How can I convert 'System.Windows.Input.Key' to 'System.Windows.Forms.Keys'?

Keys formsKey = ...;
Key wpfKey = ...;
wpfKey = KeyInterop.KeyFromVirtualKey((int)formsKey);
formsKey = (Keys)KeyInterop.VirtualKeyFromKey(wpfKey);

The KeyInterop class is the "key," plus the fact that the Windows Forms Keys enumeration has the same integer values as the Win 32 virtual key codes.

If you want to convert modifiers, use the SystemKey if you're looking at a KeyEventArgs:

System.Windows.Input.KeyEventArgs args;
System.Windows.Input.Key wpfKey= args.Key == Key.System ? args.SystemKey : args.Key;
formsKey = (System.Windows.Forms.Keys)KeyInterop.VirtualKeyFromKey(wpfKey);

Just in case people still encounter the modifier problem 7 years later, here's my solution that worked so far :

public static class KeyEventExts
{
    public static System.Windows.Forms.KeyEventArgs ToWinforms(this System.Windows.Input.KeyEventArgs keyEventArgs)
    {
        // So far this ternary remained pointless, might be useful in some very specific cases though
        var wpfKey = keyEventArgs.Key == System.Windows.Input.Key.System ? keyEventArgs.SystemKey : keyEventArgs.Key;
        var winformModifiers = keyEventArgs.KeyboardDevice.Modifiers.ToWinforms();
        var winformKeys = (System.Windows.Forms.Keys)System.Windows.Input.KeyInterop.VirtualKeyFromKey(wpfKey);
        return new System.Windows.Forms.KeyEventArgs(winformKeys | winformModifiers);
    }

    public static System.Windows.Forms.Keys ToWinforms(this System.Windows.Input.ModifierKeys modifier)
    {
        var retVal = System.Windows.Forms.Keys.None;
        if(modifier.HasFlag(System.Windows.Input.ModifierKeys.Alt))
        {
            retVal |= System.Windows.Forms.Keys.Alt;
        }
        if (modifier.HasFlag(System.Windows.Input.ModifierKeys.Control))
        {
            retVal |= System.Windows.Forms.Keys.Control;
        }
        if (modifier.HasFlag(System.Windows.Input.ModifierKeys.None))
        {
            // Pointless I know
            retVal |= System.Windows.Forms.Keys.None;
        }
        if (modifier.HasFlag(System.Windows.Input.ModifierKeys.Shift))
        {
            retVal |= System.Windows.Forms.Keys.Shift;
        }
        if (modifier.HasFlag(System.Windows.Input.ModifierKeys.Windows))
        {
            // Not supported lel
        }
        return retVal;
    }
}