Logging

Virtually every production level application should utilize some form of logging. During development, especially when developing WASM applications where using a debugger can be more challenging, diagnostic logs can be invaluable for identifying issues. Once in production, capturing information about critical failures can be invaluable.

Important

If you created your app using the Recommended preset, please refer to our Uno.Extensions.Logging documentation instead.

For more information about the difference between the Blank and Recommended presets, please see the Preset selection section of our Solution Template documentation.

Logging in Uno Platform

The Uno platform makes use of the Microsoft logging NuGet packages to provide comprehensive logging support.

Configuring logging

The standard Uno template configures logging in the App.xaml.cs file.

Add the Uno.UI.Adapter.Microsoft.Extensions.Logging NuGet package to your platform projects.
In the iOS project, add the Uno.Extensions.Logging.OSLog NuGet package to your platform projects.
In the WebAssembly project, add the Uno.Extensions.Logging.WebAssembly.Console NuGet package to your platform projects.
Open the App.xaml.cs file.
Locate the App constructor and note that logging is configured first:
```
public App()
{
    InitializeLogging();
```
Notice that Uno provides a logger factory implementation.

Locate the ConfigureFilters method and review the code:


private static void InitializeLogging()
{
#if DEBUG

// Logging is disabled by default for release builds, as it incurs a significant
// initialization cost from Microsoft.Extensions.Logging setup. If startup performance
// is a concern for your application, keep this disabled. If you're running on web or
// desktop targets, you can use url or command line parameters to enable it yourself.
//
// For more performance documentation: https://platform.uno/docs/articles/Uno-UI-Performance.html

    var factory = LoggerFactory.Create(builder =>
    {
#if __WASM__
        builder.AddProvider(new global::Uno.Extensions.Logging.WebAssembly.WebAssemblyConsoleLoggerProvider());
#elif __IOS__
        builder.AddProvider(new global::Uno.Extensions.Logging.OSLogLoggerProvider());
#elif NETFX_CORE
        builder.AddDebug();
#else
        builder.AddConsole();
#endif

        // Exclude logs below this level
        builder.SetMinimumLevel(LogLevel.Information);

        // Default filters for Uno Platform namespaces
        builder.AddFilter("Uno", LogLevel.Warning);
        builder.AddFilter("Windows", LogLevel.Warning);
        builder.AddFilter("Microsoft", LogLevel.Warning);

        // Generic Xaml events
        // builder.AddFilter("Windows.UI.Xaml", LogLevel.Debug );
        // builder.AddFilter("Windows.UI.Xaml.VisualStateGroup", LogLevel.Debug );
        // builder.AddFilter("Windows.UI.Xaml.StateTriggerBase", LogLevel.Debug );
        // builder.AddFilter("Windows.UI.Xaml.UIElement", LogLevel.Debug );
        // builder.AddFilter("Windows.UI.Xaml.FrameworkElement", LogLevel.Trace );

        // Layouter specific messages
        // builder.AddFilter("Windows.UI.Xaml.Controls", LogLevel.Debug );
        // builder.AddFilter("Windows.UI.Xaml.Controls.Layouter", LogLevel.Debug );
        // builder.AddFilter("Windows.UI.Xaml.Controls.Panel", LogLevel.Debug );

        // builder.AddFilter("Windows.Storage", LogLevel.Debug );

        // Binding related messages
        // builder.AddFilter("Windows.UI.Xaml.Data", LogLevel.Debug );
        // builder.AddFilter("Windows.UI.Xaml.Data", LogLevel.Debug );

        // Binder memory references tracking
        // builder.AddFilter("Uno.UI.DataBinding.BinderReferenceHolder", LogLevel.Debug );

        // RemoteControl and HotReload related
        // builder.AddFilter("Uno.UI.RemoteControl", LogLevel.Information);

        // Debug JS interop
        // builder.AddFilter("Uno.Foundation.WebAssemblyRuntime", LogLevel.Debug );
    });

    global::Uno.Extensions.LogExtensionPoint.AmbientLoggerFactory = factory;

#if HAS_UNO
    global::Uno.UI.Adapter.Microsoft.Extensions.Logging.LoggingAdapter.Initialize();
#endif

#endif // DEBUG
}

Notice that the logging levels of various categories can be added and configured.

Note

Notice that console logging is configured by default- .AddConsole();. This does not log output to the Visual Studio console when running a UWP app. The next task details how to add UWP logging.

Adding UWP logging

In order to support logging to the debug output view in Visual Studio, complete the following steps

To install the, Microsoft.Extensions.Logging.Debug NuGet package, right-click the solution, and select Manage NuGet packages for solution...
In the Manage Packages for Solution UI, select the Browse tab, search for Microsoft.Extensions.Logging.Debug and select it in the search results.

Important

Only version 1.1.1 of the nuget package is supported by Uno at present.
On the right-side of the Manage Packages for Solution UI, ensure the UWP and WASM projects are selected, and then click Install.

In order to add UWP logging, configure AddDebug instead of (or in addition to) AddConsole. Review the code below for an example.

Configuring logging App.xaml.cs

/// <summary>
/// Configures global logging
/// </summary>
/// <param name="factory"></param>
static void ConfigureFilters(ILoggerFactory factory)
{
    factory
        .WithFilter(new FilterLoggerSettings
            {
                { "Uno", LogLevel.Warning },
                { "Windows", LogLevel.Warning },

                // Debug JS interop
                // { "Uno.Foundation.WebAssemblyRuntime", LogLevel.Debug },

                // Generic XAML events
                // { "Windows.UI.Xaml", LogLevel.Debug },
                // { "Windows.UI.Xaml.VisualStateGroup", LogLevel.Debug },
                // { "Windows.UI.Xaml.StateTriggerBase", LogLevel.Debug },
                // { "Windows.UI.Xaml.UIElement", LogLevel.Debug },

                // Layouter specific messages
                // { "Windows.UI.Xaml.Controls", LogLevel.Debug },
                // { "Windows.UI.Xaml.Controls.Layouter", LogLevel.Debug },
                // { "Windows.UI.Xaml.Controls.Panel", LogLevel.Debug },
                // { "Windows.Storage", LogLevel.Debug },

                // Binding related messages
                // { "Windows.UI.Xaml.Data", LogLevel.Debug },

                // DependencyObject memory references tracking
                // { "ReferenceHolder", LogLevel.Debug },

                // ListView-related messages
                // { "Windows.UI.Xaml.Controls.ListViewBase", LogLevel.Debug },
                // { "Windows.UI.Xaml.Controls.ListView", LogLevel.Debug },
                // { "Windows.UI.Xaml.Controls.GridView", LogLevel.Debug },
                // { "Windows.UI.Xaml.Controls.VirtualizingPanelLayout", LogLevel.Debug },
                // { "Windows.UI.Xaml.Controls.NativeListViewBase", LogLevel.Debug },
                // { "Windows.UI.Xaml.Controls.ListViewBaseSource", LogLevel.Debug }, //iOS
                // { "Windows.UI.Xaml.Controls.ListViewBaseInternalContainer", LogLevel.Debug }, //iOS
                // { "Windows.UI.Xaml.Controls.NativeListViewBaseAdapter", LogLevel.Debug }, //Android
                // { "Windows.UI.Xaml.Controls.BufferViewCache", LogLevel.Debug }, //Android
                // { "Windows.UI.Xaml.Controls.VirtualizingPanelGenerator", LogLevel.Debug }, //WASM
            }
        )
        //.AddConsole(LogLevel.Information);
#if DEBUG
        .AddConsole(LogLevel.Debug)
        .AddDebug(LogLevel.Debug);

#else
        .AddConsole(LogLevel.Information);
#endif
}

Notice the use of the pre-compiler directives to change the logging level when debugging.

Uno logging extensions

The Uno platform also provides an extension that simplifies the use of logging by making it simple to retrieve a reference to the logger.

In order to add logging to a class, add the following using statements namespaces:
```
using Microsoft.Extensions.Logging;
using Uno.Extensions;
```
In order to write output to a log, the Log extension method is available on an object instance, which then exposes the operations available from Microsoft.Extensions.Logging.LoggerExtensions. Here is an example on how to write error and information level messages:
```
if (discoveryClient.IsError)
{
    this.Log().LogError(discoveryClient.Error);
    throw new Exception(discoveryClient.Error);
}
this.Log().LogInformation($"UserInfoEndpoint: {discoveryClient.UserInfoEndpoint}");
```
Tip

To learn more about the logging capabilities, review the Microsoft logging reference materials here:
- Logging in .NET

Log output for not implemented member usage

By default, when a member is invoked at runtime that's not implemented by Uno (ie, marked with the [NotImplemented] attribute), an error message is logged.

Important

This feature flag must be set before the base.InitializeComponent() call within the App.xaml.cs constructor.

The logging behavior can be configured using feature flags:

By default, a message is only logged on the first usage of a given member. To log every time the member is invoked:
```
Uno.UI.FeatureConfiguration.ApiInformation.AlwaysLogNotImplementedMessages = true;
```
By default the message is logged as an error. To change the logging level:
```
Uno.UI.FeatureConfiguration.ApiInformation.NotImplementedLogLevel = LogLevel.Debug; // Raise not implemented usages as Debug messages
```
This can be used to suppress the not implemented output, if it's not useful.

IOS Unhandled Exception

System.InvalidOperationException: A suitable constructor for type 'Microsoft.Extensions.Options.OptionsMonitor`1[Microsoft.Extensions.Logging.LoggerFilterOptions]' could not be located. Ensure the type is concrete and services are registered for all parameters of a public constructor.

If you are getting the above exception when running on iOS and the Linker Behavior is set to "Link All" it is likely that the IL linker is removing some logging classes. See Linking Xamarin.iOS Apps

One option is to use linkskip file to exclude the assemblies causing issues. Add the following to your mtouch arguments:

--linkskip=Uno.Extensions.Logging.OSLog 
--linkskip=Microsoft.Extensions.Options

The other option is to add a custom linker definition file

<linker> 
  <assembly fullname="YOUR PROJECT ASSEMBLIES" />
  
  <assembly fullname="Microsoft.Extensions.Options" />

  <assembly fullname="System.Net.Http" />

  <assembly fullname="System.Core">
    <!-- This is required by JSon.NET and any expression.Compile caller -->
    <type fullname="System.Linq.Expressions*" />
  </assembly>
</linker>

Logging

Important

Logging in Uno Platform

Configuring logging

Note

Adding UWP logging

Important

Uno logging extensions

Tip

Log output for not implemented member usage

Important

IOS Unhandled Exception