Cast Generic to Generic

Question

I have a base WPF UserControl that handles some common functionality for derived UserControls. In the code-behind of any derived UserControl I call an event

private void SomeClick(object sender, RoutedEventArgs e) {
    HandleClick(sender);
    MyDataGrid.Items.Refresh();
}

In my base UserControl I do

public class BaseUserControl : UserControl {
   protected void HandleClick(object sender) {
       var vm = (BaseViewModel<Part>)DataContext;
       ...
   }
}

This throws an InvalidCastException since DataContext is of type BaseViewModel but of a derived type like BaseViewModel<Wire> or BaseViewModel<Connector>.

How can I cast that?

What does your generic class look like and what is T declared as? Related: https://msdn.microsoft.com/en-US/library/dd799517(v=vs.110).aspx — nvoigt, Dec 16 '16 at 07:24
You can't - a `BaseViewModel` simply *isn't* a `BaseViewModel`. Assuming `BaseViewModel` is a class, you can't make this work. If it were an interface, you could use covariance. — Jon Skeet, Dec 16 '16 at 07:25
Just because `A` and `B` have a particular inheritance relationship does *not* imply that `G` and `G` have the *same* inheritance relationship. — Damien_The_Unbeliever, Dec 16 '16 at 07:58

score 32 · Accepted Answer · answered Dec 16 '16 at 07:53

32

You cannot cast a Generic<Derived> to a Generic<Base>.

Just imagine if you could. You have a List<Wolf> and cast it to a List<Animal>. Now you could .Add() a Sheep to your List<Animal>. But wait... now your List<Wolf> contains a Sheep. What a mess.

This would only work out if you could make sure that the thing you cast to is read-only in all possible forms. This is was co- and contravariance is all about. It only works for interfaces though.

answered Dec 16 '16 at 07:53

nvoigt

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5

As an example where covariance would work, consider the interface `IReadOnlyList`. Here if you have an `IReadOnlyList` it would convert _implicitly_ to `IReadOnlyList`. The `out` signifies covariance (`in` would be contravariance) in this context. It is correct that `class` types cannot be co- and contravariant like this (as of the current version). Co- and contravariance of this type is only supported for `interface` and `delegate` types. – Jeppe Stig Nielsen Dec 16 '16 at 08:01
1

List of wolfs with sheeps in it, best explanation ever :) – Matthias Auswöger Aug 28 '18 at 11:24
3

Sorry, I disagree with the "mess"... You wouldn't have a list of wolves with sheep in it, you'd have a list of _animals_ containing both wolves and sheep. Which seems perfectly reasonable. – Basic Dec 09 '19 at 02:17
4

@Basic Casting from a `List` to a `List` would not create a new list neither would it change the type of the original variable or instance. That is not what casting does. You still have only one list, originally declared as `List` and it would have a sheep in it. – nvoigt Dec 09 '19 at 05:42

Cast Generic to Generic

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