Ensure NULL string value is written as quoted string in YAML

src/Common/YamlUtils.cs

@@ -19,7 +19,10 @@ internal class YamlUtils
     {
         private static Deserializer deserializer = (Deserializer)new DeserializerBuilder().Build();
         private static Deserializer camelCaseDeserializer = (Deserializer)new DeserializerBuilder().WithNamingConvention(CamelCaseNamingConvention.Instance).Build();
-        private static Serializer serializer = (Serializer)new SerializerBuilder().WithNamingConvention(CamelCaseNamingConvention.Instance).Build();
+        private static Serializer serializer = (Serializer)new SerializerBuilder()
+            .WithEventEmitter(nextEmitter => new QuotedNullStringEvenEmitter(nextEmitter))
+            .WithNamingConvention(CamelCaseNamingConvention.Instance)
+            .Build();
 
         // we have a specific serializer for metadata as it is not a simple string/string dictionary
         // and we want it to look like: no-loc: [Cmdlet, -Parameter]

src/YamlWriter/QuotedNullStringEvenEmitter.cs

@@ -0,0 +1,28 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using System.Runtime.Remoting.Metadata.W3cXsd2001;
+using YamlDotNet.Core;
+using YamlDotNet.Core.Events;
+using YamlDotNet.Serialization;
+using YamlDotNet.Serialization.EventEmitters;
+
+public class QuotedNullStringEvenEmitter : ChainedEventEmitter
+{
+    public QuotedNullStringEvenEmitter(IEventEmitter nextEmitter) : base(nextEmitter)
+    {
+    }
+
+    public override void Emit(ScalarEventInfo eventInfo, YamlDotNet.Core.IEmitter emitter)
+    {
+       if (eventInfo.Source.Type == typeof(string) && string.Equals(eventInfo.Source.Value?.ToString() , "NULL", System.StringComparison.OrdinalIgnoreCase))
+        {
+            emitter.Emit(new Scalar(@"'NULL'"));
+        }
+        else
+        {
+            // For all other values, use the default emitter
+            base.Emit(eventInfo, emitter);
+        }
+    }
+}

test/Pester/ExportYamlCommandHelp.Tests.ps1

@@ -164,6 +164,12 @@ Describe "Export-YamlCommandHelp tests" {
                 }
         }
 
+        It 'Should be able to handle accepted values with NULL' {
+            $cmd = Import-MarkdownCommandHelp -Path "$PSScriptRoot/assets/Resolve-DNSName.md"
+            $yamlFile = $cmd | Export-YamlCommandHelp -outputfolder $TestDrive -Force
+            $yamlDict = Import-YamlCommandHelp $yamlFile.FullName
+            $yamlDict.Parameters[-1].AcceptedValues | Should -Contain "'NULL'"
+        }
     }
 
     Context "Input" {

test/Pester/MeasurePlatyPSMarkdown.Tests.ps1

@@ -13,9 +13,9 @@ Describe "Export-MarkdownModuleFile" {
 
     It "Should identify all the '<fileType>' assets" -TestCases @(
         @{ fileType = "unknown"; expectedCount = 2 }
-        @{ fileType = "CommandHelp"; expectedCount = 40 }
+        @{ fileType = "CommandHelp"; expectedCount = 41 }
         @{ fileType = "ModuleFile"; expectedCount = 15 }
-        @{ fileType = "V1Schema"; expectedCount = 50 }
+        @{ fileType = "V1Schema"; expectedCount = 51 }
         @{ fileType = "V2Schema"; expectedCount = 5 }
     ) {
         param ($fileType, $expectedCount)