What is the function of DLL in C#

Question

https://www.youtube.com/watch?v=BFO_luxmo5U&feature=em-uploademail

I wish to know the function of Dynamically Linked Library (DLL) in C#, particularly related to above reference. In the above link around 1:59 position author is talking about DLL.

Vulpes · Answer

A .NET DLL provides functionality which .NET executables and other .NET DLLs can use. All you need to do is to add a reference to the DLL and you can then use its public types and methods as though they had been defined in the current application itself. A DLL itself is not executable since it has no Main method. In the video the DLL is being used to house a static class containing extension methods. Extension methods don't have to be included in a separate DLL but it's a good way of sharing the functionality between a number of applications.

Vulpes · Answer

Well you won't be able to do this with the DLL alone as it's not executable. There's no entry-point i.e. Main() method in a DLL.

Maha · Answer

Thank you for your help. Suppose I want to provide only DLL file to write above program. What do have I to do?

Vulpes · Answer

OK let's build an executable now to test the DLL. Type this at the command prompt: notepad DemoExecutable.cs and when notepad opens up press OK to create a new file paste this code into it and save it: using System; using DemoLibrary; class Program { static void Main() { Demo d = new Demo(); d.SayHello(); Console.ReadKey(); } } Now compile the executable whilst referencing the DLL: csc /r:DemoLibrary.dll DemoExecutable.cs If you now type: dir you'll see DemoExecutable.exe has been added to the folder. Finally run the executable by typing: DemoExecutable and the output will be: Hello from DemoLibrary And basically that's all there is to DLLs though the ones you'll encounter in practice will of course be much more complicated than this.

Maha · Answer

Yes I have done everything you showed me now you can proceed further.

Vulpes · Answer

Well if you want to really see what's going on it's easiest to create the DLL from the Visual Studio Command Prompt or Developer Command Prompt from VS 2012 onwards. You should be able to find this from the Start menu under Visual Studio tools. First of all create a folder for your C# projects by typing this (always press return after each line when in the command prompt): md c:\csprojects Next type this to change the working folder to the one you've just created: cd c:\csprojects Next type: notepad DemoLibrary.cs and when notepad opens up press OK to create a new file paste this code into it and save it: using System; namespace DemoLibrary { public class Demo { public void SayHello() { Console.WriteLine(Hello from DemoLibrary); } } } Now type the following at the command line to compile the DLL: csc /t:library DemoLibrary.cs If you now type: dir You should see DemoLibrary.dll in the listing with a size of 3072 bytes. Although it may not mean much to you you can inspect the CIL and metadata in this DLL by typing the following: ildasm DemoLibrary.dll Once you've got this far I can show you to how to reference the dll from an exe if you like.

Maha · Answer

How to convert this into DLL format. using System; namespace DemoLibrary { public class Demo { public string Test; } }

Vulpes · Answer

Tools such as the object browser in Visual Studio will tell you what's in the DLL and Intellisense will also help when writing code. The writer of the DLL might also want to provide written documentation including examples on similar lines to what Microsoft do for the .NET framework. It's possible to generate documentation automatically if you include documentation comments (three forward slashes instead of two) in your source code.

Maha · Answer

If DLL file is in a non-readable format how can programmer write an appropriate program (in the above case DemoExtension program) to make use the DLL file.

Vulpes · Answer

No it's stored in binary form like an .exe is. Also as it's a .NET DLL it contains CIL (common intermediate language) and metadata rather than native code. The CIL is then converted to native code method by method when they are invoked within the DLL for the first time. However unless it's been obfuscated it's possible to use a decompiler such as Redgate .NET reflector to recover the source code in a readable form. The only things of importance that can't be recovered are the original local variable names and any comments.

Maha · Answer

I wish to know whether DLL file is human readable format or not.

Vulpes · Answer

Well that's the class library project which he started with in the video. He then compiled it to a DLL and added a reference to the DLL to his console project. This enabled him to instantiate the Demo class and access its Test field from the console project. So the source code is already in class library (i.e. DLL) form and can't be executed directly because there's no Main method. To test a class library you always have to compile it to a DLL and then reference it from an executable project. However the executable project can be in the same solution as the class library and I think he did an example of that later in the video.

Maha · Answer

In the above example following source code (Class1.cs) is assumed to be in DLL form therefore programmer does not have access to alter it. As a solution DemoExtension class is created. I wish to know how below source code is look like in DLL form. using System; namespace DemoLibrary { public class Demo { public string Test; } }

Vulpes · Answer

Sorry Maha but I'm not sure what you're looking for here. Can you clarify please.

Maha · Answer

In the above example Class1.cs is assumed to be in the DLL form. Could you show me please how it looks like in the DLL form.

Maha · Answer

Thank you for your explanation.