Cannot compare object from Arraylist to the actual object-CodePudding

im trying to select a random user control within an array list. I get the index of the array but it tells me it "cannot simply convert int to UserControl." Anyone know what I did wrong?

    ArrayList notiList = new ArrayList();

    int count = 0;
    int i;
    public MainPage()
    {
        this.InitializeComponent();

        foreach (NotiObject noti in itemsPanel.Children.Where(c => c is NotiObject))
        {
            notiList.Add(noti);
            System.Diagnostics.Debug.WriteLine(noti);

        }
        i = new Random().Next(0, notiList.Count);

    }

   
    void sendNotification()
    {
        NotiObject randomNoti = notiList.IndexOf(i);



    }

CodePudding user response：

As Dai has hinted, ArrayList is a particularly old thing, from back in the days when .net was relatively new and didn't have the incredibly useful feature known as generics.

The manual page for ArrayList says this (my emphasis):

Important We don't recommend that you use the ArrayList class for new development. Instead, we recommend that you use the generic List class.

Even the manufacturer is saying "don't use this product"

The big problem with ArrayList is that because it wants to be able to store anything, it holds its contents in an object array

This means you can put two completely unrelated things in next to each other, you have to inspect the type of them if you do, and you always have to cast to turn the object back into what you want

notiList.Add(123); //whoops, that's not a NotiObject

foreach(var o in notiList)
  var n = (NotiObject)notiList[0]; //whoops, that's a crash
}

So, working with it is pretty wearisome, particularly the part where you have to cast all the time.. This gets boring very quickly:

object o = "hello";
object p = "world";
object q = (string)o   (string)p;
object r = ((string)q).Substring(3).IndexOf((stribg)p);
r = (int)r   ((int)r)/2;

Storing everything in an object can be done, but look at what a mess it is. You'd have to start putting the type name into the variable name just to help remember that r was an int, and q was a string - Hungarian notation's another relic of the past.

When you put things in an ArrayList, this is what you're doing; storing them in object

So generics were invented and List was invented. A list that can be custom made to store a single type of objects like a string, int or NotiObject

var nums = new List<int>();
nums.Add(123); //works

var notiList = new List<NotiObject>();
notiList.Add(123); //compiler refuses this one

Now I've said all that, it's possible to answer your question. This code doesn't make sense:

NotiObject randomNoti = notiList.IndexOf(i);

i is an integer. IndexOf is a method that finds the numeric index of an item in the list. If the list was "a","b","c" and you asked for IndexOf("b") the result is 1 because b is at the second index, and indexing starts from 0.

IndexOf is not "get me the object at index blahblah", it's "tell me the index of this object blahblah"

The code doesn't make sense because you've passed an integer in and the list stores NotiObject. IndexOf will never find an integer in a list of NotiObject. This was the first mistake. You were allowed to make it because ArrayList stores everything as objects so you're allowed to pass an integer into IndexOf even if there are no integers in the list

IndexOf returns an integer. You cannot assign an integer to a variable of type NotiObject. This is the thing the compiler is complaining about

Even if you form the code correctly, you still have to cast:

NotiObject randomNoti = (NotiObject)notiList[i];

It's all very wearisome and if you persist with ArrayList probably not the last mistake you'll make with it either

If you used a List<NotiObject> you wouldn't have been allowed to pass an integer to IndexOf; the compiler would have stopped you which would hopefully then have made you assess IndexOf in the docs, and see that it's for finding the int index from the object, not the object at int index

You'd write code like:

List<NotiObject> notiList = new List<NotiList>();
...
NotiObject randomNoti = notiList[i];

without the cast. If you want to read more into why there is no cast, check out some introductory articles to generics. In a nutshell generics (any time you see something like <T> or <TBlahBlah>) allow you to specify something like a template code skeleton that the compiler uses to create code for you; code that substitutes the for the kind of object you want to work with. There isn't any casting any more because the compiler will write a whole List class that dedicatedly only works with NotiObjects