From a0e57f5ff68a9f0b60fe2257205a1d4062fd1961 Mon Sep 17 00:00:00 2001
From: Stephen Toub <stoub@microsoft.com>
Date: Sat, 5 Jul 2025 22:22:11 -0400
Subject: [PATCH] Seal NullLogger

Sealing it makes type checks against it cheaper. Some libraries special-case ILogger instances that are NullLogger in order to avoid some logging-related overheads.

While normally sealing a previously shipped public type would be a breaking change, NullLogger's only constructor is private, so types can't currently derive from it.
---
 .../ref/Microsoft.Extensions.Logging.Abstractions.cs            | 2 +-
 .../Microsoft.Extensions.Logging.Abstractions/src/NullLogger.cs | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/libraries/Microsoft.Extensions.Logging.Abstractions/ref/Microsoft.Extensions.Logging.Abstractions.cs b/src/libraries/Microsoft.Extensions.Logging.Abstractions/ref/Microsoft.Extensions.Logging.Abstractions.cs
index 4e973443ef8257..cd1125477953f3 100644
--- a/src/libraries/Microsoft.Extensions.Logging.Abstractions/ref/Microsoft.Extensions.Logging.Abstractions.cs
+++ b/src/libraries/Microsoft.Extensions.Logging.Abstractions/ref/Microsoft.Extensions.Logging.Abstractions.cs
@@ -177,7 +177,7 @@ public readonly partial struct LogEntry<TState>
         public Microsoft.Extensions.Logging.LogLevel LogLevel { get { throw null; } }
         public TState State { get { throw null; } }
     }
-    public partial class NullLogger : Microsoft.Extensions.Logging.ILogger
+    public sealed partial class NullLogger : Microsoft.Extensions.Logging.ILogger
     {
         internal NullLogger() { }
         public static Microsoft.Extensions.Logging.Abstractions.NullLogger Instance { get { throw null; } }
diff --git a/src/libraries/Microsoft.Extensions.Logging.Abstractions/src/NullLogger.cs b/src/libraries/Microsoft.Extensions.Logging.Abstractions/src/NullLogger.cs
index e2b1cd8b264eb8..d77a41315a0ec7 100644
--- a/src/libraries/Microsoft.Extensions.Logging.Abstractions/src/NullLogger.cs
+++ b/src/libraries/Microsoft.Extensions.Logging.Abstractions/src/NullLogger.cs
@@ -8,7 +8,7 @@ namespace Microsoft.Extensions.Logging.Abstractions
     /// <summary>
     /// Minimalistic logger that does nothing.
     /// </summary>
-    public class NullLogger : ILogger
+    public sealed class NullLogger : ILogger
     {
         /// <summary>
         /// Returns the shared instance of <see cref="NullLogger"/>.