Generating initialization code for .dll

Question

I recently heard that it is possible for a .dll to run code as soon as it is loaded, when an application which references the .dll is loaded, for example. Event though I made some tests of my own and tried looking for answers here and on Google I was unable to find some way to generate an initialization method for a .dll.

I would like to know if it is really possible to run code from a .dll when it is loaded by an application.

If so, how can I do so?

Answer 1

WARNING (thanks Ben Voigt for the catch :)): the below code applies only to C# that guarantees that the generated class won't be beforefieldinit. But with C++/CLI should not work as easily: Managed C++ Static Constructor not called in .net4

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So as stated in my comment you may use something like this:

using System;

class MyAwesomeLibrary
{
    static MyAwesomeLibrary()
    {
        Console.WriteLine(string.Format("Hey {0} is using me!", Environment.UserName));
    }
    
    public static int GetTheAnswer()
    {
        return 42;
    }
}

class Client
{
    static void Main()
    {
        Console.WriteLine("The answer is: " + MyAwesomeLibrary.GetTheAnswer());
    }
}

In the static constructor you can do advanced things like checking the registry, communicating with a server...

And if you're a bad guy (or just a developer/company who wants to protect its rights) you can throw an exception:

throw new Exception("The library has not been correctly registered...");

This will result in a TypeInitializationException preventing the use of the whole library class.

Or you can implement a CheckMe method ans asks all the user to call it before using the library or even to authenticate and get a security token they'll use each time they use something...

EDIT:

Whatever the protection you use a determined attacker could circumvent all the plumbing by decompiling the library so if you can you should obfuscate your library too to be (a little) more protected.