fix: fixed #1237.

src/Shared/HandyControl_Shared/Controls/Window/MessageBox.cs

@@ -470,6 +470,7 @@ public sealed class MessageBox : Window
         switch (messageBoxButton)
         {
             case MessageBoxButton.OK:
+                messageBox._messageBoxResult = MessageBoxResult.Yes;
                 messageBox._showOk = true;
                 messageBox._buttonOk = new Button
                 {
@@ -482,6 +483,7 @@ public sealed class MessageBox : Window
 
                 break;
             case MessageBoxButton.OKCancel:
+                messageBox._messageBoxResult = MessageBoxResult.Cancel;
                 messageBox._showOk = true;
                 messageBox._buttonOk = new Button
                 {
@@ -512,6 +514,7 @@ public sealed class MessageBox : Window
 
                 break;
             case MessageBoxButton.YesNo:
+                messageBox._messageBoxResult = MessageBoxResult.Cancel;
                 messageBox._showYes = true;
                 messageBox._buttonYes = new Button
                 {
@@ -541,6 +544,7 @@ public sealed class MessageBox : Window
 
                 break;
             case MessageBoxButton.YesNoCancel:
+                messageBox._messageBoxResult = MessageBoxResult.Cancel;
                 messageBox._showYes = true;
                 messageBox._buttonYes = new Button
                 {