throw CommandParsingException rather than crash on missing single value

src/dotnet/CommandLine/CommandLineApplication.cs

@@ -156,7 +156,10 @@ namespace Microsoft.DotNet.Cli.CommandLine
                 else if (isLongOption || arg.StartsWith("-"))
                 {
                     CommandOption option;
+
                     var result = ParseOption(isLongOption, command, args, ref index, out option);
+                  
+
                     if (result == ParseOptionResult.ShowHelp)
                     {
                         command.ShowHelp();
@@ -285,14 +288,24 @@ namespace Microsoft.DotNet.Cli.CommandLine
                     else
                     {
                         index++;
-                        arg = args[index];
+
-                        if (!option.TryParse(arg))
+                        if (index < args.Length)
+                        {
+                            arg = args[index];
+                            if (!option.TryParse(arg))
+                            {
+                                command.ShowHint();
+                                throw new CommandParsingException(
+                                    command,
+                                    String.Format(LocalizableStrings.UnexpectedValueForOptionError, arg, optionName));
+                            }
+                        }
+                        else
                         {
                             command.ShowHint();
                             throw new CommandParsingException(
                                 command,
-                                String.Format(LocalizableStrings.UnexpectedValueForOptionError, arg, optionName));
+                                String.Format(LocalizableStrings.OptionRequiresSingleValueWhichIsMissing, arg, optionName));
-
                         }
                     }
                 }

src/dotnet/CommandLine/LocalizableStrings.cs

@@ -4,6 +4,8 @@ namespace Microsoft.DotNet.Cli.CommandLine
     {
         public const string LastArgumentMultiValueError = "The last argument '{0}' accepts multiple values. No more argument can be added.";
 
+        public const string OptionRequiresSingleValueWhichIsMissing = "Required value for option '{0}' was not provided.";
+
         public const string UnexpectedValueForOptionError = "Unexpected value '{0}' for option '{1}'";
 
         public const string UnexpectedArgumentError = "Unrecognized {0} '{1}'";

test/dotnet.Tests/CommandLineApplicationTests.cs

@@ -0,0 +1,26 @@
+using System;
+using FluentAssertions;
+using Microsoft.DotNet.Cli.CommandLine;
+using Xunit;
+
+namespace Microsoft.DotNet.Tests
+{
+    public class CommandLineApplicationTests
+    {
+        [Fact]
+        public void WhenAnOptionRequiresASingleValueThatIsNotSuppliedItThrowsCommandParsingException()
+        {
+            var app = new CommandLineApplication();
+
+            app.Option("-v|--verbosity", "be verbose", CommandOptionType.SingleValue);
+
+            Action execute = () => app.Execute("-v");
+
+            execute.ShouldThrow<CommandParsingException>()
+                   .Which
+                   .Message
+                   .Should()
+                   .Be("Required value for option '-v' was not provided.");
+        }
+    }
+}