Add null check (#2192)

* Add test

* Resolve comments

Author: vicancy

src/Microsoft.Azure.SignalR/ServerConnections/ServiceConnectionManager.cs

@@ -10,11 +10,12 @@
 using Microsoft.Azure.SignalR.Common;
 using Microsoft.Azure.SignalR.Protocol;
 
+#nullable enable
 namespace Microsoft.Azure.SignalR;
 
 internal class ServiceConnectionManager<THub> : IDisposable, IServiceConnectionManager<THub> where THub : Hub
 {
-    private IServiceConnectionContainer _serviceConnection;
+    private IServiceConnectionContainer? _serviceConnection;
 
     public void SetServiceConnection(IServiceConnectionContainer serviceConnection)
     {
@@ -23,22 +24,22 @@ public void SetServiceConnection(IServiceConnectionContainer serviceConnection)
 
     public Task StartAsync()
     {
-        return _serviceConnection.StartAsync();
+        return _serviceConnection?.StartAsync() ?? throw new InvalidOperationException();
     }
 
     public Task StopAsync()
     {
-        return _serviceConnection.StopAsync();
+        return _serviceConnection?.StopAsync() ?? Task.CompletedTask;
     }
 
-    public async Task OfflineAsync(GracefulShutdownMode mode, CancellationToken token)
+    public Task OfflineAsync(GracefulShutdownMode mode, CancellationToken token)
     {
-        await _serviceConnection.OfflineAsync(mode, token);
+        return _serviceConnection?.OfflineAsync(mode, token) ?? Task.CompletedTask;
     }
 
-    public async Task CloseClientConnections(CancellationToken cancellationToken)
+    public Task CloseClientConnections(CancellationToken cancellationToken)
     {
-        await _serviceConnection.CloseClientConnections(cancellationToken);
+        return _serviceConnection?.CloseClientConnections(cancellationToken) ?? Task.CompletedTask;
     }
 
     public Task WriteAsync(ServiceMessage serviceMessage)
@@ -68,6 +69,11 @@ public void Dispose()
 
     public IAsyncEnumerable<GroupMember> ListConnectionsInGroupAsync(string groupName, int? top = null, ulong? tracingId = null, CancellationToken token = default)
     {
+        if (_serviceConnection == null)
+        {
+            throw new AzureSignalRNotConnectedException();
+        }
+
         return _serviceConnection.ListConnectionsInGroupAsync(groupName, top, tracingId, token);
     }
 }

test/Microsoft.Azure.SignalR.Tests/ServiceLifetimeManagerFacts.cs

@@ -110,6 +110,13 @@ public async Task ServiceLifetimeManagerGroupTest(string functionName, Type type
         Assert.False(blazorDetector.IsBlazor(nameof(TestHub)));
     }
 
+    [Fact]
+    public async Task ServiceLifetimeManagerStopShouldNotThrow()
+    {
+        var serviceConnectionManager = new ServiceConnectionManager<TestHub>();
+        await serviceConnectionManager.StopAsync();
+    }
+
     [Theory]
     [InlineData("SendAllAsync", typeof(BroadcastDataMessage))]
     [InlineData("SendAllExceptAsync", typeof(BroadcastDataMessage))]