How to package DLL class library correctly?

I encapsulated a DLL class library for common function calls, but I put it in the Managed folder and restart the server
After waiting for the server to complete the restart, put the plug-in written through the DLL class library into the Plugins folder, and the server prompts me that the assembly reference is missing.
Class library: Oxide.MyClass
Plugin reference: using Oxide.MyClass;
Error message: Error while compiling: PluginTest.cs(5,20): error CS0234: The type or namespace name `MyClass' does not exist in the namespace `Oxide'. Are you missing an assembly reference?

by using harmony 

You should be able to reference external libraries by adding a comment like so to the top of your plugin:

// Reference: <DLL NAME>
Thank so much

Merged post

One thing I really don't understand is why some functions keep prompting that they are not clear? I double-checked the function definition, and I'm pretty sure there's only one possibility, but it keeps saying I've included an error:
error CS0121: The call is ambiguous between the following methods or properties: `Oxide.Ext.Acdiu.UI.AddPanel(this string, ref Oxide.Game.Rust.Cui.CuiElementContainer, string, Oxide.Ext.Acdiu.UI. Rect, bool)' and `Oxide.Ext.Acdiu.UI.AddPanel(this string, ref Oxide.Game.Rust.Cui.CuiElementContainer, string, Oxide.Ext.Acdiu.UI.Rect, bool)'

These prompts generally appear when the UI function is called, and other functions can run normally when called, and no similar error prompts have appeared yet.
wYRgJoDjF1Ocj5I.png Rick7851

by using harmony 

what is harmony ?