(System.IO.FileNotFoundException) .Net core Web API cannot find child dependency (Dependencies of dependency) when added DLL by “Add Reference”

问题

I am getting System.IO.FileNotFoundException in my .Net Core Web API. So I've set up the below project to demonstrate the problem.

I created a.Net Standard library named DemoLibrary and added QRCoder dependency via NuGet.

Disclaimer: Reason for choosing the QRCoder is that the Web API doesn't use it by default. I don't use it in my project. In fact, I'm getting this exception for EntityFrameworkCore.

I created a new .Net Core Web API DemoWebAPI which has no other dependencies.

Then added the DemoLibrary to DemoWebAPI via Add Reference -> Browse -> DemoLibrary.dll.

This is my solution:

The DemoMethod method in Calculate class just creates the object of QRCodeGenerator.

public class Calculate
{
    public static string DemoMethod()
    {
        QRCodeGenerator qrGenerator = new QRCodeGenerator();
        return "";
    }
}

And my ValuesController in DemoWebAPI just calls the method:

[HttpGet]
    public ActionResult<IEnumerable<string>> Get()
    {
        return new string[] { "value1", "value2", DemoLibrary.Calculate.DemoMethod()  };
    }

Now, when I run the DemoWebAPI project I get below exception upon the call to the DemoMethod:

System.IO.FileNotFoundException: 'Could not load file or assembly 'QRCoder, Version=1.3.5.0, Culture=neutral, PublicKeyToken=null'. The system cannot find the file specified.'

I understand the fact that I have to copy the QRCoder.dll file somewhere. But I fail to understand where to put it. I've already tried putting it in "bin/debug/netcoreapp2.2" of the DemoWebAPI and "bin/debug/netstandard2.0" of the DemoLibrary.

But I couldn't get it working.

Request: Please post your answer as descriptive as you can because I am new to .Net Core.

Edit: I am aware of the NuGet servers. I have read topics like hosting a NuGet server in IIS and Azure. The reason behind DLL reference is I want to use my DLLs in two projects one of them is a .net core API and the other is .net framework class library which is compiled by NMAKE. I couldn't find any way to restore NuGet packages in the .MAK files.

回答1:

It looks like you've merely added the DLL for DemoLibrary to your DemoWebApi project. That's not how you should be adding references. Since these are in the same solution, you should add a project reference. That will fix your issue.

Now, let me explain what's actually going on here. Your DemoLibrary has a dependency on QRCoder. It's a NuGet reference, which means that package will be restored (i.e. downloaded) and included in your DemoLibrary build output. However, it will be included as one or more DLLs along side your DemoLibrary.dll. When you then just reference DemoLibrary.dll, you're missing all these other DLLs that are part of DemoLibrary and thus, things don't work properly.

Now, when it comes to a project reference, things are little more complex. A project reference essentially wraps the referenced project into your other project. You can think of it as sort of a sub project. For all intents and purposes, it's like any dependency of the sub project becomes a dependency of the main project. That means that DemoWebAPI now technically has a NuGet package reference to QRCoder even though there's no explicit package reference in its project file. The dependency comes from your DemoLibary project. As such, with a project reference, all the necessary dependencies will be included, because it's as if the main project included those itself, by way of the sub project.

For what it's worth, you should virtually never include a DLL as a reference directly. That used to be required, but the concept of NuGet packages has all but eliminated the practice. Even if DemoLibrary was not in the same solution as DemoWebAPI (meaning you could no longer do a project reference), the correct way to use it would be to turn DemoLibary into a NuGet package, and then reference it in DemoWebAPI via a package reference, like any other NuGet package. You do not simply add the DLL.

来源：https://stackoverflow.com/questions/55417837/system-io-filenotfoundexception-net-core-web-api-cannot-find-child-dependency