Register Windows program with the mailto protocol programmatically

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@Dillie-O: Your answer put me in the right direction (I should have expected it to just be a registry change) and I got this working. But I’m going to mark this as the answer because I’m going to put some additional information that I found while working on this.

The solution to this question really doesn’t depend on what programming language you’re using, as long as there’s some way to modify Windows registry settings.

Finally, here’s the answer:

  • To associate a program with the mailto protocol for all users on a computer, change the HKEY_CLASSES_ROOT\mailto\shell\open\command Default value to:
    “Your program’s executable” “%1”
  • To associate a program with the mailto protocol for the current user, change the HKEY_CURRENT_USER\Software\Classes\mailto\shell\open\command Default value to:
    “Your program’s executable” “%1”

The %1 will be replaced with the entire mailto URL. For example, given the link:

<a href="mailto:[email protected]">Email me</a>

The following will be executed:
“Your program’s executable” “mailto:[email protected]”

Update (via comment by shellscape):
As of Windows 8, this method no longer works as expected. Win8 enforces the following key: HKEY_CURRENT_USER\Software\Microsoft\Windows\Shell\Associati‌​ons\URLAssociations\‌​MAILTO\UserChoice for which the ProgID of the selected app is hashed and can’t be forged. It’s a royal PITA.

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