Better exception for Generic methods on Hub (#15388)

src/SignalR/server/Core/src/Internal/DefaultHubDispatcher.cs

@@ -547,6 +547,11 @@ namespace Microsoft.AspNetCore.SignalR.Internal
 
             foreach (var methodInfo in HubReflectionHelper.GetHubMethods(hubType))
             {
+                if (methodInfo.IsGenericMethod)
+                {
+                    throw new NotSupportedException($"Method '{methodInfo.Name}' is a generic method which is not supported on a Hub.");
+                }
+
                 var methodName =
                     methodInfo.GetCustomAttribute<HubMethodNameAttribute>()?.Name ??
                     methodInfo.Name;

src/SignalR/server/SignalR/test/HubConnectionHandlerTestUtils/Hubs.cs

@@ -569,6 +569,13 @@ namespace Microsoft.AspNetCore.SignalR.Tests
         }
     }
 
+    public class GenericMethodHub : Hub
+    {
+        public void GenericMethod<T>()
+        {
+        }
+    }
+
     public class DisposeTrackingHub : TestHub
     {
         private readonly TrackDispose _trackDispose;

src/SignalR/server/SignalR/test/HubConnectionHandlerTests.cs

@@ -1327,6 +1327,19 @@ namespace Microsoft.AspNetCore.SignalR.Tests
             }
         }
 
+        [Fact]
+        public void CannotHaveGenericMethodOnHub()
+        {
+            using (StartVerifiableLog())
+            {
+                var serviceProvider = HubConnectionHandlerTestUtils.CreateServiceProvider(null, LoggerFactory);
+
+                var exception = Assert.Throws<NotSupportedException>(() => serviceProvider.GetService<HubConnectionHandler<GenericMethodHub>>());
+
+                Assert.Equal("Method 'GenericMethod' is a generic method which is not supported on a Hub.", exception.Message);
+            }
+        }
+
         [Theory]
         [MemberData(nameof(HubTypes))]
         public async Task BroadcastHubMethodSendsToAllClients(Type hubType)