Question

I’m still learning how to use Events in C# so I’ve come up with what should be a fairly simple practice example. I have an object that reads from a database into a List. When an item in the database gets deleted an event is triggered. How do I have a DBWrapper object listen for an event triggered by another class?

public class DBWrapper
{
    private List<DBStruct>  _List;

    public List<DBStruct> GetList
    {
         get
         {
              return _List;
         }
    }

    void OnDeleted(object sender, EventArgs e)
    {
         //Re-read the list from db.  I could probably do
         //something with Lazy<T> loading to eliminate the event stuff
         //but that's not the point.
         _List = (from d in db select d).ToList();
    }
}

I then have another class with a Delete() method that triggers an event:

public class DeleteDB
{
     public void Delete(int index)
     {
          //delete from db where d.ID = index

          //fire deletion event to a DBWrapper object
     }
}

This is essentially what I want my output to be:

void Test()
{
    DBWrapper dbw = new DBWrapper();

    Console.WriteLine(dbw.GetList);
    //Apple=1,Banana=2,Cake=3

   DeleteDB ddb = new DeleteDB();
   ddb.Delete(1);

   Console.WriteLine(dbw.GetList);
   //Banana=2,Cake=3
}

How do I have one object listen to an event triggered by another object?

Answer 1

You wire up the event handler in the DbWrapper to the event on the DeleteDB. Here is some code to explain, using what you provided.

public class DeleteEventArgs:EventArgs{
    public DeleteEventArgs(int index){
        _index = index;
    }
    private readonly int _index;
    public int Index{get{return _index;}}
}

public class DeleteDB
{
     public event EventHandler<DeleteEventArgs> Deleted;

     public void Delete(int index)
     {
            if(Deleted!=null)  {
                  Deleted(this,new DeletedEventArgs(index));
            }
      }
 }

  void Test()
  {
      DBWrapper dbw = new DBWrapper();
      Console.WriteLine(dbw.GetList);
      //Apple=1,Banana=2,Cake=3

      DeleteDB ddb = new DeleteDB();
      ddb.Deleted += dbw.OnDeleted;            

      ddb.Delete(1);

      Console.WriteLine(dbw.GetList);
      //Banana=2,Cake=3
   }

Please note that your OnDeleted method of the DBWrapper would need to made public to assign it like this, unless the Test() method is inside of the DBWrapper class.