Allow custom filtering logic for FakeLogger

src/Libraries/Microsoft.Extensions.Diagnostics.Testing/Logging/FakeLogCollector.cs

@@ -129,6 +129,15 @@ internal void AddRecord(FakeLogRecord record)
             return;
         }
 
+        foreach (var condition in _options.CustomFilters)
+        {
+            if (!condition(record))
+            {
+                // record was filtered out by a custom filter
+                return;
+            }
+        }
+
         lock (_records)
         {
             _records.Add(record);

src/Libraries/Microsoft.Extensions.Diagnostics.Testing/Logging/FakeLogCollectorOptions.cs

@@ -3,7 +3,8 @@
 
 using System;
 using System.Collections.Generic;
-using Microsoft.Extensions.Logging;
+using System.Diagnostics.CodeAnalysis;
+using Microsoft.Shared.DiagnosticIds;
 
 #pragma warning disable CA2227 // Collection properties should be read only
 
@@ -34,6 +35,17 @@ public class FakeLogCollectorOptions
     /// </remarks>
     public ISet<LogLevel> FilteredLevels { get; set; } = new HashSet<LogLevel>();
 
+    /// <summary>
+    /// Gets or sets custom filters used to filter out records to be collected.
+    /// </summary>
+    /// <value>Default is an empty array.</value>
+    /// <remarks>
+    /// Defaults to an empty array, which doesn't filter any records.
+    /// If not empty, only records for which the filter function returns <see langword="true" /> will be collected by the fake logger.
+    /// </remarks>
+    [Experimental(DiagnosticIds.Experiments.Telemetry)]
+    public IEnumerable<Func<FakeLogRecord, bool>> CustomFilters { get; set; } = Array.Empty<Func<FakeLogRecord, bool>>();
+
     /// <summary>
     /// Gets or sets a value indicating whether to collect records that are logged when the associated log level is currently disabled.
     /// </summary>

test/Libraries/Microsoft.Extensions.Diagnostics.Testing.Tests/Logging/FakeLoggerTests.cs

@@ -6,8 +6,6 @@
 using System.Globalization;
 using System.Linq;
 using System.Threading;
-using Microsoft.Extensions.Logging;
-using Microsoft.Extensions.Logging.Testing;
 using Microsoft.Extensions.Time.Testing;
 using Xunit;
 
@@ -283,4 +281,59 @@ public void Scopes()
         Assert.Equal(42, (int)logger.LatestRecord.Scopes[0]!);
         Assert.Equal("Hello World", (string)logger.LatestRecord.Scopes[1]!);
     }
+
+    [Fact]
+    public void FilterByCustomFilter()
+    {
+        const string NotIgnoredMessage1 = "Not ignored message 1";
+        const string NotIgnoredMessage2 = "Not ignored message 2";
+        const string IgnoredMessage = "Ignored message";
+
+        // Given
+        var options = new FakeLogCollectorOptions
+        {
+            CustomFilters = [
+                r => !r.Message.Equals(IgnoredMessage, StringComparison.Ordinal),
+            ],
+        };
+
+        var collector = FakeLogCollector.Create(options);
+        var logger = new FakeLogger(collector);
+
+        // When
+        logger.LogInformation(NotIgnoredMessage1);
+        logger.LogInformation(IgnoredMessage);
+        logger.LogError(IgnoredMessage);
+        logger.LogCritical(IgnoredMessage);
+        logger.LogError(NotIgnoredMessage2);
+
+        var records = logger.Collector.GetSnapshot();
+
+        // Then
+        Assert.Equal(2, records.Count);
+        Assert.Equal(2, logger.Collector.Count);
+
+        var firstLogRecordFromSnapshot = records[0];
+
+        Assert.Equal(NotIgnoredMessage1, firstLogRecordFromSnapshot.Message);
+        Assert.Equal(LogLevel.Information, firstLogRecordFromSnapshot.Level);
+        Assert.Null(firstLogRecordFromSnapshot.Exception);
+        Assert.Null(firstLogRecordFromSnapshot.Category);
+        Assert.True(firstLogRecordFromSnapshot.LevelEnabled);
+        Assert.Empty(firstLogRecordFromSnapshot.Scopes);
+        Assert.Equal(0, firstLogRecordFromSnapshot.Id.Id);
+        Assert.EndsWith($"info] {NotIgnoredMessage1}", firstLogRecordFromSnapshot.ToString());
+
+        var secondLogRecordFromSnapshot = records[1];
+        Assert.Equal(NotIgnoredMessage2, secondLogRecordFromSnapshot.Message);
+        Assert.Equal(LogLevel.Error, secondLogRecordFromSnapshot.Level);
+        Assert.Null(secondLogRecordFromSnapshot.Exception);
+        Assert.Null(secondLogRecordFromSnapshot.Category);
+        Assert.Empty(secondLogRecordFromSnapshot.Scopes);
+        Assert.True(secondLogRecordFromSnapshot.LevelEnabled);
+        Assert.Equal(0, secondLogRecordFromSnapshot.Id.Id);
+        Assert.EndsWith($"error] {NotIgnoredMessage2}", secondLogRecordFromSnapshot.ToString());
+
+        Assert.Equivalent(secondLogRecordFromSnapshot, logger.LatestRecord);
+    }
 }