我关闭和重开了视觉工作室，它工作。

我不得不去BUILD ->配置管理器，然后——咳咳——勾选我的项目旁边的复选框，以确保它实际上得到了构建。

重新创建虚拟目录为我解决了这个问题。

(在此之前，我尝试了这里列出的其他建议，如清理解决方案，删除bin文件夹，重建解决方案等。没有一个工作，直到我重新创建虚拟目录)

Debug ->调试属性-> Web ->创建虚拟目录

I faced the problem in VS 2017 out of no where, took my whole day going through 100 of solutions any of these didnt work. I then took my whole solution and started it on different machine having VS 2017 installation.. IT FINALLY WORKED. Than reinstalled VS 2017 on the original one and it started running on that too. I strongly recommend you try your solution on a different machine before wasting much time to debug it, bcz it is a BUG in VS 2017 as it seemed for me, have reported same to VS TEAM. Hope this helps any one in grief. Cheers.

TL;DR:对我来说，在尝试了3天我能在这里找到的所有其他答案之后，最终的解决方案是在我的IIS应用程序池高级设置中将启用32位应用程序设置为True，然后重新创建临时ASP。并为我的IIS_IUSRS组增加“修改”和“写入”权限。

我尝试的步骤:

Completely clean and rebuild solution. Shut down and restart Visual Studio. Delete Temporary ASP.NET Files directories (C:\Windows\Microsoft.NET\Framework*). This is where my problems really started: I deleted too much. Reboot development machine. Remove .vs folder and rebuild solution. Recreate C:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files. My projects are 64-bit applications, even though build platform is AnyCPU. Verify output directory was set to bin/ on web projects. Wipe out and check out a clean copy of my version control branch. Wipe out and clone a fresh copy of my version control repository. Run Windows update to latest versions of .NET Frameworks. Repair, uninstall, and reinstall .NET Frameworks 3.5 and 4.8. Uninstall and reinstall Visual Studio 2019 Professional. Uninstall and reinstall the Internet Information Services feature in Windows. Downgrade ASP.NET NuGet package and rebuild solution (issued reference errors due to changes in assemblies). Upgrade ASP.NET back to what my projects were using and rebuild solution. Recreate IIS virtual directory through web project's Web properties. Created "empty" Global.asax files. Restore my files after no change. Replace Codebehind="Global.asax.cs" with Codefile="Global.asax.cs". This started showing me load errors for references to assemblies that actually existed in the system. Enable 32-bit applications in IIS application pool. This started displaying an access error to a 32-bit temporary directory. Restore permissions to all Temporary ASP.NET Files directories (add all but Full Control). This finally got my web application working again.

我能够从网上找到的无数答案中拼凑出足够的信息。在搜索iis找不到引用的。net dll后找到了这个答案，我尝试了最后的几个步骤，让我再次工作，在我的第四天开始与这个令人难以置信的模糊问题作斗争。

如果有办法诊断出为什么全球。asax无法加载，而不是太通用的无法加载类型错误，没有详细信息。

这个故事的寓意:当清理临时ASP。NET Files目录，删除目录的内容，而不是目录本身。否则，可能会导致权限丢失。

您还可以在IIS中检查站点的属性。(在IIS中，右键单击站点，选择“属性”。)确保物理路径设置指向您的应用程序的正确路径，而不是其他应用程序。(这为我解决了这个错误。)