Fix null reference exception for Streaming null object (#14004)

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

@@ -265,7 +265,8 @@ await SendInvocationError(hubMethodInvocationMessage.InvocationId, connection,
                         for (var parameterPointer = 0; parameterPointer < arguments.Length; parameterPointer++)
                         {
                             if (hubMethodInvocationMessage.Arguments.Length > hubInvocationArgumentPointer &&
-                                descriptor.OriginalParameterTypes[parameterPointer].IsAssignableFrom(hubMethodInvocationMessage.Arguments[hubInvocationArgumentPointer].GetType()))
+                                (hubMethodInvocationMessage.Arguments[hubInvocationArgumentPointer] == null ||
+                                descriptor.OriginalParameterTypes[parameterPointer].IsAssignableFrom(hubMethodInvocationMessage.Arguments[hubInvocationArgumentPointer].GetType())))
                             {
                                 // The types match so it isn't a synthetic argument, just copy it into the arguments array
                                 arguments[parameterPointer] = hubMethodInvocationMessage.Arguments[hubInvocationArgumentPointer];

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

@@ -933,6 +933,36 @@ public ChannelReader<int> CancelableStreamMultiParameter(int ignore, int ignore2
             return channel.Reader;
         }
 
+        public ChannelReader<int> CancelableStreamNullableParameter(int x, string y, CancellationToken token)
+        {
+            var channel = Channel.CreateBounded<int>(10);
+
+            Task.Run(async () =>
+            {
+                _tcsService.StartedMethod.SetResult(x);
+                await token.WaitForCancellationAsync();
+                channel.Writer.TryComplete();
+                _tcsService.EndMethod.SetResult(y);
+            });
+
+            return channel.Reader;
+        }
+
+        public ChannelReader<int> StreamNullableParameter(int x, int? input)
+        {
+            var channel = Channel.CreateBounded<int>(10);
+
+            Task.Run(() =>
+            {
+                _tcsService.StartedMethod.SetResult(x);
+                channel.Writer.TryComplete();
+                _tcsService.EndMethod.SetResult(input);
+                return Task.CompletedTask;
+            });
+
+            return channel.Reader;
+        }
+
         public ChannelReader<int> CancelableStreamMiddleParameter(int ignore, CancellationToken token, int ignore2)
         {
             var channel = Channel.CreateBounded<int>(10);

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

@@ -3591,6 +3591,64 @@ public async Task StreamHubMethodCanAcceptCancellationTokenAsArgumentAndBeTrigge
             }
         }
 
+        [Fact]
+        public async Task StreamHubMethodCanAcceptNullableParameter()
+        {
+            using (StartVerifiableLog())
+            {
+                var tcsService = new TcsService();
+                var serviceProvider = HubConnectionHandlerTestUtils.CreateServiceProvider(builder =>
+                {
+                    builder.AddSingleton(tcsService);
+                }, LoggerFactory);
+                var connectionHandler = serviceProvider.GetService<HubConnectionHandler<LongRunningHub>>();
+
+                using (var client = new TestClient())
+                {
+                    var connectionHandlerTask = await client.ConnectAsync(connectionHandler).OrTimeout();
+
+                    var streamInvocationId = await client.SendStreamInvocationAsync(nameof(LongRunningHub.StreamNullableParameter), 5, null).OrTimeout();
+                    // Wait for the stream method to start
+                    var firstArgument = await tcsService.StartedMethod.Task.OrTimeout();
+                    Assert.Equal(5, firstArgument);
+
+                    var secondArgument = await tcsService.EndMethod.Task.OrTimeout();
+                    Assert.Null(secondArgument);
+                }
+            }
+        }
+
+
+        [Fact]
+        public async Task StreamHubMethodCanAcceptNullableParameterWithCancellationToken()
+        {
+            using (StartVerifiableLog())
+            {
+                var tcsService = new TcsService();
+                var serviceProvider = HubConnectionHandlerTestUtils.CreateServiceProvider(builder =>
+                {
+                    builder.AddSingleton(tcsService);
+                }, LoggerFactory);
+                var connectionHandler = serviceProvider.GetService<HubConnectionHandler<LongRunningHub>>();
+
+                using (var client = new TestClient())
+                {
+                    var connectionHandlerTask = await client.ConnectAsync(connectionHandler).OrTimeout();
+
+                    var streamInvocationId = await client.SendStreamInvocationAsync(nameof(LongRunningHub.CancelableStreamNullableParameter), 5, null).OrTimeout();
+                    // Wait for the stream method to start
+                    var firstArgument = await tcsService.StartedMethod.Task.OrTimeout();
+                    Assert.Equal(5, firstArgument);
+
+                    // Cancel the stream which should trigger the CancellationToken in the hub method
+                    await client.SendHubMessageAsync(new CancelInvocationMessage(streamInvocationId)).OrTimeout();
+
+                    var secondArgument = await tcsService.EndMethod.Task.OrTimeout();
+                    Assert.Null(secondArgument);
+                }
+            }
+        }
+
         [Fact]
         public async Task InvokeHubMethodCannotAcceptCancellationTokenAsArgument()
         {