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    9. Logging
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    11. Set up logging

    Set up logging

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    To enable logging you can use ConsoleLogger, FileLogger, or custom logger.

    To build your custom logger you have to implemnt ILogger interface that defines the methods that are used to perform logging.

    There are 3 types of messages in a log file:

    • Error - for unrecoverable exceptions
    • Warning - for recoverable/expected exceptions
    • Trace - for general information

    Write logs to a file

    The following example shows how to use FileLogger class to write logs to output.log file.

    // Create logger and specify the output file
FileLogger fileLogger = new FileLogger("output.log");

// Create ViewerSettings and specify FileLogger
ViewerSettings viewerSettings = new ViewerSettings(fileLogger);

using (Viewer viewer = new Viewer("sample.docx", viewerSettings))
{
    ViewOptions viewOptions = HtmlViewOptions.ForEmbeddedResources();
    viewer.View(viewOptions);
}

    The sample contents of the output.log file:

    [TRACE] Rendering into HTML with embedded resources.
[TRACE] Detecting file type by extension.
[TRACE] File type detected successfully: Microsoft Word Open XML Document.
[TRACE] Opening document.
[TRACE] Document is loaded.
[TRACE] Applying options.
[TRACE] Converting page 1.
[TRACE] Page 1 conversion completed.
[TRACE] Converting page 2.
[TRACE] Page 2 conversion completed.
[TRACE] Releasing resources.

    Write logs to a console

    The following example shows how to use ConsoleLogger class and write logs to console.

    // Create console logger
ConsoleLogger consoleLogger = new ConsoleLogger();

// Create ViewerSettings and specify FileLogger
ViewerSettings viewerSettings = new ViewerSettings(consoleLogger);

using (Viewer viewer = new Viewer("sample.docx", viewerSettings))
{
    ViewOptions viewOptions = HtmlViewOptions.ForEmbeddedResources();
    viewer.View(viewOptions);
}

    The following image demonstrates sample console output:

    Output when using ConsoleLogger

    Implement custom logger

    To implement your logger use ILogger interface.

    For trace messages - implement public void Trace(string message) method
    For warning messages - implement public void Warning(string message) method
    For error messages - implement public void Error(string message) method

    The following example demonstrates how to implement custom logger:

    // Create logger and specify the output file
MyFileLogger logger = new MyFileLogger("log.txt");

// Create ViewerSettings and pass logger
ViewerSettings viewerSettings = new ViewerSettings(logger);

using (Viewer viewer = new Viewer("sample.docx", viewerSettings))
{
    ViewOptions viewOptions = HtmlViewOptions.ForEmbeddedResources();
    viewer.View(viewOptions);
}

/// <summary>
/// Writes log messages to the file.
/// </summary>
public class MyFileLogger : ILogger
{
    private readonly string _fileName;

    /// <summary>
    /// Create logger to file.
    /// </summary>
    /// <param name="fileName">Full file name with path</param>
    public MyFileLogger(string fileName)
    {
        _fileName = fileName;
    }

    /// <summary>
    /// Writes trace message to file.
    /// Trace log messages provide generally useful information about application flow.
    /// </summary>
    /// <param name="message">The trace message.</param>
    /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="message"/> is null.</exception>
    public void Trace(string message) =>
        File.AppendAllText(_fileName, $"[TRACE] {message}{Environment.NewLine}");

    /// <summary>
    /// Writes warning message to file.
    /// Warning log messages provide information about the unexpected and recoverable event in application flow.
    /// </summary>
    /// <param name="message">The warning message.</param>
    /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="message"/> is null.</exception>
    public void Warning(string message) =>
        File.AppendAllText(_fileName, $"[WARN] {message}{Environment.NewLine}");

    /// <summary>
    /// Writes an error message to file.
    /// Error log messages provide information about unrecoverable events in application flow.
    /// </summary>
    /// <param name="message">The error message.</param>
    /// <param name="exception">The exception.</param>
    /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="message"/> is null.</exception>
    /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="exception"/> is null.</exception>
    public void Error(string message, Exception exception) =>
        File.AppendAllText(_fileName, $"[ERROR] {message}, exception: {exception}{Environment.NewLine}");
}

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