Unable to Save/SaveAs .xls and .xlt files using Microsoft.Office.Interop.Excel in VB.NET despite File Block Settings being unchecked in Trust Center

Question

Unable to Save/SaveAs .xls and .xlt files using Microsoft.Office.Interop.Excel in VB.NET despite File Block Settings being unchecked in Trust Center

Akbar Husain 10

I have a Windows Forms application developed in VB.NET. I am trying to open an Excel file, manipulate it by adding new sheets and data, and then save the file using Microsoft.Office.Interop.Excel.

All Excel file extensions are working as expected except .xls and .xlt. For these two extensions, the application throws the following error:

Error: Exception from HRESULT: 0x800A03EC

The error occurs on the following line of code:

wb.SaveAs(pstrDestFileName, Excel.XlFileFormat.xlExcel8)

Code

Sub Main()

    Dim xlApp As New Microsoft.Office.Interop.Excel.Application

    Dim wb As Microsoft.Office.Interop.Excel.Workbook

    Try

        xlApp = CreateObject("Excel.Application")

        xlApp.Interactive = False

        xlApp.DisplayAlerts = False

        xlApp.EnableEvents = False

        Dim pstrSrcFileName = "C:\Files\Book1.xls"

        Dim pstrDestFileName = "C:\Files\einfotree Copy of Book1.xls"

        wb = xlApp.Workbooks.Open(pstrSrcFileName, False, False, , "", "", , , , , , , False)

        wb.SaveAs(pstrDestFileName, Excel.XlFileFormat.xlExcel8)

        wb.Close(False)

        xlApp.Quit()

    Catch ex As Exception

       '

    End Try

End Sub

Note: Has there been any change in Microsoft Office versions or security settings that now restricts saving .xls or .xlt files using Interop?

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1 answer

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Answer 1

Killian N 1,140 Independent Advisor

Hi @Akbar Husain,

In this scenario, the 0x800A03EC error is typically raised by Excel itself declining the save operation, rather than an issue within the VB.NET code.

Since .xls and .xlt are legacy binary formats, many Microsoft 365 environments apply security restrictions through GPO, Intune, or Cloud Policy that prevent saving files in Excel 97–2003 formats. When these restrictions are in place, Excel Interop often reports only the generic error HRESULT: 0x800A03EC, without additional detail.

To help narrow this down, you may want to review the following:

In Excel > Options > Trust Center > File Block Settings, confirm that the Save checkbox (not just Open) is unchecked for "Excel 97–2003 workbooks and templates".
If your organization manages these settings centrally, be aware that policy enforcement can override any local Trust Center changes.
If you are saving an .xlt file, ensure the correct format is used:

Use XlFileFormat.xlTemplate8 for templates

Use XlFileFormat.xlExcel8 for workbooks (.xls)

As a simple validation step, you can try saving the file as .xlsx first. If that succeeds, it usually confirms that the automation itself is working correctly and that the issue is specific to legacy formats being blocked by policy.

For reference, you may find the following documentation helpful:

For a cleaner VB.NET approach, you could use something like the following:

Dim ext = IO.Path.GetExtension(pstrDestFileName).ToLowerInvariant()
 
Dim fmt As Excel.XlFileFormat =
    If(ext = ".xlt", Excel.XlFileFormat.xlTemplate8,
       Excel.XlFileFormat.xlExcel8)  ' .xls
wb.SaveAs(pstrDestFileName, fmt)

While this will not bypass any organizational policies, it does ensure the correct format is applied and avoids potential mismatches when saving .xlt files.

I hope these suggestions help move things in the right direction. Please let me know your results. If the issue persists, it would also be useful to know whether this is a work or school–managed device (Intune/GPO/security baseline) or a personal, unmanaged Microsoft 365 installation, as that distinction can affect the behavior significantly.

If you have any questions or need further assistance, please feel free to reach out. I’m happy to continue helping and look forward to your update.

If the answer is helpful, click on "Accept Answer" and please vote positively. If you have additional questions about this answer, click on "Comment".

Note: Follow the steps in our documentation to enable email notifications if you want to receive the email notification related to this thread.

Akbar Husain 10 Reputation points

2026-05-07T08:53:35.01+00:00

Hello Killian N,

Thanks for your reply.

In the File Block Settings, I have already unchecked all the checkboxes, but I am still unable to save or save as the file.

Additionally, when I try to save or save as the same file using a VBA macro, the operation works successfully. However, the issue occurs only when performing the operation through Interop in VB.NET.

In our organization, we do not have any Intune policies configured, and there are no known group policies or security policies restricting .xls or .xlt files from being saved or saved as.

Could you please let me know if there are any recent Microsoft Office updates, compatibility changes, or Interop-specific restrictions related to legacy Excel formats that might be causing this issue?
Killian N 1,140 Reputation points Independent Advisor

2026-05-07T09:56:48.9233333+00:00
Hi @Akbar Husain,

If the file saves correctly when using VBA but fails with error 0x800A03EC when using Interop, this typically indicates that Excel is waiting for a hidden prompt. In most cases, this is related to the Compatibility Checker or a similar dialog. Because the Interop session is running with Interactive = False and alerts are suppressed, the automation process is unable to respond to that prompt, causing the save operation to fail.

With that in mind, here are a few things you could try:

First, temporarily set Visible = True, Interactive = True, and DisplayAlerts = True, then run the same SaveAs operation again. If Excel displays a Compatibility Checker, loss of functionality, or conversion warning, that would confirm the root cause.

Next, you can disable compatibility checking before saving by setting wb.CheckCompatibility = False. Microsoft documents that this flag controls whether the compatibility checker runs automatically during save.

Additionally, please ensure that the correct FileFormat is being used for the target extension. For example, .xls should use xlExcel8, and .xlt should use xlTemplate8.

Finally, consider using the more explicit SaveAs overload that includes parameters such as AccessMode, ConflictResolution, and Local. These options are part of the Interop SaveAs signature and can make the save operation more predictable in automation scenarios.

Regarding recent Office updates, I’m not aware of any change that would block .xls or .xlt files specifically for Interop while still allowing them through VBA. When legacy formats are restricted, this is usually enforced through File Block settings or policies, which affect Excel as a whole rather than Interop alone. Microsoft also notes that these restrictions are often policy-driven.

I hope this helps clarify the issue.

Wishing you a great day ahead.
Akbar Husain 10 Reputation points

2026-05-07T10:23:26.13+00:00
Hello @Killian N

This is a great option to check. I applied the properties you suggested and tried the SaveAs operation again, but I identified a very unusual behavior.

Before executing the program, I manually checked the File Block Settings in Excel, and all the related checkboxes were unchecked and enabled, as shown in the below image.
After that, I executed my VB.NET Interop program. The file opened successfully, but during the SaveAs operation it threw the following error:

"You are attempting to save a file type (Excel 97-2003 Workbooks and Templates) that has been blocked by your File Block Settings in the Trust Center."

After clicking OK on the message, I rechecked the File Block Settings and noticed that all the related options had become disabled, as shown in the below image.

However, after closing all Excel instances and reopening Excel again, the File Block Settings returned to normal, where the checkboxes were again unchecked and enabled.

So it appears that while the Interop application is running, Excel is temporarily treating the Excel 97-2003 formats as blocked, even though no policy or Trust Center restriction is configured permanently.

Could this behavior be related to:

Protected View,

Office security hardening,

COM automation restrictions,

registry-based temporary policy enforcement,

or changes introduced in recent Microsoft Office updates?

Thanks,

Akbar
Killian N 1,140 Reputation points Independent Advisor

2026-05-07T13:55:47.3+00:00
Hi @Akbar Husain,

Thank you for the detailed follow‑up and for sharing your observations, that behavior is indeed unusual, and you’ve done an excellent job narrowing it down.

Based on what you described, this does not appear to be a permanent Trust Center or policy configuration issue. Instead, it strongly suggests that Excel is applying a temporary, session‑scoped security state when launched through COM automation (Interop). In other words, while Excel is started programmatically, it behaves differently than when launched interactively by a user, even though the same Trust Center settings remain unchanged on disk.

A few important points worth noting:

Excel Interop launches Excel in a non‑standard automation context, which can trigger stricter handling of legacy binary formats such as .xls and .xlt.

This behavior is runtime‑only and scoped to the lifetime of the Interop session, which aligns with your finding that the File Block Settings revert to normal once all Excel instances are closed.

Because VBA runs inside an already trusted Excel instance, it does not hit the same automation boundary or security evaluation that Interop does.

The resulting File Block dialog appears to be internally enforced by Excel, not written back to registry or policy, which explains why no permanent setting changes are visible.

At this point, this goes beyond standard Trust Center configuration and falls into Excel COM automation internals, legacy format handling, and possibly recent Office security hardening changes. Unfortunately, Microsoft does not publicly document these automation‑specific behaviors in detail, and Interop often surfaces only the generic 0x800A03EC error when Excel itself declines an operation.

Because this is now clearly a deep Excel / VBA / Interop behavior, you may get much stronger technical insight by posting this scenario to a specialized forum such as Stack Overflow (tags: excel-interop, vb.net, excel-vba, office-interop).

This community tends to have contributors who work extensively with Excel’s COM object model and can investigate undocumented edge cases like this one, including any subtle changes introduced in newer Microsoft 365 builds.

This link is shared by community members for your convenience. It points to a third-party site that is not managed or verified by Microsoft. I can’t guarantee the quality, safety, or suitability of any content or software found there. Please review carefully and make sure you understand any potential risks before using it.

From a practical standpoint, many teams encountering this behavior choose one of the following workarounds:

Save using .xlsx and convert only when absolutely required

Perform the legacy .xls save via a VBA macro executed inside Excel

Avoid .xls/.xlt formats entirely where possible, as they are increasingly restricted in modern Office environments

I wish I could provide a more definitive answer here, but given the nature of this issue, a VBA / Excel automation‑specialized forum will likely be the best path forward.

Thank you again for the clear diagnostics and for sharing your findings. I genuinely hope the broader Excel developer community can help you get to the bottom of this.

Kind regards,
Akbar Husain 10 Reputation points

2026-05-08T04:21:39.2366667+00:00

Hi @Killian N

Thanks a lot for this detailed explanation of the issue and for providing us with a roadmap for the next steps. This will greatly help us identify and reach the root cause of the issue.

Once again, thank you very much for your support and guidance.

Thanks,

[Moderator note: Personally Identifiable Information removed]
Killian N 1,140 Reputation points Independent Advisor

2026-05-12T04:01:29.1866667+00:00

Hi @Akbar Husain,

Thank you very much for your kind and thoughtful message. I truly appreciate you taking the time to share your feedback, and I’m glad to hear that the explanation and proposed next steps were helpful in moving you closer to identifying the root cause of the issue.

It’s always my goal to provide clear guidance and meaningful support, so your confirmation means a great deal to me.

If you feel that the information provided has addressed your questions, I would sincerely appreciate it if you could mark the response as Accepted Answer in the forum. Doing so will help other community members who may be facing a similar issue more easily find relevant guidance and solutions.

Once again, thank you for your collaboration and trust. Please don’t hesitate to reach out if you have any further questions or need additional assistance.

Warm regards,

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Unable to Save/SaveAs .xls and .xlt files using Microsoft.Office.Interop.Excel in VB.NET despite File Block Settings being unchecked in Trust Center

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