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

[SimonZhao888]

Fixes #14831

Root Cause

TaskDialogPage.GetBoundButtonByID and GetBoundRadioButtonByID map the ID returned by the native API directly to an array index without performing bounds checking.
If TaskDialogIndirect returns an ID outside the current page configuration during an abnormal closure (e.g., an ID in the "custom" range when no custom buttons are defined), an IndexOutOfRangeException is triggered, preventing the intended fallback logic from executing.

Proposed changes

• Add bounds checking for the custom-button index in GetBoundButtonByID; return null if the index is out of bounds.
• Add bounds checking for the radio-button index in GetBoundRadioButtonByID; return null if the index is out of bounds.
• Handle the null return value in the TaskDialog TDN_RADIO_BUTTON_CLICKED callback to prevent potential dereferencing issues.
• Add unit tests to cover the scenario where an out-of-bounds ID returns null instead of throwing an exception.

Customer Impact

• Eliminated the risk of process crashes (IndexOutOfRangeException) in this code path.
• Improved the robustness of TaskDialog when encountering exceptional or unexpected native return values, ensuring it correctly enters existing fault-tolerance branches.
• Maintained the behavior of standard execution paths and public APIs, focusing solely on enhancing reliability and fault tolerance.

Regression?

• No

Risk

• Mini

Screenshots

Before

2026-08-04.142359.mp4

After

2026-08-04.142529.mp4

Test methodology

• Manually
• Automated test cases

Test environment(s)

• 11.0.0-preview.7.26381.103

Test Project

TaskDialogIssue14831.zip

Microsoft Reviewers: Open in CodeFlow