You can create a class library Project, Once it build, it will create DLL and then reuse your C# code in other projects.
Step :
File
|
New Project
| Select Template Type "
ClassLibrary
"
This will have a class file, you can use the same / rename or add a new files. Build it. It will create a dll.
Here is one step by step guide, which may help you.
How to Create a Class Library in C# .NET[
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