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Logging errors and warnings

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

Enabling logs​

The code to achieve logging is added to your project by the Avalonia 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, see the LogEventLevel enum reference.

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

Verbose log output in the IDE Debug Output window

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 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);

Alternative log targets​

LogToDelegate​

Route log messages to a custom callback function:

public static AppBuilder BuildAvaloniaApp()
    => AppBuilder.Configure<App>()
        .UsePlatformDetect()
        .LogToDelegate((level, area, source, messageTemplate, propertyValues) =>
        {
            Console.WriteLine($"[{area}] {messageTemplate}");
        });

LogToTextWriter​

Write log messages to any TextWriter, such as a file or Console.Out:

public static AppBuilder BuildAvaloniaApp()
    => AppBuilder.Configure<App>()
        .UsePlatformDetect()
        .LogToTextWriter(File.CreateText("avalonia.log"));

Log sinks​

The LogToTrace extension method uses StringLogSink. 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 StringLogSink.cs

See also​

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