How can I pass a function to a C# class dll, that I am calling from another C# app? The function is defined in the app where the dll is called

I am calling a C# class dll in my C# service app. But in the class dll, at some point there has to be executed a method that is defined on the caller app. So I want to pass, like a parameter, a whole method to the dll, that has to be executed at a specific time.

The challainging point is that the function has to be executed in the dll, in a timer event. How can I pass my function in this case?

My C# app where I call the dll.

using MyClassLibrary; // my dll
namespace Concheetah_Service_Bahmuller
{
   public partial class Service1 : ServiceBase
     {
       public Service1()
       {
         InitializeComponent();
       }

       protected override void OnStart(string[] args)
       {
         MyClassLibrary.Main_Prog main_Prog = new MyClassLibrary.Main_Prog();
         main_Prog.Main_Start(); // starting point of my dll            
       }

       public void func_passed()
       {
       // some supplementary code
       }

     }
   }

MyClassLibrary

System.Timers.Timer timer1 = new  System.Timers.Timer();

public void Main_Start()
{    
     Initialize_timer1();  // starting point of the dll
}

public void Initialize_timer1()
{
     timer1.Elapsed += new ElapsedEventHandler(OnTimedEvent_timer1);
     timer1 = 35;
     timer1.Start();
}

private void OnTimedEvent_timer1(object sender, EventArgs e)
{
     //some code
     func_passed(); // at this point my passed function should be executed.
}

Solution

Have a look at Delegates, Anonymous Methods and Lambda Expressions.

Note that it makes no difference whether the code is in another DLL (in another assembly in C# terms) or not as long as you have a reference to this other project or assembly and the things you want to access are public.

Change the library like this (showing only changed things):

private Action _timerAction;

public void Main_Start(Action timerAction)
{    
     _timerAction = timerAction;
     Initialize_timer1();
}

private void OnTimedEvent_timer1(object sender, EventArgs e)
{
     //some code
     _timerAction();

     // If _timerAction can be null, call it like this instead:
     _timerAction?.Invoke();
}

Then call it like this from the application:

main_Prog.Main_Start(func_passed);

Make sure not to add braces () after func_passed since we don't want to call the function here, we want to pass the function itself as an argument.

There are different Action and Func Delegates having a different number of parameters. Unlike Action, Func has a return type other than void.

Another way to solve the problem is to have the library expose an event the application can subscribe to.