Lazy-init formatter resolvers in MessagePackHubProtocolOptions (#10929)

src/SignalR/common/Protocols.MessagePack/ref/Microsoft.AspNetCore.SignalR.Protocols.MessagePack.netstandard2.0.cs

@@ -6,7 +6,7 @@ namespace Microsoft.AspNetCore.SignalR
     public partial class MessagePackHubProtocolOptions
     {
         public MessagePackHubProtocolOptions() { }
-        public System.Collections.Generic.IList<MessagePack.IFormatterResolver> FormatterResolvers { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } [System.Runtime.CompilerServices.CompilerGeneratedAttribute]set { } }
+        public System.Collections.Generic.IList<MessagePack.IFormatterResolver> FormatterResolvers { get { throw null; } set { } }
     }
 }
 namespace Microsoft.AspNetCore.SignalR.Protocol

src/SignalR/common/Protocols.MessagePack/src/MessagePackHubProtocolOptions.cs

@@ -9,6 +9,25 @@ namespace Microsoft.AspNetCore.SignalR
 {
     public class MessagePackHubProtocolOptions
     {
-        public IList<IFormatterResolver> FormatterResolvers { get; set; } = MessagePackHubProtocol.CreateDefaultFormatterResolvers();
+        private IList<IFormatterResolver> _formatterResolvers;
+
+        public IList<IFormatterResolver> FormatterResolvers
+        {
+            get
+            {
+                if (_formatterResolvers == null)
+                {
+                    // The default set of resolvers trigger a static constructor that throws on AOT environments.
+                    // This gives users the chance to use an AOT friendly formatter.
+                    _formatterResolvers = MessagePackHubProtocol.CreateDefaultFormatterResolvers();
+                }
+
+                return _formatterResolvers;
+            }
+            set
+            {
+                _formatterResolvers = value;
+            }
+        }
     }
 }