Int wont work

Why are you trying to convert a string to an int just so you can reconvert it to a string to write it on the console?

You may also note here[^] that SelectedItems returns a collection and not a single string.

Abhinav is right...which is why I marked his answer as a 5, but I wanted to elaborate a bit more on what he said and provide (hopefully) some helpful tips for the future.

When you get an error like this, the best thing to do is go through your code step by step in the debugger. If you are using Visual Studio, use your Watch window. If something doesn't work the way you expected, put each of the variables into the watch window and see the results.

In your case, if you had done this, you could have started by putting listBox1.SelectedItems.ToString() in the watch window. If you had done that, you would have seen that it equaled something like:

System.Windows.Forms.ListBox+SelectedObjectCollection

That's probably not what you were expecting. Then, when you used the Parse function, it would have thrown an error because it is not a numeric value.

My other suggestion is only use the Parse methods if you are assured that it will work. Otherwise, use TryParse instead.

Console.WriteLine();Takes String i think if u write like this it will work
Console.WriteLine(listBox1.SelectedItems.ToString());