I have a DotNetCore 2.1 Web App (just created it today from the VS 2017 scaffold). This solution also includes a DotNet Core 2.1 Library DLL project. The solution builds ok.

我有一个DotNetCore2.1 Web App(今天刚从VS 2017脚手架上创建)。该解决方案还包括一个DotNet Core 2.1库DLL项目。解决方案构建得很好。

I have another (brand new) DotNetCore 2.1 Web App that wants to use the Library DLL. But when I try to add a new reference (on the Browse tab), it complains:

我还有另一个(全新的)DotNetCore2.1Web应用程序想要使用库DLL。但当我尝试添加新引用时(在浏览选项卡上)，它抱怨：

One or more errors occurred.

The reference is invalid or unsupported.

Any suggestions?

有什么建议吗？

(Curiously, the DLL doesn't show up directly under the Bin\Debug folder like it does in classic .Net; It's under Bin\Debug\netcoreapp2.1)

(奇怪的是，DLL不像在经典的.Net中那样直接显示在Bin\Debug文件夹下；它在Bin\Debug\netcoreapp2.1下)

I got this error after I moved some of my code repos around and suddenly my .NET Core 2.1 console app wouldn't recognize a .NET 4.5 DLL anymore when I added it.

在我移动了一些代码报告后，我得到了这个错误，当我添加它时，我的.NET Core2.1控制台应用程序突然不能识别.NET 4.5DLL了。

Turned out that although the broken reference did not show in the solution explorer, the project file still contained it, along with the now broken HintPath to the DLL's old location. Manually removing this reference from the project file solved it.

原来，尽管解决方案资源管理器中没有显示损坏的引用，但项目文件仍然包含它，以及指向DLL旧位置的现已损坏的HintPath。手动从项目文件中删除此引用已解决此问题。

Go into your .csproj and make sure the <Reference Include=... is pointing to the correct directory.

进入您的.csproj并确保

For example, my references were pointing to the bin\Debug path of another project when the only build that was actually there was the bin\Release

例如，我的引用指向另一个项目的bin\Debug路径，而实际存在的唯一版本是bin\Release

When you create a new project from the Visual Studio 2017 Class Library (.NET Core) template, the csproj file contains:

当您从Visual Studio 2017类库(.NET Core)模板创建新项目时，csproj文件包含：

<TargetFramework>netcoreapp2.1</TargetFramework>

Since this is a library, it (apparently) needs to target netstandard2.0. (BTW, creating a library project from the dotnet command line command correctly targets netstandard2.0. Go figure.)

由于这是一个库，它(显然)需要以netStandard2.0为目标。(顺便说一句，从DotNet命令行命令创建库项目正确地针对netStandard2.0。想一想吧。)

So changing the line in the library's csproj file to

因此，将库的csproj文件中的行更改为

<TargetFramework>netstandard2.0</TargetFramework>

and rebuilding the library now allows me to reference the library from the DotNet Core 2.1 (netcoreapp2.1) application.

现在，重新构建库允许我从DotNet Core 2.1(netcoreapp2.1)应用程序引用该库。

I am converting lots of projects to .net6, and one of the projects suddenly refused to let me add .dll files. The solution was to remove the old references first, then add the new references. (The project had references to old .dll files with the same name as the new converted ones.)

我正在将许多项目转换为.net6，其中一个项目突然拒绝让我添加.dll文件。解决方案是首先删除旧引用，然后添加新引用。(该项目引用了与新转换的文件同名的旧.dll文件。)

Normally Visual Studio lets me add two dlls with the same name but different path, so I later can erase the old one, but in this particular case this did not work. A screenshot and Paint to the rescue for my memory!

通常，Visual Studio允许我添加两个同名但路径不同的dll，这样我以后就可以删除旧的dll，但在这种情况下，这不起作用。一张截图和画作来拯救我的记忆！

In my case, I had a reference to duplicated .dll files. After removing duplication and changing references to different folders got the error above. Cleaning obj, bin and removing the .vs folder from the root directory helped

在我的例子中，我引用了复制的.dll文件。删除重复项并更改对不同文件夹的引用后，出现上述错误。清除obj、bin并从根目录中删除.vs文件夹有帮助

For me I had to add a nuget package instead of manually adding a reference. I was using .net 6 and needed System.Drawing so I added the System.Drawing.Common package.

对于我来说，我必须添加一个Nuget包，而不是手动添加引用。我使用的是.NET6，需要System.Drawing，所以我添加了System.Drawing.Common包。

https://www.nuget.org/packages/System.Drawing.Common/

Https://www.nuget.org/packages/System.Drawing.Common/