Before Windows 8, the method of adding a mailto: protocol handler was straightforward (as outlined here Register Windows program with the mailto protocol programmatically)
As of Windows 8, the old method no longer works. It would seem that Win8 enforces the following key: HKEY_CURRENT_USER\Software\Microsoft\Windows\Shell\Associations\URLAssociations\MAILTO\UserChoice.
It also appears the ProgID of the selected app is hashed and can't be forged, at least not that I can tell.
Does anyone have a working method for this, or can point me at a utility class/code that'll outline how to accomplish this programmatically?
For code, any language will do.
Edit
I've been asked from other discussions to specify a use-case, so I think it'd be helpful in the context of this question. Please consider this screenshot https://github.com/shellscape/Gmail-Notifier-Plus/raw/master/Promotional/prefs-account.png and the checkbox allowing the user to specify mailto handling. In this use-case, no one is forcing the user, the user is in control and makes the decision. While windows 8 store apps (metro/modern) have an available app manifest entry that automates the missing process described above, nothing seems to readily exist for desktop apps.
You can set your application to be activated by a custom protocol (like mailto:). When the user installs your app, if there is no other app supporting that protocol, they are not prompted and you are automatically assigned to that protocol.
If, however, the user already has an app that handles that protocol, then they will be prompted with a list of apps who support that protocol with the option to select the default. You cannot force the user to make a specific selection.
Also, if the user clicks on a protocol (like myprotocol:) and they have no app installed that handles that protocol then they will be sent to the store (app) which automatically searches for all apps that support that protocol. The user then installs whatever they want. You cannot force the user to make a specific selection (if any at all).
I wrote an article on protocol activation. It might be interesting to you: http://blog.jerrynixon.com/2012/10/walkthrough-using-windows-8-custom.html
So, I made Desktop Firefox my default mailto handler today in Windows 8 by adding the string value "mailto" to the HKCU\Software\Clients\StartMenuInternet\FIREFOX.EXE\Capabilities\URLAssociations and setting the value of "mailto" equal to the ProgID or "FirefoxURL". I then deleted the keys at HKEY_CURRENT_USER\Software\Microsoft\Windows\Shell\Associations\URLAssociations\MAILTO\UserChoice to allow me to choose the default client again and this time Firefox was available for me to choose.
The essence of this question seems to be that one cannot take over the default client for any protocol anymore (post Windows 8). The user must choose. However, if you wanted to break the OS convention you could hook the call to create the choose default dialog, which would take research, effort, and be only a temporary kludge and would require "breaking" the OS, or you could send a double click to the dialog to choose for the user, assuming your program has elevated rights so that it can send clicks to Admin windows. That would probably be the easiest way, the user would never know what happened, just a quick flash. Really though, after registering itself as a protocol handler, I don't think any program should go beyond deleting the default protocol handler registry entry, thereby forcing the user to re-choose.
You can't have your app directly take over file associations anymore in Windows 8. There are guidelines for how to handle this for both Windows Store and desktop applications here: http://msdn.microsoft.com/en-us/library/windows/apps/hh700321.aspx
This is how to set mailto protocol manually and simply in Windows 8, 8.1, 2012, 2012R2
Add a new registry branch HKEY_CURRENT_USER\Software\Microsoft\Windows\Shell\Associations\UrlAssociations\mailto\UserChoice
Then click any mailto: link in your web browser, say IE
and finally there in a program list for you to choose which was not available before.
Also MAILTO appears in Control Panel\All Control Panel Items\Default Programs now. There is no other option to add/remove a protocol from there.
来源：https://stackoverflow.com/questions/14238446/setting-mailto-protocol-handler-programmatically-in-windows-8