Add InvocationContext.GetHost extension method (#1221)

* Add InvocationContext.GetHost extension method

* Add tests for GetHost extension on InvocationContext

Author: fredrikhr

src/System.CommandLine.Hosting.Tests/HostingTests.cs

@@ -366,5 +366,46 @@ public static void GetInvocationContext_in_ConfigureServices_throws_if_not_withi
             })
                 .Should().Throw<InvalidOperationException>();
         }
+
+        [Fact]
+        public static void GetHost_returns_non_null_instance_in_subsequent_middleware()
+        {
+            bool hostAsserted = false;
+            var parser = new CommandLineBuilder()
+                .UseHost()
+                .UseMiddleware((invCtx, next) =>
+                {
+                    IHost host = invCtx.GetHost();
+                    host.Should().NotBeNull();
+                    hostAsserted = true;
+
+                    return next(invCtx);
+                })
+                .Build();
+
+            _ = parser.Invoke(string.Empty);
+
+            hostAsserted.Should().BeTrue();
+        }
+
+        [Fact]
+        public static void GetHost_returns_null_when_no_host_in_invocation()
+        {
+            bool hostAsserted = false;
+            var parser = new CommandLineBuilder()
+                .UseMiddleware((invCtx, next) =>
+                {
+                    IHost host = invCtx.GetHost();
+                    host.Should().BeNull();
+                    hostAsserted = true;
+
+                    return next(invCtx);
+                })
+                .Build();
+
+            _ = parser.Invoke(string.Empty);
+
+            hostAsserted.Should().BeTrue();
+        }
     }
 }

src/System.CommandLine.Hosting/HostingExtensions.cs

@@ -138,5 +138,12 @@ public static InvocationContext GetInvocationContext(this HostBuilderContext con
 
             throw new InvalidOperationException("Host builder has no Invocation Context registered to it.");
         }
+
+        public static IHost GetHost(this InvocationContext invocationContext)
+        {
+            _ = invocationContext ?? throw new ArgumentNullException(paramName: nameof(invocationContext));
+            var hostModelBinder = new ModelBinder<IHost>();
+            return (IHost)hostModelBinder.CreateInstance(invocationContext.BindingContext);
+        }
     }
 }