Add a callback on the reader options to expose the log filter switches

Just like it was done for log level switches in #352.

README.md

@@ -275,6 +275,22 @@ Using this package you can also declare `LoggingFilterSwitch`-es in custom secti
 
 Level updates to switches are also respected for a dynamic update.
 
+Since version 4.0.0, filter switches are exposed through a callback on the reader options so that a reference can be kept:
+
+```csharp
+var filterSwitches = new Dictionary<string, ILoggingFilterSwitch>();
+var options = new ConfigurationReaderOptions
+{
+    OnFilterSwitchCreated = (switchName, filterSwitch) => filterSwitches[switchName] = filterSwitch
+};
+
+var logger = new LoggerConfiguration()
+    .ReadFrom.Configuration(configuration, options)
+    .CreateLogger();
+
+ILoggingFilterSwitch filterSwitch = filterSwitches["$filterSwitch"];
+```
+
 ### Nested configuration sections
 
 Some Serilog packages require a reference to a logger configuration object. The sample program in this project illustrates this with the following entry configuring the _[Serilog.Sinks.Async](https://github.com/serilog/serilog-sinks-async)_ package to wrap the _[Serilog.Sinks.File](https://github.com/serilog/serilog-sinks-file)_ package. The `configure` parameter references the File sink configuration:

src/Serilog.Settings.Configuration/Settings/Configuration/ConfigurationReader.cs

@@ -75,7 +75,8 @@ void ProcessFilterSwitchDeclarations()
             SetFilterSwitch(throwOnError: true);
             SubscribeToFilterExpressionChanges();
 
-            _resolutionContext.AddFilterSwitch(switchName, filterSwitch);
+            var referenceName = _resolutionContext.AddFilterSwitch(switchName, filterSwitch);
+            _resolutionContext.ReaderOptions.OnFilterSwitchCreated?.Invoke(referenceName, filterSwitch);
 
             void SubscribeToFilterExpressionChanges()
             {

src/Serilog.Settings.Configuration/Settings/Configuration/ConfigurationReaderOptions.cs

@@ -77,6 +77,12 @@ public ConfigurationReaderOptions() : this(dependencyContext: null)
     /// </summary>
     public Action<string, LoggingLevelSwitch>? OnLevelSwitchCreated { get; init; }
 
+    /// <summary>
+    /// Called when a log filter switch is created while reading the <c>Serilog:FilterSwitches</c> section of the configuration.
+    /// The switch name includes the leading <c>$</c> character.
+    /// </summary>
+    public Action<string, ILoggingFilterSwitch>? OnFilterSwitchCreated { get; init; }
+
     internal Assembly[]? Assemblies { get; }
     internal DependencyContext? DependencyContext { get; }
     internal ConfigurationAssemblySource? ConfigurationAssemblySource { get; }

src/Serilog.Settings.Configuration/Settings/Configuration/ILoggingFilterSwitch.cs

@@ -0,0 +1,16 @@
+namespace Serilog.Settings.Configuration;
+
+/// <summary>
+/// A log event filter that can be modified at runtime.
+/// </summary>
+/// <remarks>
+/// Under the hood, the logging filter switch is either a <c>Serilog.Expressions.LoggingFilterSwitch</c> or a <c>Serilog.Filters.Expressions.LoggingFilterSwitch</c> instance.
+/// </remarks>
+public interface ILoggingFilterSwitch
+{
+    /// <summary>
+    /// A filter expression against which log events will be tested.
+    /// Only expressions that evaluate to <c>true</c> are included by the filter. A <c>null</c> expression will accept all events.
+    /// </summary>
+    public string? Expression { get; set; }
+}

src/Serilog.Settings.Configuration/Settings/Configuration/LoggingFilterSwitchProxy.cs

@@ -1,6 +1,6 @@
 namespace Serilog.Settings.Configuration;
 
-class LoggingFilterSwitchProxy
+class LoggingFilterSwitchProxy : ILoggingFilterSwitch
 {
     readonly Action<string?> _setProxy;
     readonly Func<string?> _getProxy;

test/Serilog.Settings.Configuration.Tests/ConfigurationSettingsTests.cs

@@ -1547,4 +1547,35 @@ public void TestLogLevelSwitchesCallback(string switchName)
         var systemThreading = Assert.Contains("System.Threading", switches);
         Assert.Equal(LogEventLevel.Debug, systemThreading.MinimumLevel);
     }
+
+    [Theory]
+    [InlineData("$switch1", "$switch2")]
+    [InlineData("$switch1", "switch2")]
+    [InlineData("switch1", "$switch2")]
+    [InlineData("switch1", "switch2")]
+    public void TestLogFilterSwitchesCallback(string switch1Name, string switch2Name)
+    {
+        var json = $$"""
+            {
+                'Serilog': {
+                    'FilterSwitches': {
+                        '{{switch1Name}}': 'Prop = 1',
+                        '{{switch2Name}}': 'Prop = 2'
+                    }
+                }
+            }
+            """;
+
+        IDictionary<string, ILoggingFilterSwitch> switches = new Dictionary<string, ILoggingFilterSwitch>();
+        var readerOptions = new ConfigurationReaderOptions { OnFilterSwitchCreated = (name, filterSwitch) => switches[name] = filterSwitch };
+        ConfigFromJson(json, options: readerOptions);
+
+        Assert.Equal(2, switches.Count);
+
+        var switch1 = Assert.Contains("$switch1", switches);
+        Assert.Equal("Prop = 1", switch1.Expression);
+
+        var switch2 = Assert.Contains("$switch2", switches);
+        Assert.Equal("Prop = 2", switch2.Expression);
+    }
 }

test/Serilog.Settings.Configuration.Tests/Serilog.Settings.Configuration.approved.txt

@@ -46,7 +46,12 @@ namespace Serilog.Settings.Configuration
         public bool AllowInternalMethods { get; init; }
         public bool AllowInternalTypes { get; init; }
         public System.IFormatProvider? FormatProvider { get; init; }
+        public System.Action<string, Serilog.Settings.Configuration.ILoggingFilterSwitch>? OnFilterSwitchCreated { get; init; }
         public System.Action<string, Serilog.Core.LoggingLevelSwitch>? OnLevelSwitchCreated { get; init; }
         public string? SectionName { get; init; }
     }
+    public interface ILoggingFilterSwitch
+    {
+        string? Expression { get; set; }
+    }
 }