The behavior of the FileLoggingSession is different than described.

[krzysztofFC]

Issue Observed:

• Despite the documentation indicating that "The FileLoggingSession class sends logged messages to disk files as they are logged," I cannot see any logs added to the file once the app is running, but the logs appear to be saved only after the application exits. This observation raises questions:

Questions:

• Is the behavior observed (logs saved only upon application exit) an inconsistency with the documentation, or is it the expected behavior and the documentation might be misleading or misinterpreted?

• Is there a way to send the log to the file once it is logged?

Here is a minimal working demo code to illustrate the problem:

#include "pch.h"
#include <iostream>
#include <winrt/Windows.Foundation.h>
#include <winrt/Windows.Foundation.Diagnostics.h>
#include <chrono>
#include <thread>
#include <conio.h>

using namespace winrt;
using namespace Windows::Foundation;
using namespace Windows::Foundation::Diagnostics;

/*
This application creates a FileLoggingSession and a LoggingChannel to log messages
to an ETL file. The Windows.Foundation.Diagnostics documentation states that the
FileLoggingSession class automatically saves logged messages to disk files as they
are logged.

The log files are supposed to be stored in the application's local folder, which is
typically located at:
\AppData\Local\Packages\[PackageFamilyName]\LocalState\Logs

What the application does:
1. Initializes a WinRT apartment.
2. Creates a FileLoggingSession named "MySession".
3. Creates a LoggingChannel named "MyChannel" and adds it to the session.
4. Enters a loop that logs messages every second, appending a counter to indicate the message number.
5. If a key is pressed, the loop exits, stops logging, and triggers the saving of the log file.
6. According to the documentation, the log file should be saved immediately as per the FileLoggingSession's behavior.
*/

int main()
{
    init_apartment();

    // Create a new FileLoggingSession instance.
    FileLoggingSession session(L"MySession");

    // Add a logging channel to the session.
    LoggingChannel channel(L"MyChannel");
    session.AddLoggingChannel(channel);

    // Counter for log messages.
    int counter = 0;

    std::cout << "Logging started. Press any key to stop..." << std::endl;

    // Start a loop to generate logs.
    while (true)
    {
        if (_kbhit()) // Check if a key has been pressed.
        {
            _getch(); // Consume the key press.
            break;    // Exit the loop.
        }

        // Log a message with a counter.
        channel.LogMessage(L"Log message " + std::to_wstring(counter++));

        // Wait for 1 second.
        std::this_thread::sleep_for(std::chrono::seconds(1));
    }

    std::cout << "Stopping logging session. Please wait..." << std::endl;

    // Handle the async operation to save the log file.
    auto saveOperation = session.CloseAndSaveToFileAsync();
    saveOperation.get(); // Wait for the async operation to complete.

    std::cout << "Logging session closed and file saved." << std::endl;

    return 0;
}

Kind regards, Krzysztof

---

Document Details

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• ID: 0f5e03ad-d932-d83c-58e6-359dfe5ec5f4
• Version Independent ID: cb6b60a2-2b3c-89bb-fd69-72c755e719c3
• Content: FileLoggingSession Class (Windows.Foundation.Diagnostics) - Windows UWP applications
• Content Source: winrt-api-build/xml/Windows.Foundation.Diagnostics/FileLoggingSession.xml
• Product: uwp
• Technology: winrt-reference
• GitHub Login: @alvinashcraft
• Microsoft Alias: aashcraft

[alvinashcraft]

Hi @krzysztofFC. Thanks for reaching out about this API. I'll try to find someone on the product team who can explain the observed behavior? Have you tried asking about this on the Q&A forums? The folks there can sometimes provide answers more quickly: Microsoft Q&A

If you do create a thread over at Microsoft Q&A, we would appreciate if you could share that link here so we can track the progress and incorporate any technical details into our documentation.

[alvinashcraft]

I got some info from the product team, @krzysztofFC. They said:

Events go to disk when a buffer is filled or when the user calls FileLoggingSession.CloseAndSaveToFileAsync.

I've updated the docs with this clarification. The changes should appear on the live site soon. Thanks for helping to improve the docs!