Show help on command parsing error

src/Cli/dotnet/CommandLineValidation/CommandParsingException.cs

@@ -2,19 +2,20 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 using System;
+using System.CommandLine.Parsing;
 
 namespace Microsoft.DotNet.Cli
 {
     internal class CommandParsingException : Exception
     {
         public CommandParsingException(
             string message, 
-            string helpText = null) : base(message)
+            ParseResult parseResult = null) : base(message)
         {
-            HelpText = helpText ?? "";
+            ParseResult = parseResult;
             Data.Add("CLI_User_Displayed_Exception", true);
         }
 
-        public string HelpText { get; } = "";
+        public ParseResult ParseResult;
     }
 }

src/Cli/dotnet/ParseResultExtensions.cs

@@ -26,7 +26,8 @@ public static void ShowHelpOrErrorIfAppropriate(this ParseResult parseResult)
                 {
                     throw new CommandParsingException(
                         message: string.Join(Environment.NewLine,
-                                             parseResult.Errors.Select(e => e.Message)));
+                                             parseResult.Errors.Select(e => e.Message)), 
+                        parseResult: parseResult);
                 }
             }
             else if (parseResult.HasOption("--help"))

src/Cli/dotnet/Program.cs

@@ -70,9 +70,9 @@ public static int Main(string[] args)
                         : e.Message.Red().Bold());
 
                     var commandParsingException = e as CommandParsingException;
-                    if (commandParsingException != null)
+                    if (commandParsingException != null && commandParsingException.ParseResult != null)
                     {
-                        Reporter.Output.WriteLine(commandParsingException.HelpText);
+                        commandParsingException.ParseResult.ShowHelp();
                     }
 
                     return 1;

src/Tests/dotnet-add-reference.Tests/GivenDotnetAddReference.cs

@@ -127,6 +127,7 @@ public void WhenNoCommandIsPassedItPrintsError(string commandName)
                 .Execute($"add", commandName);
             cmd.Should().Fail();
             cmd.StdErr.Should().Be(CommonLocalizableStrings.RequiredCommandNotPassed);
+            cmd.StdOut.Should().BeVisuallyEquivalentToIfNotLocalized(AddCommandHelpText(Directory.GetCurrentDirectory()));
         }
 
         [Fact]