question about Dll in C#

check this out:

How to: Create and Use C# DLLs[^]

hope it helps :)

start new project c# Class Library.
C# .dll cannot be executed: it needs to be called/referenced by an application.
Then in same solution,
make a second project such as Windows or Console application that will
build and run as an .exe program.
then add reference to the .dll library project.
then add
using [ClassLibraryName];
to .exe code files

Please refer following threads:

In this article you will get information about how to create and use DLLs in C Sharp :
Creating and Using C# DLLs[^]

The process of creating class libraries is quite straightforward, you can get some more details over here,
How to Create a Class Library in C# .NET[^]

Here you can get complete details with detailed description of Libraries in C#,
Please refer:
Introduction to Custom Libraries[^]

Hope it helps!