Method that accepts only classes that implement a specific interface

Question

I have the following method

    private void PushToMainWindow(UserControl child) // where child is IMainWindowPlugin
    {
        _navigationStack.Push((IMainWindowPlugin)child);
        ...
        // Here I do stuff that makes use of the child as a usercontrol
        child.Width = 500;
        child.Margin = new Thickness(0,0,0,0);
        ...
    }

What I would like to do is inform the compiler that I'll only be accepting UserControl objects that also implement the IMainWindowPlugin interface.

I know I can do a if statement and throw or cast and check for null, but those are both run-time solutions, and I'm looking for a way to tell the developer up front that there is a restriction on the type of UserControl he can add. Is there a way to say this in c# ?

UPDATE: Added more code to show that the usercontrol is used as a usercontrol, so I can't just pass the child as the interface.

If IMainWindowPlugin is your interface you can create base class that inherits from UserControl and have empty implementation for this interface. Also check this one: http://stackoverflow.com/questions/178333/multiple-inheritance-in-c-sharp — Amittai Shapira, Sep 03 '15 at 08:27

jjczopek · Accepted Answer · 2015-09-03T08:47:16.243

3

Have you consider generics? Something like this should work:

    private void PushToMainWindow<T>(T child) where T: UserControl, IMainWindowPlugin
    {
        var windowPlugin = child as IMainWindowPlugin;
        _navigationStack.Push(windowPlugin);
        ...
        // Here I do stuff that makes use of the child as a usercontrol
        child.Width = 500;
        child.Margin = new Thickness(0,0,0,0);
        ...
    }

Compiler will not allow for passing to the PushToMainWindow() method objects that are not meeting the where clause, which means that the class you are passing has to be UserControl (or derived) and implement IMainWindowPlugin.

Other thing is, that probably passing the interface itself is better idea, instead of basing on concrete implementation.

edited Sep 03 '15 at 08:47

answered Sep 03 '15 at 08:39

jjczopek

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this really looks like a good solution, with the system automatically completing T – user230910 Sep 03 '15 at 08:51
Generics are very useful. Refer to MSDN to dig into the topic: https://msdn.microsoft.com/en-us/library/512aeb7t.aspx?f=255&MSPPError=-2147217396 – jjczopek Sep 03 '15 at 09:25
Thank you, I have implemented this solution, and its working 100% – user230910 Sep 03 '15 at 10:41

score 0 · Answer 2 · answered Sep 03 '15 at 08:21

0

Why not

void PushToMainWindow(IMainWindowPlugin child) { ... }

answered Sep 03 '15 at 08:21

Richard Schneider

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ok, that would make sense, let me add more detail to the question – user230910 Sep 03 '15 at 08:22

score 0 · Answer 3 · answered Sep 03 '15 at 08:29

0

 private void PushToMainWindow(IMainWindowPlugin child) 
    {
        _navigationStack.Push(child);
        var newChild=(UserControl )child
        newChild.Width = 500;
        newChild.Margin = new Thickness(0,0,0,0);
    }

answered Sep 03 '15 at 08:29

Arash

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It just inverts the problem, I would prefer that both conditions are met, thanks tho! – user230910 Sep 03 '15 at 08:52

Method that accepts only classes that implement a specific interface

3 Answers3