扩展方法和 generics - 我应该怎么做？

Question

I'm in the middle of overhauling some code and I hit a bit of a snag.

This is the method which I currently have, it needs reworking to support some structure changes:

/// <summary>
/// Recreates a dashboard control based off of its settings.
/// </summary>
/// <typeparam name="T"> The type of control to be recreated. </typeparam>
/// <param name="settings"> The known settings needed to recreate the control.</param>
/// <returns> The recreated control. </returns>
public static T Recreate<T>(ISetting<T> settings) where T : new()
{
    T _control = new T();
    settings.SetSettings(_control);
    Logger.DebugFormat("Recreated control {0}", (_control as Control).ID);
    return _control;
}

ISetting is being removed completely in favor of an extension method known to _control.

So, I have now:

public static class RadControlExtensions
{
    public static RadDockZoneSetting GetSettings(this RadDockZone dockZone)
    {
        RadDockZoneSetting radDockZoneSetting = new RadDockZoneSetting(dockZone.UniqueName, dockZone.ID, dockZone.Skin, dockZone.MinHeight, 
            dockZone.HighlightedCssClass, dockZone.BorderWidth, dockZone.Parent.ID);

        return radDockZoneSetting;
    }

    public static RadTabSetting GetSettings(this RadTab tab, int index)
    {
        RadTabSetting radTabSetting = new RadTabSetting(tab.Text, tab.Value, index);
        return radTabSetting;
    }

    //Continued
}

The control that is being recreated is guaranteed to have this extension method (would be nice to enforce this, though.)

I'm now at:

public static T Recreate<T>() where T : new()
{
    T _control = new T();
    //Not right -- you can't cast a control to an extension method, obviously, but
    //this captures the essence of what I would like to accomplish.
    (_control as RadControlExtension).SetSettings();
    Logger.DebugFormat("Recreated control {0}", (_control as Control).ID);
    return _control;
}

What should I be looking into to support this, if possible?

Answer 1

If you know that every _control that gets passed will be a RadDockZone (or derived from RadDockZone) just do this:

T _control = new T();
(RadDockZone)_control.SetSettings();
Logger.DebugFormat("Recreated control ... //rest of code here

If it's not always going to be a RadDockZone, you'll need to do some type checking to get the right type to call the extension method. I'm presuming, there, that you have a.SetSettings() extension method on all the possible Types that could be passed to your Recreate method.

Answer 2

You need to cast your T to something that is supported by your extension method.

(_control as RadDockZone).GetSettings

Extension methods operate on a type they are not a type in the traditional sense. The 'SomeFn(string this)' makes your extension work on things that are strings which would be strings and anything derived from them.

Answer 3

If I understand correctly what you are trying to do, just put a constraint on T :

public static T Recreate<T>() where T : RadControl, new() {
    // etc.
}

You might have to use double dispatch and define

public static RadControl GetSettings(this RadControl control) {

}

which will invoke the appropriate GetSettings method.

Answer 4

No, Jason's answer is the cleaner way to go. The accepted solution killed the type safety and made use of generics pointless. You could switch to generic-less design (having a RadControlFactory ) and had the job done.