Refactor Clipboard to make it testable

[zsd4yr]

Related to #416

Ideally we would refactor clipboard code so that all calls to win32 OLE* methods are exposed by a single class (OleClipboard) and published as a public interface and a constructor that takes object that implements that interface as a parameter. Then the test will mock the IOleClipboard interface and use constructor that takes this mock as a parameter.

[RussKie]

I'm happy to attempt to tackle this, but I'd like to get a better understanding how you envisage the refactor.

The current implementation of the Clipboard exposes a number of static methods, that pretty much bar the extensibility. How would the current Clipboard implementation make use of OleClipboard?
For example, how would the following implementation look:

winforms/src/System.Windows.Forms/src/System/Windows/Forms/Clipboard.cs

Lines 225 to 227 in d2b5e20

	public static void Clear() {
	Clipboard.SetDataObject(new DataObject());
	}

Suppose we create a class OleClipboard : IOleClipboard, that looks something like this:

public interface IClipboard // there may be other types provided by non-Windows platforms
{
    void SetDataObject(object data, bool copy, int retryTimes, int retryDelay);
}
internal sealed class OleClipboard : IClipboard
{
    // implementations
}

NB: we could add all helper methods like string GetText(), Image GetImage() etc to the interface as well, so each implementation could address its nuances and idiosyncrasies, and dumb down the Clipboard implementation.

Then we would need to open up the class and make the constructor public. We would also probably want to expose all static methods as instance methods, and to provide a backwards compatibility have some sort of a singleton instance that fulfils the statics.
E.g. something like

public sealed class Clipboard
{
    private readonly IClipboard _clipboard;

    public Clipboard(IClipboard clipboard) 
    {
        _clipboard = clipboard;
    }

    private static Clipboard Instance => new Clipboard(new OleClipboard);

    public void Clear() => _clipboard.SetDataObject(new DataObject()); // or _clipboard.Clear()
    public static void Clear() => Instance.Clear();
}

Is this something along your thinking, or am I wildly off the mark?

[merriemcgaw]

@dreddy-work can you take a look

[RussKie]

@AdamYoblick @Tanya-Solyanik would love to hear your thoughts on the proposal

[JeremyKuhne]

@RussKie I believe the designer has some abstractions with the clipboard that might be worth looking at.

[weltkante]

Also keep in mind that soonish the Clipboard/DataObject implementation needs to be replaced entirely due to phasing out BinaryFormatter as serialization system. If this means that API changes are required then it might be worth to already consider them here.