Combo box not showing full Unicode text when an item is selected

Question

Combo box not showing full Unicode text when an item is selected

David H 0

Issue Description

When using an ANSI application that uses the UTF-8 code page, and items in a ComboBox list contain non-English text (i.e. include characters that span multiple bytes), selecting those items results in the text being truncated in the selection field. This truncation also occurs when setting text in the ComboBox using the SetWindowTextA function.

example

Here is a sample program that demonstrates this issue for your reference. Compile it as 32 bit (x86) mode.

Upon further investigation and debugging into the assembly code, it appears that Windows is truncating the text before setting it in the Edit control.

Once this truncated text is present, calling the GetWindowTextLengthA function retrieves the length of the truncated text. However, there are instances where, after pasting in non-English text (like the testString2 in the sample program), the GetWindowTextLengthA function returns a value that is less than the actual number of bytes (or code units), but more than the number of actual characters (or code points). Additionally, the size returned by GetWindowTextA sometimes exceeds the value returned by GetWindowTextLengthA. This discrepancy poses a potential buffer overrun risk if GetWindowTextA attempts to write more bytes than the caller allocated.

Versions Tested

Windows 10 Pro 22H2
Windows 11 Pro 22H2

Workarounds

Avoid subclassing the controls. Although Microsoft's own documentation says to subclass the edit control if you want to customize behaviour of a combo box.
Directly set the Edit control's text using the SetWindowTextA function. One approach could be to handle the CBN_SELCHANGE notification, retrieve the string associated with the selected list item, and then set the Edit control to display that text.

Jeanine Zhang-MSFT 11,356 Reputation points Microsoft External Staff

2023-09-04T07:27:48.9733333+00:00

According to our policy, there are some websites that we cannot access. Could you please share your sample via github? And please check the link of the documentation.
David Lowndes 2,640 Reputation points MVP

2023-09-04T07:58:19.94+00:00

Your code specifically uses ANSI APIs, for example, using CreateWindowExA.
Why are you hoping that an ANSI window would work with Unicode texts?
David H 0 Reputation points

2023-09-05T03:51:22.3+00:00

@Jeanine Zhang-MSFT , I uploaded the code to GitHub and changed the link in the issue description.

@David Lowndes , sorry I forgot to mention that it is using the new UTF-8 code page introduced in Windows 10. I have updated the issue description.
David Lowndes 2,640 Reputation points MVP

2023-09-05T11:34:02.6+00:00

Ah, so you're using the beta facility "Use Unicode UTF-8 for worldwide language support" to have the ANSI APIs work with UTF8? Since it's "beta", you've possibly encountered some aspect that's not been converted to handle it.
RLWA32 49,636 Reputation points

2023-09-05T15:44:15.3266667+00:00

@David H, With respect to subclassing the edit control if you use the SetWindowSubclass function instead of old style subclassing then text is not truncated.

And, notwithstanding the size values returned by GetWindowTextLengthA and GetWindowTextA the debug CRT did not detect heap corruption when the non-English strings were returned.

Finally, adding a call to MessageBoxA to display the non-English strings returned by GetWindowTextA seemed to display the expected data.
David H 0 Reputation points

2023-09-06T00:19:56.6066667+00:00

Thanks @RLWA32 , I can confirm that SetWindowSubclass works correctly in my test app. Unfortunately, it may not be so easy to use that in my real application as it requires two additional parameters to the window procedure and I do not have the ability to change that in my application.

3 answers

Your answer

Jeanine Zhang-MSFT 11,356 Reputation points Microsoft External Staff

2023-09-04T07:27:48.9733333+00:00

According to our policy, there are some websites that we cannot access. Could you please share your sample via github? And please check the link of the documentation.
David Lowndes 2,640 Reputation points MVP

2023-09-04T07:58:19.94+00:00

Your code specifically uses ANSI APIs, for example, using CreateWindowExA.
Why are you hoping that an ANSI window would work with Unicode texts?
David H 0 Reputation points

2023-09-05T03:51:22.3+00:00

@Jeanine Zhang-MSFT , I uploaded the code to GitHub and changed the link in the issue description.

@David Lowndes , sorry I forgot to mention that it is using the new UTF-8 code page introduced in Windows 10. I have updated the issue description.
David Lowndes 2,640 Reputation points MVP

2023-09-05T11:34:02.6+00:00

Ah, so you're using the beta facility "Use Unicode UTF-8 for worldwide language support" to have the ANSI APIs work with UTF8? Since it's "beta", you've possibly encountered some aspect that's not been converted to handle it.
RLWA32 49,636 Reputation points

2023-09-05T15:44:15.3266667+00:00

@David H, With respect to subclassing the edit control if you use the SetWindowSubclass function instead of old style subclassing then text is not truncated.

And, notwithstanding the size values returned by GetWindowTextLengthA and GetWindowTextA the debug CRT did not detect heap corruption when the non-English strings were returned.

Finally, adding a call to MessageBoxA to display the non-English strings returned by GetWindowTextA seemed to display the expected data.
David H 0 Reputation points

2023-09-06T00:19:56.6066667+00:00

Thanks @RLWA32 , I can confirm that SetWindowSubclass works correctly in my test app. Unfortunately, it may not be so easy to use that in my real application as it requires two additional parameters to the window procedure and I do not have the ability to change that in my application.

Answer 1

Castorix31 90,686

Use Unicode (Use Unicode Character Set in Properties) and it will work (remove all ANSI APIs and replace char by WCHAR (and add L" for strings)) :

User's image

David H 0 Reputation points

2023-09-05T03:52:39.13+00:00

That will work, but I cannot change my application to use WCHAR.
Castorix31 90,686 Reputation points

2023-09-05T07:19:59.59+00:00

I cannot change my application to use WCHAR.

Why ?

I replaced them in your sample in ~10 seconds and it worked...

Answer 2

Jeanine Zhang-MSFT 11,356 Microsoft External Staff

Hello,

Welcome to Microsoft Q&A!

I agree with the communities. You should use Unicode versions instead of ANSI versions.

Refer to the Doc: Working with Strings

"but I cannot change my application to use WCHAR."

I suggest you could try to use the MultiByteToWideChar function converts an ANSI string to a Unicode string.

You could refer to the Doc: Converting Unicode and ANSI Strings

Thank you.

Jeanine

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David H 0 Reputation points

2023-09-05T23:38:46.36+00:00

Hi Jeanine,

Converting to wide char is not possible in this case since it's an ANSI control (created with CreateWindowA, using an ANSI window procedure). It is not possible (or rather, very difficult) for me to change my the window procedure my application uses.

The problem has been demonstrated as happening within the Windows code, outside of my control.
Jeanine Zhang-MSFT 11,356 Reputation points Microsoft External Staff

2023-09-06T09:04:20.0166667+00:00

It seems to be an issue in Explorer that prevents mixed ANSI/Unicode subclassing from working properly. I suggest you should try to use SetWindowLongW instead of "SetWindowLongA" and use CallWindowProcW instesd of CallWindowProcA.
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Jeanine Zhang-MSFT 11,356 Reputation points Microsoft External Staff

2023-09-15T03:22:05.4333333+00:00

Have you got any updates about this issue? Could you please check my answer?

Answer 3

RLWA32 49,636

@David H, Since I was able to reproduce the truncated text issue with your demo when using old style subclassing it seems to me that is the core problem. Without subclassing or subclassing with SetWindowSubclass the demo seems to function as intended. I did not experience heap corruption due to buffer overruns with the demo.

Since this does not seem to be a Visual Studio or C++ issue to me I suggest you consider using the Feedback Hub to report this Windows issue to Microsoft.

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Combo box not showing full Unicode text when an item is selected

3 answers

Your answer