我也遇到过这个问题。首先，您必须手动构建您的DLL项目，右键单击“构建”。那么它就会起作用。

在我的例子中，我得到这个错误消息的原因很简单，从TFS获得项目的最新版本后，解决方案中错误的项目被标记为启动项目。选择正确的项目作为启动项目为我解决了这个问题。

The Visual Studio IDE doesn't do any building behind the scenes, the Msbuild application does. The VS IDE essentially just constructs the project file that is used by Msbuild and quite often it makes mistakes if you leave it up to the IDE to figure things out on it's own. If you are getting the Metadata file '.dll' could not be found error it is likely due to the fact that the correct/expected assemblies are not being found. So perhaps Visual Studio might be creating a project file for a 4.5 framework app, and expecting 4,5 assemblies, while you are referencing 4.0 assemblies. So look at your Visual Studio settings for incompatibilities or go into the project file yourself, and manually fix it by specifying the correct path <Reference Include="C:\\correct path to assembly\\yourAssembly.dll" />.

我的项目也面临着同样的问题，我在一个解决方案中添加了控制台应用程序，但那些其他解决方案不起作用，并显示未找到.dll，所以我尝试了这个方法:

右键单击有问题的项目 单击应用程序选项卡，这是默认的 将输出类型控制台应用程序更改为类库

在我的案例中，解决方案中的一些项目针对任何CPU，其中一些针对x86。在整个解决方案统一了平台目标后，编译错误消失了。

对我来说，问题是我打开了两个Visual Studio窗口，我的项目在一个窗口中运行调试，而我试图在另一个窗口中构建它。

我不得不停止调试，然后它让我成功构建。