Fix issue 14831: TaskDialogPage.GetBoundButtonByID throws IndexOutOfRangeException when TaskDialogIndirect returns unexpected button ID

src/System.Windows.Forms/System/Windows/Forms/Dialogs/TaskDialog/TaskDialog.cs

@@ -1132,7 +1132,11 @@ private HRESULT HandleTaskDialogCallback(
 
                 case TASKDIALOG_NOTIFICATIONS.TDN_RADIO_BUTTON_CLICKED:
                     int radioButtonID = (int)wParam;
-                    TaskDialogRadioButton radioButton = _boundPage.GetBoundRadioButtonByID(radioButtonID)!;
+                    TaskDialogRadioButton? radioButton = _boundPage.GetBoundRadioButtonByID(radioButtonID);
+                    if (radioButton is null)
+                    {
+                        break;
+                    }
 
                     checked
                     {

src/System.Windows.Forms/System/Windows/Forms/Dialogs/TaskDialog/TaskDialogPage.cs

@@ -717,7 +717,10 @@ internal void DenyIfWaitingForInitialization()
         // Check if the button is part of the custom buttons.
         if (buttonID >= CustomButtonStartID)
         {
-            return _boundCustomButtons![buttonID - CustomButtonStartID];
+            int customButtonIndex = buttonID - CustomButtonStartID;
+            return (uint)customButtonIndex < (uint)_boundCustomButtons!.Length
+                ? _boundCustomButtons[customButtonIndex]
+                : null;
         }
         else
         {
@@ -736,7 +739,15 @@ internal void DenyIfWaitingForInitialization()
             throw new InvalidOperationException();
         }
 
-        return buttonID == 0 ? null : _radioButtons[buttonID - RadioButtonStartID];
+        if (buttonID == 0)
+        {
+            return null;
+        }
+
+        int radioButtonIndex = buttonID - RadioButtonStartID;
+        return (uint)radioButtonIndex < (uint)_radioButtons.Count
+            ? _radioButtons[radioButtonIndex]
+            : null;
     }
 
     internal void Validate()

src/test/unit/System.Windows.Forms/System/Windows/Forms/TaskDialogTests.cs

@@ -4,6 +4,7 @@
 #nullable disable
 
 using Microsoft.DotNet.RemoteExecutor;
+using System.Reflection;
 
 namespace System.Windows.Forms.Tests;
 
@@ -58,4 +59,51 @@ public void TaskDialog_ShowDialog_SetProperty_DifferentThread_ThrowsInvalidOpera
         // verify the remote process succeeded
         Assert.Equal(RemoteExecutor.SuccessExitCode, invokerHandle.ExitCode);
     }
+
+    [WinFormsFact]
+    public void TaskDialogPage_GetBoundButtonByID_CustomRangeOutOfBounds_ReturnsNull()
+    {
+        TaskDialogPage page = new();
+        PrepareBoundLikeState(page);
+        dynamic access = page.TestAccessor.Dynamic;
+        access._boundCustomButtons = Array.Empty<TaskDialogButton>();
+        access._boundStandardButtonsByID = new Dictionary<int, TaskDialogButton>();
+
+        TaskDialogButton button = page.GetBoundButtonByID(buttonID: 100);
+
+        Assert.Null(button);
+    }
+
+    [WinFormsFact]
+    public void TaskDialogPage_GetBoundRadioButtonByID_OutOfBounds_ReturnsNull()
+    {
+        TaskDialogPage page = new();
+        PrepareBoundLikeState(page);
+
+        TaskDialogRadioButton radioButton = page.GetBoundRadioButtonByID(buttonID: 1);
+
+        Assert.Null(radioButton);
+    }
+
+    private static void PrepareBoundLikeState(TaskDialogPage page)
+    {
+        ConstructorInfo constructor = typeof(TaskDialog).GetConstructor(
+            BindingFlags.Instance | BindingFlags.NonPublic,
+            binder: null,
+            Type.EmptyTypes,
+            modifiers: null);
+        Assert.NotNull(constructor);
+
+        TaskDialog dialog = (TaskDialog)constructor.Invoke(null);
+        SetPrivateField(page, "<BoundDialog>k__BackingField", dialog);
+    }
+
+    private static void SetPrivateField(object instance, string fieldName, object value)
+    {
+        FieldInfo field = instance.GetType().GetField(
+            fieldName,
+            BindingFlags.Instance | BindingFlags.NonPublic);
+        Assert.NotNull(field);
+        field.SetValue(instance, value);
+    }
 }