() at System.Web.Compilation.BuildProvidersCompiler.PerformBuild() at System.Web.Compilation.BuildManager.CompileWebFile(VirtualPath virtualPath) at System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean ensureIsUpToDate) at System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean ensureIsUpToDate) at System.Web.Compilation.BuildManager.GetVPathBuildResult(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean ensureIsUpToDate) at System.Web.Compilation.BuildManager.GetCompiledAssembly(String virtualPath) at ToSic.SexyContent.WebApi.AppApiControllerSelector.SelectController(HttpRequestMessage request) in C:\Projects\2SexyContent\Web\DesktopModules\ToSIC_SexyContent\Sxc WebApi\AppApiControllerSelector.cs:line 77
The reason is that the compiler has a hard time figuring out where to locate this class. We believe it's because Dnn has gone through various .net versions and that makes these things a bit harder.
The solution is to tell the Compiler that you're using System.Net.Http in the desired version. Add the add tag to the web.config in the location below:
<configuration>
<system.web>
<compilation>
<assemblies>
<add assembly="System.Net.Http, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
...
Note: you shouldn't add the whole block - it should just show you where it must go