C#.Net --- creating DLL files in c#

  DLL means Dynamic Linked Library. the dll is supportive file which gives a pre-defined 

methods to executable file. by using dll file we can reduce code complexity 
  The DLL are major part of BCL( base class library) which is one of component of .NET framework.(another part is CLR common language Runtime)
  if a program generate a execution file with extension '.exe' is exe file 
     a program generate a support file with extension '.dll'  is know as dll file .

 the dll files acts as libraries which contains one or more pre defined methods 
 the methods are used later where we want .
for examples :
       In a college, a library contains so many books by using these books we read or write or do anything what we want 

if we are creating dll means  we can use these methods in another project without deriving that class like with out doing inheritance 

How to Create a DLL file

    => open visual studio and 
      => select file->new->project
       => in project window select -> c# and select template "class library file" instead of Console Application
                            give a name like MathOperations and click ok
        => it will generate a new project with out main method . and type the following 

code 

After executing this program your can see in computer a dll file created in project path C:/dotnet/MathOperations/MathOperations/bin/debug/'-------here a file with MathOperations.dll name will created .