using Dynamic Link Library (DLL)

Actually static libraries were the original technology that is about half a century old; older than the C language. Back then compilers generated one or more object code files that got combined into a single executable. A Windows lib file originally (in DOS days) was a library of object files and each object file was the result of compiling one input file; in other words, each c/cpp file was compiled into one object file. I said was but this is the way that most non-managed compilers work now. Static linking is what was done by compilers when I started learning programming in 1972. In the 1980's IBM provided dynamic linking for COBOL in their "mainframe" computers but it was not part of the operating system. I have read that Windows had dynamic linking before Unix did; the C compiler certainly originally created object files that were statically linked together because that is the way compilers worked before Unix and C were designed.

Yes Sam is correct. Just use term "Class Library" or Assembly.

Prior to .NET, a DLL file with physical extension .dll was called a dynamic link library, hence DLL. and then there was something called Static Linked Library. But that was C++.

In .NET, everything (exe or dll) is an assembly and term DLL is replaced with a class library.

If you look at the right side of this post, you will see two tutorials on how to create and use a class library.

I hope it helps and clears a few concepts.

Cheers!

I was trying to make things easier for you. My suggestion was a way to make development easier. You don't have to start with the classes in the application project. I was suggesting to do that to make things easier. You can start with both projects from the beginning; it is a matter of your choice. I am sorry I confused you.

I would also suggest that it is more meaningful to use the term Class Library instead of DLL but if that is confusing then ignore that. The problem is that the term DLL can be ambiguous and confusing.

If you put your Shape, Rectangle, Triangle and Circle classes in the .dll and declare them as public classes then, when a reference to the .dll is added to your .exe,  you'll be able to create objects of these types from the .exe just as if they'd been declared in the .exe itself:

Rectangle rec = new Rectangle(0,0,0,0); // or whatever the constructor requires
// do something with rec

In addition to what Vulpes said, note that there are very many types of DLLs. Originally DLLs were only "regular DLLs". The term regular in this context is not officially defined and the definition is controversial since older versions of VB could not create regular DLLs and some VB programmers would not accept that as the truth. When we write a program that uses the Windows API directly, we are calling reguloar DLLs. The Windows API are functions in regular DLLs. You cannot make a regular DLL using managed code, and C# is always managed code.

So what you want to do is to make a project for a "Class Library". That will create a DLL using managed code that you can easily use with other managed code. What you can do is to create an application that has the classes in it; you can put everything in one Windows Forms application or whatever type of application you need to create. Then later after you have things working you can create a new Class Library project and move the relevant classes to the Class Library. Then add a reference to the Class Library to your application and then replace use of the classes in the application with calls to the Class Library. So do you understand that so far?