Ignore null loggers returned by LogProviders in ExtendedLoggerFactory (#6585)

Author: petrroll

src/Libraries/Microsoft.Extensions.Telemetry/Logging/ExtendedLoggerFactory.cs

@@ -11,6 +11,7 @@
 using Microsoft.Extensions.Diagnostics.Buffering;
 #endif
 using Microsoft.Extensions.Diagnostics.Enrichment;
+using Microsoft.Extensions.Logging.Abstractions;
 using Microsoft.Extensions.Options;
 using Microsoft.Shared.Diagnostics;
 
@@ -223,13 +224,20 @@ private void AddProviderRegistration(ILoggerProvider provider, bool dispose)
 
     private LoggerInformation[] CreateLoggers(string categoryName)
     {
-        var loggers = new LoggerInformation[_providerRegistrations.Count];
+        var loggers = new List<LoggerInformation>(_providerRegistrations.Count);
         for (int i = 0; i < _providerRegistrations.Count; i++)
         {
-            loggers[i] = new LoggerInformation(_providerRegistrations[i].Provider, categoryName);
+            var loggerInformation = new LoggerInformation(_providerRegistrations[i].Provider, categoryName);
+
+            // We do not need to check for NullLogger<T>.Instance as no provider would reasonably return it (the <T> handling is at
+            // outer loggers level, not inner level loggers in Logger/LoggerProvider).
+            if (loggerInformation.Logger != NullLogger.Instance)
+            {
+                loggers.Add(loggerInformation);
+            }
         }
 
-        return loggers;
+        return loggers.ToArray();
     }
 
     private (MessageLogger[] messageLoggers, ScopeLogger[] scopeLoggers) ApplyFilters(LoggerInformation[] loggers)

test/Libraries/Microsoft.Extensions.Telemetry.Tests/Logging/ExtendedLoggerFactoryTests.cs

@@ -8,6 +8,7 @@
 using System.Text;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
 using Microsoft.Extensions.Logging.Testing;
 using Moq;
 using Xunit;
@@ -536,6 +537,20 @@ public static void CreateDisposeDisposesInnerServiceProvider()
         Assert.True(disposed);
     }
 
+    [Fact]
+    public static void NullLoggerByProviderIsIgnored()
+    {
+        using var factory = Utils.CreateLoggerFactory(builder =>
+        {
+            builder.AddProvider(NullLoggerProvider.Instance);
+            builder.AddProvider(new Provider());
+        });
+        var logger1 = (ExtendedLogger)factory.CreateLogger("C1");
+        Assert.Single(logger1.MessageLoggers);
+
+        logger1.LogInformation("This should not throw an exception.");
+    }
+
     private class InternalScopeLoggerProvider : ILoggerProvider, ILogger
     {
         private IExternalScopeProvider _scopeProvider = new LoggerExternalScopeProvider();