Is there an equivalent of Application_Start for a class library in c#

Question

I would like to execute certain code in a class library when it is instantiated from another assembly. Is there an entry point or bootstrap for a class library? I thought that a static method Main would do the trick but I was wrong.

Applications for this might be configuring and instantiating a logger singleton, unhandled exception handler, etc.

You're looking for a module initializer, which isn't supported by C#. — SLaks, Oct 04 '11 at 23:40
Can you provide a single point of entry into the class library, say a static class factory that you have to call in order to get any useful instance from the library? Then you could do your initialization on the first call to the static class factory. — mellamokb, Oct 04 '11 at 23:47
Can't you just use a static constructor of some class that you know will be loaded from that assembly? — svick, Oct 05 '11 at 00:08

score 2 · Answer 1 · answered Jan 02 '20 at 17:55

2

A library as it is, has not an starting point. When you are instancing a class of a library the first instruction you call is the constructor of the class (new) and its base constructors if they are on the constructor definition.

answered Jan 02 '20 at 17:55

Ixer

47
4

score 0 · Answer 2 · answered Oct 28 '19 at 20:26

The best and safe way is to design your library so that the caller will initialize your library when they know they can.

When an assembly is loaded, the CLR employes a lot of machinery to have the job done, starting from how the inner platform is designed to load modules up to the CLR itself. Each involved actor has their own limitations.

Executing code when a module is loaded is not the best practice on Win32 for the same reason: you cannot know what your caller is doing; also, the changes your doing could possibly alter the current AppDomain, but may not plbe propagated in all the other AppDomain of the application.

A conscious initialization method is the cleanest way to let the caller to control the initialization of your assembly.

All the other answers partially address the issue, but could introduce unwanted side effects and non deterministic behaviors.

score 0 · Answer 3 · edited May 23 '17 at 11:54

0

AppDomain.AssemblyLoad Event which occurs when an assembly is loaded. Probably that can be used to call an initialize method in your class library.

public static void Main() 
{
    AppDomain currentDomain = AppDomain.CurrentDomain;
    currentDomain.AssemblyLoad += new AssemblyLoadEventHandler(MyAssemblyLoadEventHandler);
}

static void MyAssemblyLoadEventHandler(object sender, AssemblyLoadEventArgs args) 
{
      Console.WriteLine("ASSEMBLY LOADED: " + args.LoadedAssembly.FullName);
      //If this is the assembly that you want to call an initialize method..
}

Below are two similar threads

how to write class lib's assembly load/init event handler

.Net: Running code when assembly is loaded

edited May 23 '17 at 11:54

Community

1
1

answered Oct 04 '11 at 23:51

CharithJ

46,289
20
116
131

Are you sure that will work? I assume you will subscribe to it in a static constructor. If that's the case, I think the event will fire before the static constructor runs. – svick Oct 05 '11 at 00:08
Not too sure in which order they would fire. However this can be used to set a flag in the application or to call the initialize method in the class library. – CharithJ Oct 05 '11 at 00:47

score -1 · Answer 4 · answered Dec 30 '13 at 07:40

-1

Have you looked into the PreApplicationStartMethodAttribute?

using System.Web;

[assembly: PreApplicationStartMethod(typeof(ClassLibrary.Startup), "Start")]

namespace ClassLibrary
{
    public class Startup
    {
        public static void Start()
        {
            // do some awesome stuff here!
        }
    }
}

More detail: http://dochoffiday.com/professional/simulate-application-start-in-class-library

answered Dec 30 '13 at 07:40

dochoffiday

5,515
6
32
41

4

this will work only within asp.net application and will not work in context of class library test project for example. There is nothing about it in documentation, but it became obvious after I tried this out and exploration of Support for invoking the start methods outside of ASP.NET part of [WebActivator documentation](https://github.com/davidebbo/WebActivator/blob/master/README.markdown) which uses `PreApplicationStartMethodAttribute` internally – Leonid Vasilev Jul 01 '15 at 16:08
3

Voted down because the question wasn't about ASP.NET. – StefanLundmark Jul 30 '16 at 20:51

Is there an equivalent of Application_Start for a class library in c#

4 Answers4

Linked