What is the function of DLL in C#

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.

OK, let's build an executable now to test the DLL.

Type this at the command prompt:

   notepad DemoExecutable.cs

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.

How to convert this into DLL format.

using System;

namespace DemoLibrary
{
public class Demo
{
public string Test;
}
}

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.

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.

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.

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;
}
}

Sorry, Maha, but I'm not sure what you're looking for here.

Can you clarify please.