Add test for supported scenario (#12931)

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

@@ -1050,4 +1050,32 @@ namespace Microsoft.AspNetCore.SignalR.Tests
 
         public bool TokenStateInDisconnected { get; set; }
     }
+
+    public class CallerServiceHub : Hub
+    {
+        private readonly CallerService _service;
+
+        public CallerServiceHub(CallerService service)
+        {
+            _service = service;
+        }
+
+        public override Task OnConnectedAsync()
+        {
+            _service.SetCaller(Clients.Caller);
+            var tcs = (TaskCompletionSource<bool>)Context.Items["ConnectedTask"];
+            tcs?.TrySetResult(true);
+            return base.OnConnectedAsync();
+        }
+    }
+
+    public class CallerService
+    {
+        public IClientProxy Caller { get; private set; }
+
+        public void SetCaller(IClientProxy caller)
+        {
+            Caller = caller;
+        }
+    }
 }

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

@@ -3625,6 +3625,35 @@ namespace Microsoft.AspNetCore.SignalR.Tests
             }
         }
 
+        [Fact]
+        public async Task ClientsCallerPropertyCanBeUsedOutsideOfHub()
+        {
+            CallerService callerService = new CallerService();
+            var serviceProvider = HubConnectionHandlerTestUtils.CreateServiceProvider(services =>
+            {
+                services.AddSingleton(callerService);
+            });
+            var connectionHandler = serviceProvider.GetService<HubConnectionHandler<CallerServiceHub>>();
+
+            using (StartVerifiableLog())
+            {
+                using (var client = new TestClient())
+                {
+                    var connectionHandlerTask = await client.ConnectAsync(connectionHandler);
+
+                    // Wait for a connection, or for the endpoint to fail.
+                    await client.Connected.OrThrowIfOtherFails(connectionHandlerTask).OrTimeout();
+
+                    await callerService.Caller.SendAsync("Echo", "message").OrTimeout();
+
+                    var message = Assert.IsType<InvocationMessage>(await client.ReadAsync().OrTimeout());
+
+                    Assert.Equal("Echo", message.Target);
+                    Assert.Equal("message", message.Arguments[0]);
+                }
+            }
+        }
+
         private class CustomHubActivator<THub> : IHubActivator<THub> where THub : Hub
         {
             public int ReleaseCount;