Haw can i create DLL file

There is no "convert". Especially if you already have "class library".

And, in .NET, there is no essential difference between "DLL" and "EXE". It's just a name pattern and nothing else. "EXE" files have an entry point, but you can use them in exact same way as "DLL". Essentially, this is also a class library, a bit specializes.

The central object of .NET is "Assembly", not "EXE" or "DLL".

With Visual Studio, you can create a class library if you choose the application option "Class Library", in the very first tab, "Application" of your project properties. And you can change this option any time. By default, the main (and the only one, with Visual Studio) executable module of your assembly will be named *.DLL, but you can name it in any other way, it just does not matter.

This is all you need.

—SA

when you build you application then at time DLL created
just check your application's BIN Folder, there is DLL.