How To Log Errors and Warnings

This guide shows you how to can log warnings and errors in Avalonia UI using the standard (Microsoft)System.Diagnostics.Trace component.

The code to achieve logging is added to your project by the Avalonia UI solution templates if you use them.

To enable, or to check that logging is enabled, follow this procedure:

• Locate the Program.cs file for your application.
• Check that the BuildAvaloniaApp method calls LogToTrace, for example:

public static AppBuilder BuildAvaloniaApp()
    => AppBuilder.Configure<App>()
        .UsePlatformDetect()
        .LogToTrace();

Without parameters, LogToTrace will log messages with a severity of Warning or higher. You can change this to another level by passing a LogLevel parameter to the LogToTrace call. For example:

using Avalonia.Logging;
...
public static AppBuilder BuildAvaloniaApp()
    => AppBuilder.Configure<App>()
        .UsePlatformDetect()
        .LogToTrace(LogEventLevel.Verbose)

info

For the full API documentation on the LogEventLevel enum, see here.

Log messages are then shown in the Debug view of the Output window of your IDE. For example, with verbose logging enabled:

If you want to re-route these messages to different location, you can use the methods on the System.Diagnostics.Trace component.

Log Area

Each message from Avalonia UI is assigned an area that can be used to filter the log. These are described by the members of Avalonia.Logging.LogArea static class:

• Property
• Binding
• Animations
• Visual
• Layout
• Control

You can restrict the log to a specific area, or areas by adding arguments of type Avalonia.Logging.LogArea after the LogEventLevel argument in the LogToTrace call. For example, this will log only property and layout messages:

public static AppBuilder BuildAvaloniaApp()
    => AppBuilder.Configure<App>()
        .UsePlatformDetect()
        .LogToTrace(LogEventLevel.Debug, LogArea.Property, LogArea.Layout);

Log Sinks

The LogToTrace extension method uses TraceLogSink which writes its messages to Trace. Avalonia supports custom sinks by implementing ILogSink. Assigning your custom sink to Avalonia.Logging.Logger.Sink will allow Avalonia to use it.

Extension method to assign Logger.Sink

using Avalonia.Controls;
using Avalonia.Logging;

namespace MyNamespace;
public static class MyLogExtensions
{
    public static AppBuilder LogToMySink(this AppBuilder builder, 
        LogEventLevel level = LogEventLevel.Warning, 
        params string[] areas)
    {
        Logger.Sink = new MyLogSink(level, areas);
        return builder;
    }
}

Startup with custom sink

public static AppBuilder BuildAvaloniaApp()
    => AppBuilder.Configure<App>()
        .UsePlatformDetect()
        .LogToMySink();

info

View the source code on GitHub TraceLogSink.cs