What is in a DLL and how does it work?

Question

I'm always referencing DLLs in my C# code, but they have remained somewhat of a mystery which I would like to clarify. This is a sort of brain dump of questions regarding DLLs.

I understand a DLL is a dynamically linked library which means that another program can access this library at run time to get "functionality". However, consider the following ASP.NET project with Web.dll and Business.dll (Web.dll is the front end functionality and it references Business.dll for types and methods).

1. At what point does Web.dll dynamically link to Business.dll? You notice a lot in Windows HDD thrashing for seemingly small tasks when using Word (etc.) and I reckon that Word is going off and dynamically linking in functionality from other DLLs?

   1a. Additionally, what loads and links the DLL - the OS or some run time framework such as the .NET framework?

   1b. What is the process of "linking"? Are compatibility checks made? Loading into the same memory? What does linking actually mean?

2. What actually executes the code in the DLL? Does it get executed by the processor or is there another stage of translation or compilation before the processor will understand the code inside the DLL?

   2a. In the case of a DLL built in C# .NET, what is running this: the .NET framework or the operating system directly?

3. Does a DLL from Linux work on a Windows system (if such a thing exists), or are they operating system specific?

4. Are DLLs specific to a particular framework? Can a DLL built using C# .NET be used by a DLL built with, for example, Borland C++?

   4a. If the answer to 4 is "no" then what is the point of a DLL? Why dont the various frameworks use their own formats for linked files? For example: an .exe built in .NET knows that a file type of .abc is something that it can link into its code.

5. Going back to the Web.dll / Business.dll example - to get a class type of customer I need to reference Business.dll from Web.dll. This must mean that Business.dll contains some sort of a specification as to what a customer class actually is. If I had compiled my Business.dll file in, say, Delphi: would C# understand it and be able to create a customer class, or is there some sort of header info or something that says "hey sorry you can only use me from another Delphi DLL"?

   5a. Same applies for methods; can I write a CreateInvoice() method in a DLL, compile it in C++, and then access and run it from C#? What stops or allows me from doing this?

6. On the subject of DLL hijacking, surely the replacement (bad) DLL must contain the exact method signatures and types as the one that is being hijacked. I suppose this wouldn't be hard to do if you could find out what methods were available in the original DLL.

   6a. What in my C# program is deciding if I can access another DLL? If my hijacked DLL contained exactly the same methods and types as the original but it was compiled in another language, would it work?

What is DLL importing and DLL registration?

Answer 1

First of all, you need to understand the difference between two very different kinds of DLLs. Microsoft decided to go with the same file extensions (.exe and .dll) with both .NET (managed code) and native code, however managed code DLLs and native DLLs are very different inside.

1) At what point does web.dll dynamically link to business.dll? You notice a lot in Windows HDD thrashing for seemingly small tasks when using Word etc and I reckon that this Word going off and dynamically linking in functionality from other DLL's?

1) In the case of .NET, DLLs are usually loaded on demand when the first method trying to access anything from the DLL is executed. This is why you can get TypeNotFoundExceptions anywhere in your code if a DLL can't be loaded. When something like Word suddenly starts accessing the HDD a lot, it's likely swapping (getting data that has been swapped out to the disk to make room in the RAM)

1a) Additionally what loads and links the DLL - the O/S or some runtime framework such as the .Net framework?

1a) In the case of managed DLLs, the .NET framework is what loads, JIT compiles (compiles the .NET bytecode into native code) and links the DLLs. In the case of native DLLs it's a component of the operating system that loads and links the DLL (no compilation is necessary because native DLLs already contain native code).

1b) What is the process of "linking"? Are checks made that there is compatibility? Loading into the same memory? What does linking actually mean?

1b) Linking is when references (e.g. method calls) in the calling code to symbols (e.g. methods) in the DLL are replaced with the actual addresses of the things in the DLL. This is necessary because the eventual addresses of the things in the DLL cannot be known before it's been loaded into memory.

2) What actually executes the code in the DLL? Does it get executed by the processor or is there another stage of translation or compilation before the processor will understand the code inside the DLL?

2) On Windows, .exe files and .dll files are quite identical. Native .exe and .dll files contain native code (the same stuff the processor executes), so there's no need to translate. Managed .exe and .dll files contain .NET bytecode which is first JIT compiled (translated into native code).

2a) In the case of a DLL built from C# .net what is running this? The .Net framework or the operating system directly?

2a) After the code has been JIT compiled, it's ran in the exact same way as any code.

3) Does a DLL from say Linux work on a Windows system (if such a thing exists) or are they operating system specific?

3) Managed DLLs might work as-is, as long as the frameworks on both platforms are up to date and whoever wrote the DLL didn't deliberately break compatibility by using native calls. Native DLLs will not works as-in, as the formats are different (even though the machine code inside is the same, if they're both for the same processor platform). By the way, on Linux, "DLLs" are known as .so (shared object) files.

4) Are they specific to a particular framework? Can a DLL built using C# .Net be used by a DLL built with Borland C++ (example only)?

4) Managed DLLs are particular to the .NET framework, but naturally they work with any compatible language. Native DLLs are compatible as long as everyone uses the same conventions (calling conventions (how function arguments are passed on the machine code level), symbol naming, etc)

5) Going back to the web.dll / business.dll example. To get a class type of customer I need to reference business.dll from web.dll. This must mean that business.dll contains a specification of some sort of what a customer class actually is. If I had compiled my business.dll file in say Delphi would C# understand it and be able to create a customer class - or is there some sort of header info or something that says "hey sorry you can only use me from another delphi dll".

5) Managed DLLs contain a full description of every class, method, field, etc they contain. AFAIK Delphi doesn't support .NET, so it would create native DLLs, which can't be used in .NET straightforwadly. You will probably be able to call functions with PInvoke, but class definitions will not be found. I don't use Delphi so I don't know how it stores type information with DLLs. C++, for example, relies on header (.h) files which contain the type declarations and must be distributed with the DLL.

6) On the subject of DLL hijacking, surely the replacement (bad) DLL must contain the exact method signatures, types as the one that is being hijacked. I suppose this wouldnt be hard to do if you could find out what methods etc were available in the original DLL.

6) Indeed, it's not hard to do if you can easily switch the DLL. Code signing can be used to avoid this. In order for someone to replace a signed DLL, they would have to know the signing key, which it kept secret.

6a) A bit of a repeat question here but this goes back to what in my C# program is deciding if I can access another DLL? If my hijacked DLL contained exactly the same methods and types as the original but it was compiled in another lanugage would it work?

6a) It would work as long as it's a managed DLL, made with any .NET language.

• What is DLL importing? and dll registration?

"DLL importing" can mean many things, usually it means referencing a DLL file and using things in it.

DLL registration is something that's done on Windows to globally register DLL files as COM components to make them available to any software on the system.

Answer 2

A .dll file contains compiled code you can use in your application.

Sometimes the tool used to compile the .dll matters, sometimes not. If you can reference the .dll in your project, it doesn't matter which tool was used to code the .dll's exposed functions.

The linking happens at runtime, unlike statically linked libraries, such as your classes, which link at compile-time.

You can think of a .dll as a black box that provides something your application needs that you don't want to write yourself. Yes, someone understanding the .dll's signature could create another .dll file with different code inside it and your calling application couldn't know the difference.

HTH