how to write 'hello world' in classlibrary.cs

You have already asked it over here, want basic code for class library function and how to implement namespace.[^]
So, you could have wait for few time to some one respond.
If you check out the link I have provided, has step by step explanation.

Create a Class Library Project > Write a Method

Public Class MyClassLibrary
    {
      public static string MyHelloMethod()
         {
            return "Hello World";
         }

    }

Build the dll. Add the project as reference to the calling project.

Add the namespace if its different projects.

Then,

Console.WriteLine(MyClassLibrary.MyHelloMethod())

;

This will print, "Hello World" in your output screen.

Note, I used the static method, to directly call with out creating object. Hope you know that.