[DialogResult] Add TryAgain and Continue (result 10, and 11) respectively.

Is your feature request related to a problem? Please describe.

Nope, I am filing this because I noticed that the documentations here: https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-messagebox As such I noticed that DialogResults of 10, and 11 are possible.

Describe the solution you’d like and alternatives you’ve considered

Currently I have no other alternatives in mind other than adding the 2 members to DialogResult. Also might need to add MB_CANCELTRYCONTINUE to the button options too for Cancel, Try Again, and Continue buttons (I can file a separate issue for this one). It also seems to miss on the default buttons to also be able to make button 4 the default if they wanted to as well (I can file a separate issue for this one as well). Nowever I would logically only make it available in .NET 6+ if approved.

Will this feature affect UI controls?

I do not think it will affect them at all, it will instead allow one to handle if one pressed the continue button on a dialog or try again to do specific things for those specific results which could be seen as an enhancement.

API Changes:

namespace System.Windows.Forms
{
    public enum DialogResult
    {
-        OK = 1,
+        OK = (int)ID.OK,
        // docs stay the same.
-        Cancel = 2,
+        Cancel = (int)ID.CANCEL,
        // docs stay the same.
-        Abort = 3,
+        Abort = (int)ID.ABORT,
        // docs stay the same.
-        Retry = 4,
+        Retry = (int)ID.RETRY,
        // docs stay the same.
-        Ignore = 5,
+        Ignore = (int)ID.IGNORE,
        // docs stay the same.
-        Yes = 6,
+        Yes = (int)ID.YES,
        // docs stay the same.
-        No = 7,
+        No = (int)ID.NO,
+
+        /// <summary>
+        /// The dialog box return value is Try Again (usually sent from a button labeled Try Again).
+        /// </summary>
+        TryAgain = (int)ID.TRYAGAIN,
+
+        /// <summary>
+        /// The dialog box return value is Continue (usually sent from a button labeled Continue).
+        /// </summary>
+        Continue = (int)ID.CONTINUE,
    }
    public enum MessageBoxButtons
    {
+        /// <summary>
+        ///  Specifies that the message box contains Cancel, Try Again, and Continue buttons.
+        ///  This field is constant.
+        /// </summary>
+        CancelTryContinue = (int)MB.CANCELTRYCONTINUE,
    }
    public enum MessageBoxDefaultButton
    {
+        /// <summary>
+        ///  Specifies that the forth button on the message box should be the
+        ///  default button.
+        /// </summary>
+        Button4 = (int)MB.DEFBUTTON4,
    }
}

As for MessageBox class this code here: https://github.com/dotnet/winforms/blob/f337d18dfcf05f9764a90895358928d6c342c4f2/src/System.Windows.Forms/src/System/Windows/Forms/MessageBox.cs#L413-L427

can be replaced with:

      try
      {
          ID mbresult = MessageBoxW(new HandleRef(owner, handle), text, caption, style);
+         return mbresult switch
+         {
+             _ => (DialogResult)mbresult,
+         };
      }
      finally
      {
          Application.EndModalMessageLoop();
          ThemingScope.Deactivate(userCookie);
          // Right after the dialog box is closed, Windows sends WM_SETFOCUS back to the previously active control
          // but since we have disabled this thread main window the message is lost. So we have to send it again after
          // we enable the main window.
          User32.SendMessageW(new HandleRef(owner, handle), User32.WM.SETFOCUS);
      }

And this could be attempted on removal: https://github.com/dotnet/winforms/blob/f337d18dfcf05f9764a90895358928d6c342c4f2/src/System.Windows.Forms/src/System/Windows/Forms/MessageBox.cs#L35-L56

Edit: Shown complete changes to the enums, The changes to the MessageBox class that would be needed for this is a WIP.

[AraHaan]

opened #4746.

[RussKie]

@terrajobst no, false alarm. More details here https://github.com/dotnet/winforms/pull/4746#issuecomment-819997698