Skipping output when exception is from cancellation (#1132)

* Skipping output when exception is from cancellation

Author: Keboo

src/System.CommandLine.Tests/UseExceptionHandlerTests.cs

@@ -109,6 +109,20 @@ public async Task Declaration_of_UseExceptionHandler_can_come_before_other_middl
                     .Contain("oops!");
         }
 
+        [Fact]
+        public async Task When_thrown_exception_is_from_cancelation_no_output_is_generated()
+        {
+            int resultCode = await new CommandLineBuilder()
+                .AddCommand(new Command("the-command"))
+                .UseExceptionHandler()
+                .UseMiddleware(_ => throw new OperationCanceledException())
+                .Build()
+                .InvokeAsync("the-command", _console);
+
+            _console.Out.ToString().Should().BeEmpty();
+            resultCode.Should().NotBe(0);
+        }
+
         [Fact]
         public async Task UseExceptionHandler_output_can_be_customized()
         {

src/System.CommandLine/Builder/CommandLineBuilderExtensions.cs

@@ -290,14 +290,16 @@ public static CommandLineBuilder UseExceptionHandler(
 
             void Default(Exception exception, InvocationContext context)
             {
-                context.Console.ResetTerminalForegroundColor();
-                context.Console.SetTerminalForegroundRed();
-
-                context.Console.Error.Write("Unhandled exception: ");
-                context.Console.Error.WriteLine(exception.ToString());
+                if (!(exception is OperationCanceledException))
+                {
+                    context.Console.ResetTerminalForegroundColor();
+                    context.Console.SetTerminalForegroundRed();
 
-                context.Console.ResetTerminalForegroundColor();
+                    context.Console.Error.Write("Unhandled exception: ");
+                    context.Console.Error.WriteLine(exception.ToString());
 
+                    context.Console.ResetTerminalForegroundColor();
+                }
                 context.ResultCode = 1;
             }
         }