在我的例子中，我有一个在适当的源文件夹中列出的类，但没有在解决方案资源管理器中注册。我必须右键单击项目>添加现有项目，手动选择它说它缺少的类。然后一切都运转正常!

把我的解决方案加入到混合中，因为它有点不同，我花了一段时间来弄清楚。

In my case I added a new class to one project but because my version control bindings weren't set I needed to make the file writable outside of Visual Studio (via the VC). I had cancelled out of the save in Visual Studio but after I made the file writable outside VS I then hit Save All again in VS. This inadvertently caused the new class file to not be saved in the project..however..Intellisense still showed it up as blue and valid in the referencing projects even though when I'd try to recompile the file wasn't found and got the type not found error. Closing and opening Visual Studio still showed the issue (but if I had taken note the class file was missing upon reopening).

当我意识到这一点时，解决方法很简单:将项目文件设置为可写，将丢失的文件读入项目。现在都建好了。

在我的例子中，我有一个在适当的源文件夹中列出的类，但没有在解决方案资源管理器中注册。我必须右键单击项目>添加现有项目，手动选择它说它缺少的类。然后一切都运转正常!

在我的VS2019解决方案中添加了一个新的库项目后，我得到了“使用系统”的错误。添加包Newtonsoft. json (currentversion)到库项目修复了这个问题，因为对Newtonsoft的依赖。Json包含了NETStandard。库的依赖项下的库，并正在生成一个警告图标。主项目有一个错误，直到我安装了Newtonsoft。Json，所以我认为它也适用于库;它确实做到了。

对于那些在试图将网站发布到Azure时遇到这种错误的人来说，上面这些看起来很有希望的解决方案都没有帮助到我。我也在同一条船上——我的解决方案本身构建得很好。最后我不得不这么做

把我溶液里的鸡块都去掉。 关闭并重新打开我的溶液。 重新添加所有的nuget包。

有点痛苦，但这是我能让我的网站发布到Azure的唯一方法。

I had a similar issue: The compiler was unable to detect a folder inside the same project, so a using directive linking to that folder generated an error. In my case, the problem originated from renaming the folder. Even though I updated the namespace of all the classes inside that folder, the project info somehow failed to update. I tried everything: deleting the .suo file and the bin and obj folders, cleaning the solution, reloading the project - nothing helped. I resolved the problem by deleting the folder and the classes inside, creating a new folder and creating new classes in that new folder (simply moving the classes inside the new folder didn't help).

PS:以我为例，我正在开发一个web应用程序，但这个问题可能发生在不同类型的项目中。