Error in UnityEngine.UI.dll used by Managed plug-ins

I am creating a Managed plug-ins for use with Unity 2020.
Using UnityEngine.UI.dll found in Unity2020 will result in a build error.
Where can I find UnityEngine.UI.dll that doesn't cause a build error?
The content of the error is
"Error CS0012: Type'MonoBehaviour'is defined in an unreferenced assembly. You need to add a reference to the assembly'UnityEngine.CoreModule, Version = 0.0.0.0, Culture = neutral, PublicKeyToken = null'. "

In Unity2020, UnityEngine.UI.dll did not exist in the same folder as Unity2019.1.
(Editor \ Data \ UnityExtensions \ Unity \ GUISystem \ UnityEngine.UI.dll)
After searching, it was found in the following folder.
Editor \ Data \ Resources \ PackageManager \ ProjectTemplates \ libcache
\ Com.unity.template.2d-5.0.0 \ ScriptAssemblies \ UnityEngine.UI.dll
\ Com.unity.template.3d-5.0.4 \ ScriptAssemblies \ UnityEngine.UI.dll
\ Com.unity.template.hd-10.5.0 \ ScriptAssemblies \ UnityEngine.UI.dll
\ Com.unity.template.universal-10.5.0 \ ScriptAssemblies \ UnityEngine.UI.dll
However, both result in a build error.

Using the dll in the project folder will result in the same build error.
([project name] \ Library \ ScriptAssemblies \ UnityEngine.UI.dll)

 

Does this only happen for a certain project or does it happen also if you create a new one?

 

Thank you for your reply.
I tried the following, but I get a build error.
-Changed the .Net FrameWork version of the project at Unity 2019 to 4.7.1 and added UnityEngine.UI.dll.
-Create a new project and add UnityEngine.UI.dll.

For example, if you write the following in the test, it will not cause an error,
public bool Check (GameObject obj)
{
UnityEngine.UI.Graphic com = obj.GetComponent <Graphic> ();
return (obj);
}
If you change the return statement as follows, a build error will occur.
public bool Check (GameObject obj)
{
UnityEngine.UI.Graphic com = obj.GetComponent <Graphic> ();
return (com);
}

When I check UnityEngine.UI.Graphic in the object browser of Visual Studio, it doesn't seem to recognize "MonoBehavior".

 

Thank you for your response.
I checked what you answered, but I feel that it is a little different from what you asked.
What I'm hearing is that it's not the Unity project of the application that is causing the error.
It's happening in a project that creates a custom dll for use in an application.
This custom dll uses UnityEngine and UnityEngine.UI instructions.
Therefore, I am trying to refer to and register UnityEngine.dll and UnityEngine.UI.dll in the custom dll project.
However, when I use UnityEngine.UI.dll provided by Unity, which I know, I get an error.
At the time of Unity2019.1, I used the dll provided by (Editor \ Data \ UnityExtensions \ Unity \ GUISystem \ UnityEngine.UI.dll).
However, in 2020.3, the same folder as 2019.1 does not exist.
Please tell me UnityEngine.UI.dll to prevent error in custom dll project.
If I don't understand what you're telling me, please point it out.
thank you.

 

I was taught how to deal with it.
The dll can be solved by using the one in the following location.
UnityEngine.dll:
/[UNITY_EDITOR]/Editor/Data/Managed/UnityEngine/UnityEngine.dll
UnityEngine.CoreModule.dll:
/ [UNITY_EDITOR] /Editor/Data/Managed/UnityEngine/UnityEngine.CoreModule.dll
UnityEngine.UI.dll:
/ [PROJECT] /Library/ScriptAssemblies/UnityEngine.UI.dll

There is a commentary in the documentation.
https://docs.unity3d.com/Manual/UsingDLL.html