Extract the class file from the dll in c#

If by 'extract' you mean create an object of the class, then you need to make sure it's been declared as public in the dll source code, otherwise it's won't be accessible to the client application.

If it is public, then you can create an object of that class just as if it had been defined in the client application itself. You may need to add a ' using' directive to your code to facilitate this if the class is defined within a namespace in the dll.

Of course, you need to add a reference to the dll to the client application before you can access anything at all!

However, if by 'extract' you mean you want to extract the MSIL or original source code of the class from the dll, then you need to use a dis-assembler such as Redgate Reflector.

Even then you won't be able to extract the precise source code. All comments and local variable names will be lost - the latter will be replaced with numbered storage locations.

Also if the dll has been obfuscated, you won't be able to recover the source code at all except with the use of special 'hacker' software.