The error code 0x000005A5 translates to ERROR_NON_MDICHILD_WINDOW. This error occurs in Windows programs specifically when you use functions or operations related to Multiple Document Interface (MDI) child windows on a window handle that isn’t an MDI child window.

Understanding MDI Windows:

• MDI (Multiple Document Interface) is a UI concept for managing multiple documents within a single parent window frame.
• MDI child windows represent the individual documents displayed within the MDI parent window.

Causes of ERROR_NON_MDICHILD_WINDOW:

• Incorrect Window Handle: The most common cause is trying to use functions like MDIIsWindow or SendMessage with specific MDI child window message flags (e.g., WM_MDIACTIVATE) on a window handle that doesn’t belong to an MDI child window.

• Code Errors: Mistakes in how you’re identifying or managing window handles within your MDI application can lead to this error.

Troubleshooting Steps:

1. Verify MDI Window Type: Double-check if the window handle you’re using actually refers to an MDI child window within your application’s MDI setup.

2. Review Function Requirements: Ensure the function you’re using (e.g., MDIIsWindow) is designed to work specifically with MDI child windows. Some functions might have limitations regarding the window type.

3. Debug Window Handle: Use debugging tools to inspect the window handle you’re passing to the function. Verify if it’s a top-level window or a window from a different part of your application’s UI that isn’t an MDI child window.

4. Identify MDI Parent (if applicable): If you need to interact with a specific MDI child window, consider obtaining the handle of its MDI parent window and then using MDI-specific functions to enumerate or interact with its child windows.

5. Alternative Approaches: Depending on your goal, explore alternative approaches that might not require treating the window as an MDI child window. This could involve using functions designed for top-level windows or interacting with the window differently based on its type.

Additional Tips:

• Use meaningful variable names to differentiate between window handles, especially when working with different window types within an MDI application.
• Consider using higher-level UI libraries or frameworks (if available) that provide abstractions for MDI management, potentially simplifying identification and interaction with MDI child windows.

By following these steps and understanding the reasons behind the ERROR_NON_MDICHILD_WINDOW error, you should be able to identify the issue with your window handle and fix your code to use MDI functions appropriately on MDI child windows or explore alternative approaches for non-MDI child windows.