What exactly are you trying to do? Just debug the application? You can host it in IIS on your machine and attach your debugger to that process to debug it. But you not going to be able to fix any problems. Your best bet is to get the full source code or you will have to try and reverse engineer the dll. To do that, you can try any of these two:
I have only used .net reflector in the past when it was still free. So I am not sure how well the other works, but Telerik usually build quality tools, so I am sure it will be just as good.
Microsoft works on dll and exe files these files are compiled and ready to use with desired application or platform. If any software decompile then Microsoft Software Integrity is failed. any one can decompile application and services developed in Microsoft IDE. So it is not Possible.
It is possible to reverse engineer a dll and an exe. It gets more tricky if the developer obscured his code, but there are always ways around these things. That is why there is so much cracked games out on the market. Somebody reverse engineered them into assembly and patched it.
So if you want to safe guard you code from being decompiled, you will need to take some extra steps so it is not just as simple and 1 application. But most do not for web applications, as the dll is on a server which has it’s own security and not distributed.