From bc505d5fcb32333402d17833623ff471de459633 Mon Sep 17 00:00:00 2001
From: Steven Maillet <smaillet@users.noreply.github.com>
Date: Mon, 19 Jan 2026 10:49:48 -0800
Subject: [PATCH] Added test for properties with a nullable type that are
 marked as required. * This is, at best, a pointless NOP.     - Behavior of
 this is formally UNDEFINED.     - Generate a warning diagnostic to verify
 that * Added documentation for analyzer diagnostics     - This is published
 with the documentation for the library     - Published location is the
 HelpURI for the diagnostic to allow click through to the helps from an IDE.

---
 docfx/.editorconfig                           | 34 +++++++++
 docfx/CommandLine/Diagnostics/UNC000.md       |  6 ++
 docfx/CommandLine/Diagnostics/UNC001.md       | 38 ++++++++++
 docfx/CommandLine/Diagnostics/UNC002.md       | 45 +++++++++++
 docfx/CommandLine/Diagnostics/UNC003.md       | 24 ++++++
 docfx/CommandLine/Diagnostics/UNC004.md       | 26 +++++++
 docfx/CommandLine/index.md                    | 15 +++-
 docfx/IgnoredWords.dic                        |  1 +
 docfx/documentation.msbuildproj               |  1 +
 .../TypeSymbolExtensions.cs                   | 10 ++-
 .../CommandAnalyzerTests.cs                   | 76 ++++++++++++++++---
 .../input.cs                                  |  4 +-
 .../input.cs                                  | 11 +++
 .../input.cs                                  |  7 +-
 .../input.cs                                  | 14 ++++
 .../Ubiquity.NET.CommandLine.SrcGen.UT.csproj |  4 +
 .../AnalyzerReleases.Unshipped.md             |  5 +-
 .../CommandLineAnalyzer.cs                    | 69 +++++++++++++----
 .../Diagnostics.cs                            | 28 ++++++-
 .../Properties/Resources.Designer.cs          | 27 +++++++
 .../Properties/Resources.resx                 | 65 +++++++++-------
 21 files changed, 447 insertions(+), 63 deletions(-)
 create mode 100644 docfx/.editorconfig
 create mode 100644 docfx/CommandLine/Diagnostics/UNC000.md
 create mode 100644 docfx/CommandLine/Diagnostics/UNC001.md
 create mode 100644 docfx/CommandLine/Diagnostics/UNC002.md
 create mode 100644 docfx/CommandLine/Diagnostics/UNC003.md
 create mode 100644 docfx/CommandLine/Diagnostics/UNC004.md
 create mode 100644 docfx/IgnoredWords.dic
 create mode 100644 src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/CommandAnalyzerTests/FileValidation_with_wrong_type_produces_UNC002/input.cs
 create mode 100644 src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/CommandAnalyzerTests/Required_nullable_types_produce_diagnostic/input.cs

diff --git a/docfx/.editorconfig b/docfx/.editorconfig
new file mode 100644
index 0000000..2364720
--- /dev/null
+++ b/docfx/.editorconfig
@@ -0,0 +1,34 @@
+#Primary settings apply to all files unless overridden below
+root = true
+
+[*]
+indent_style = space
+indent_size = 4
+insert_final_newline = true
+tab_width = 4
+end_of_line = crlf
+
+# VSSPELL: Spell checker settings for all files
+vsspell_section_id = 869f688c36354e788f0a0b53ab42cf41
+
+# [VSSpellChecker bug 277](https://github.com/EWSoftware/VSSpellChecker/issues/277)
+# VSSPELL: Disable the analyzers until Bug 277 is fixed.
+vsspell_code_analyzers_enabled = false
+vsspell_ignored_words_869f688c36354e788f0a0b53ab42cf41 = File:.\IgnoredWords.dic
+
+# match VS generated formatting for MSBuild project files
+[*.*proj,*.props,*.targets]
+indent_style = space
+indent_size = 2
+tab_width = 2
+
+[*.yml,*.yaml]
+indent_style = space
+indent_size = 2
+tab_width = 2
+
+[*.md]
+# mark left margin for split screen preview of markdown files
+# requires: https://marketplace.visualstudio.com/items?itemName=PaulHarrington.EditorGuidelinesPreview
+guidelines = 92
+
diff --git a/docfx/CommandLine/Diagnostics/UNC000.md b/docfx/CommandLine/Diagnostics/UNC000.md
new file mode 100644
index 0000000..cab668f
--- /dev/null
+++ b/docfx/CommandLine/Diagnostics/UNC000.md
@@ -0,0 +1,6 @@
+# UNC0000: An internal analyzer exception occurred.
+An internal error occurred in the analyzer. Please [report](https://github.com/UbiquityDotNET/Ubiquity.NET.Utils/issues)
+the issue with as much detail as possible, ideally with a small repro to help identify the
+problem. Please include the full stack of the exception as shown in the message details.
+Occurrences of this diagnostic are ALWAYS a bug in the generator/Analyzer in question and
+should be fixed.
diff --git a/docfx/CommandLine/Diagnostics/UNC001.md b/docfx/CommandLine/Diagnostics/UNC001.md
new file mode 100644
index 0000000..3eccf60
--- /dev/null
+++ b/docfx/CommandLine/Diagnostics/UNC001.md
@@ -0,0 +1,38 @@
+# UNC001: Missing command attribute on containing type
+A command line property annotating attribute is applied to a property but the containing
+type does not contain an attribute that designates it as a command. The generator will
+***ignore*** the property and the ***entire*** class.
+
+## Example:
+``` C#
+using System.IO;
+
+using Ubiquity.NET.CommandLine.GeneratorAttributes;
+
+namespace TestNamespace;
+
+internal class testInput1
+{
+    [Option( "-o" )] // UNC001 reported here
+    public required DirectoryInfo SomePath { get; init; }
+}
+```
+
+## Fix:
+``` C#
+using System.IO;
+
+using Ubiquity.NET.CommandLine.GeneratorAttributes;
+
+namespace TestNamespace;
+
+// apply a command attribute to the type to allow generation
+// of the backing details for parsing and binding the results to this type.
+[RootCommand( Description = "Root command for tests" )]
+internal class testInput1
+{
+    [Option( "-o" )]
+    public required DirectoryInfo SomePath { get; init; }
+}
+```
+
diff --git a/docfx/CommandLine/Diagnostics/UNC002.md b/docfx/CommandLine/Diagnostics/UNC002.md
new file mode 100644
index 0000000..aebdbcc
--- /dev/null
+++ b/docfx/CommandLine/Diagnostics/UNC002.md
@@ -0,0 +1,45 @@
+# UNC002 : Property attribute not allowed standalone
+This diagnostic is reported when an attribute is detected that is a qualifier to another
+required attribute, but the required attribute is not present.
+
+## Example
+`FolderValidationAttribute` provides additional information for validation of a property. To
+generate source that operates correctly an additional attribute is required. For example,
+to specify that a Folder provided must exist already a command can apply the
+`FolderValidationAttribute`. However that attribute requires additional information to
+identify the property as an option and other behaviors. Without the constrained attribute
+this attribute and property it is attached to is ignored by source generation.
+
+``` C#
+using System.IO;
+
+using Ubiquity.NET.CommandLine.GeneratorAttributes;
+
+namespace TestNamespace;
+
+[RootCommand( Description = "Root command for tests" )]
+internal class testInput1
+{
+    // This attribute triggers UNC002 - Property attribute FolderValidation is not allowed on a property independent of a qualifying attribute such as OptionAttribute.
+    [FolderValidation( FolderValidation.CreateIfNotExist )]
+    public required DirectoryInfo SomePath { get; init; }
+}
+```
+
+## Fix
+``` C#
+using System.IO;
+
+using Ubiquity.NET.CommandLine.GeneratorAttributes;
+
+namespace TestNamespace;
+
+[RootCommand( Description = "Root command for tests" )]
+internal class testInput1
+{
+    // Fix is to apply the attribute that indicates the property being validated
+    [Option( "-o", Description = "Test SomePath" )]
+    [FolderValidation( FolderValidation.CreateIfNotExist )]
+    public required DirectoryInfo SomePath { get; init; }
+}
+```
diff --git a/docfx/CommandLine/Diagnostics/UNC003.md b/docfx/CommandLine/Diagnostics/UNC003.md
new file mode 100644
index 0000000..d3e4ba5
--- /dev/null
+++ b/docfx/CommandLine/Diagnostics/UNC003.md
@@ -0,0 +1,24 @@
+# UNC003: Property has incorrect type for attribute
+The property's type is not consistent with the requirements of the attribute applied to it.
+Some attributes (especially validation annotations) require a particular type for the
+property to generate valid code. When this diagnostic is raised it is an indication that an
+incorrect type is used. If this is ignored or suppressed the property is ignored for code
+generation.
+
+## Example
+``` C#
+using System.IO;
+
+using Ubiquity.NET.CommandLine.GeneratorAttributes;
+
+namespace TestNamespace;
+
+[RootCommand( Description = "Root command for tests" )]
+internal class testInput1
+{
+    [Option( "-o", Description = "Test SomePath" )]
+    // This attribute triggers UNC003 - Property attribute 'FileValidation' requires a property of type 'FileInfo'.
+    [FileValidation( FileValidation.ExistingOnly )]
+    public required string SomePath { get; init; }
+}
+```
diff --git a/docfx/CommandLine/Diagnostics/UNC004.md b/docfx/CommandLine/Diagnostics/UNC004.md
new file mode 100644
index 0000000..edaca69
--- /dev/null
+++ b/docfx/CommandLine/Diagnostics/UNC004.md
@@ -0,0 +1,26 @@
+# UNC004 : Property type is nullable but marked as required.
+This diagnostic is reported when a nullable type is marked as `Required`. This usually
+indicates an error in the source applying the attributes. An explicitly annotated nullable
+type has a legit value of null, therefore marking it as `Required` makes no sense. Required
+means that it is validated as specified on the command line, this validation only occurs at
+the time of ***invoking*** the command. (If a different command is parsed from the command
+line arguments then no validation occurs).
+
+
+## Example
+``` C#
+using Ubiquity.NET.CommandLine.GeneratorAttributes;
+
+namespace TestNamespace;
+
+[RootCommand( Description = "Root command for tests" )]
+internal partial class TestOptions
+{
+    // Use of `Required = true` reports diagnostic; UNC004 : Property type is nullable but marked as required.
+    [Option( "--thing1", Aliases = [ "-t" ], Required = true, Description = "Test Thing1", HelpName = "Help name for thing1" )]
+    public bool? Thing1 { get; init; }
+
+    [Option( "--thing2", Aliases = [ "-t" ], Description = "Test Thing2", HelpName = "Help name for thing2" )]
+    public bool Thing2 { get; init; }
+}
+```
diff --git a/docfx/CommandLine/index.md b/docfx/CommandLine/index.md
index e35c04e..3395438 100644
--- a/docfx/CommandLine/index.md
+++ b/docfx/CommandLine/index.md
@@ -1,3 +1,16 @@
 # About
 Ubiquity.NET.CommandLine contains general extensions for .NET. to support command line
-parsing using `System.CommandLine`
+parsing using `System.CommandLine`.
+
+A source generator is included that will generate the boilerplate code for command line
+parsing and binding. Additionally an analyzer is provided to aid in identifying problems
+with usage of the attributes for generation.
+
+## Analyzer Diagnostics
+Rule ID | Title |
+--------|-------|
+[UNC000](https://ubiquitydotnet.github.io/Ubiquity.NET.Utils/CommandLine/diagnostics/UNC000.html) | An internal analyzer exception occurred. |
+[UNC001](https://ubiquitydotnet.github.io/Ubiquity.NET.Utils/CommandLine/diagnostics/UNC001.html) | Missing command attribute on containing type. |
+[UNC002](https://ubiquitydotnet.github.io/Ubiquity.NET.Utils/CommandLine/diagnostics/UNC002.html) | Property attribute not allowed standalone. |
+[UNC003](https://ubiquitydotnet.github.io/Ubiquity.NET.Utils/CommandLine/diagnostics/UNC003.html) | Property has incorrect type for attribute. |
+[UNC004](https://ubiquitydotnet.github.io/Ubiquity.NET.Utils/CommandLine/diagnostics/UNC004.html) | Property type is nullable but marked as required. |
diff --git a/docfx/IgnoredWords.dic b/docfx/IgnoredWords.dic
new file mode 100644
index 0000000..35bce0a
--- /dev/null
+++ b/docfx/IgnoredWords.dic
@@ -0,0 +1 @@
+nullable
diff --git a/docfx/documentation.msbuildproj b/docfx/documentation.msbuildproj
index 8ca8982..928ad32 100644
--- a/docfx/documentation.msbuildproj
+++ b/docfx/documentation.msbuildproj
@@ -74,6 +74,7 @@
   <Target Name="AlwaysRun" BeforeTargets="AfterBuild">
     <Message Importance="High" Text="NOTE: Building $(MSBuildProjectFile) does NOTHING, docs are built using the docfx tool. This project is simply a convenient placeholder for organizing/editing files" />
   </Target>
+
   <!--
   Target to generate the versioning JSON file for consumption by the docs scripts. This target is explicitly called out
   by the build scripts to generate the JSON file with all the version details from Ubiquity.NET.Versioning.Build.Tasks.
diff --git a/src/Ubiquity.NET.CodeAnalysis.Utils/TypeSymbolExtensions.cs b/src/Ubiquity.NET.CodeAnalysis.Utils/TypeSymbolExtensions.cs
index 70365fd..b6c5991 100644
--- a/src/Ubiquity.NET.CodeAnalysis.Utils/TypeSymbolExtensions.cs
+++ b/src/Ubiquity.NET.CodeAnalysis.Utils/TypeSymbolExtensions.cs
@@ -9,7 +9,7 @@ public static class TypeSymbolExtensions
         /// <summary>Gets the full name of the symbol (Including namespace names)</summary>
         /// <param name="self">Symbol to get the name from</param>
         /// <returns>Enumerable collection of the name parts with outer most namespace first</returns>
-        public static IEnumerable<string> GetFullName( this ITypeSymbol self )
+        public static IEnumerable<string> GetFullNameParts( this ITypeSymbol self )
         {
             foreach(string namespacePart in GetNamespaceNames( self ))
             {
@@ -19,6 +19,14 @@ public static IEnumerable<string> GetFullName( this ITypeSymbol self )
             yield return self.Name;
         }
 
+        /// <summary>Gets the fill name (including namespaces) of a <see cref="ITypeSymbol"/></summary>
+        /// <param name="self"><see cref="ITypeSymbol"/> to get the name from</param>
+        /// <returns>full name for the type</returns>
+        public static string GetFullName(this ITypeSymbol self)
+        {
+            return string.Join(".", GetFullNameParts(self));
+        }
+
         /// <summary>Gets the sequence of namespaces names of the symbol (outermost to innermost)</summary>
         /// <param name="self">Symbol to get the name from</param>
         /// <returns>Enumerable collection of the name parts with outer most namespace first</returns>
diff --git a/src/Ubiquity.NET.CommandLine.SrcGen.UT/CommandAnalyzerTests.cs b/src/Ubiquity.NET.CommandLine.SrcGen.UT/CommandAnalyzerTests.cs
index 4b64ad7..66b3bd8 100644
--- a/src/Ubiquity.NET.CommandLine.SrcGen.UT/CommandAnalyzerTests.cs
+++ b/src/Ubiquity.NET.CommandLine.SrcGen.UT/CommandAnalyzerTests.cs
@@ -28,7 +28,9 @@ public async Task Option_attribute_without_command_triggers_diagnostic( TestRunt
             // (9,6): warning UNC001: Property attribute OptionAttribute is only allowed on a property in a type attributed with a command attribute. This use will be ignored by the generator.
             analyzerTest.ExpectedDiagnostics.AddRange(
                [
-                   new DiagnosticResult("UNC001", DiagnosticSeverity.Warning).WithLocation(9, 6),
+                   new DiagnosticResult("UNC001", DiagnosticSeverity.Error)
+                       .WithSpan(9, 6, 9, 93)
+                       .WithArguments("OptionAttribute"),
                ]
            );
 
@@ -43,12 +45,17 @@ public async Task FileValidation_attribute_without_command_triggers_diagnostic(
             SourceText txt = GetSourceText( nameof(FileValidation_attribute_without_command_triggers_diagnostic), "input.cs" );
             var analyzerTest = CreateTestRunner( txt, testRuntime );
 
+            // (9,6): warning UNC001: Property attribute OptionAttribute is only allowed on a property in a type attributed with a command attribute. This use will be ignored by the generator.
             // (10,6): warning UNC001: Property attribute FileValidationAttribute is only allowed on a property in a type attributed with a command attribute. This use will be ignored by the generator.
-            // (10,6): error UNC002: Property attribute FileValidationAttribute is not allowed on a property independent of a qualifying attribute such as OptionAttribute.
             analyzerTest.ExpectedDiagnostics.AddRange(
                [
-                   new DiagnosticResult("UNC001", DiagnosticSeverity.Warning).WithLocation(10, 6),
-                   new DiagnosticResult("UNC002", DiagnosticSeverity.Error).WithLocation(10, 6),
+                   new DiagnosticResult("UNC001", DiagnosticSeverity.Error)
+                      .WithArguments("OptionAttribute")
+                      .WithSpan(9, 6, 9, 51),
+
+                   new DiagnosticResult("UNC001", DiagnosticSeverity.Error)
+                       .WithArguments("FileValidationAttribute")
+                       .WithSpan(10, 6, 10, 51),
                ]
            );
 
@@ -63,14 +70,19 @@ public async Task FolderValidation_attribute_without_command_triggers_diagnostic
             SourceText txt = GetSourceText( nameof(FolderValidation_attribute_without_command_triggers_diagnostic), "input.cs" );
             var analyzerTest = CreateTestRunner( txt, testRuntime );
 
-            // (10,6): warning UNC001: Property attribute FolderValidationAttribute is only allowed on a property in a type attributed with a command attribute. This use will be ignored by the generator.
-            // (10,6): error UNC002: Property attribute FolderValidationAttribute is not allowed on a property independent of a qualifying attribute such as OptionAttribute.
+            // (11,6): error UNC001: Property attribute FolderValidationAttribute is only allowed on a property in a type attributed with a command attribute. This use will be ignored by the generator.
+            // (11,6): error UNC002: Property attribute FolderValidationAttribute is not allowed on a property independent of a qualifying attribute such as OptionAttribute.
             analyzerTest.ExpectedDiagnostics.AddRange(
                [
-                   new DiagnosticResult("UNC001", DiagnosticSeverity.Warning).WithLocation(10, 6),
-                   new DiagnosticResult("UNC002", DiagnosticSeverity.Error).WithLocation(10, 6),
+                   new DiagnosticResult("UNC001", DiagnosticSeverity.Error)
+                       .WithSpan(11, 6, 11, 55)
+                       .WithArguments("FolderValidationAttribute"),
+
+                   new DiagnosticResult("UNC002", DiagnosticSeverity.Error)
+                       .WithSpan(11, 6, 11, 55)
+                       .WithArguments("FolderValidationAttribute"),
                ]
-           );
+             );
 
             await analyzerTest.RunAsync( TestContext.CancellationToken );
         }
@@ -86,6 +98,45 @@ public async Task GoldenPath_produces_no_diagnostics( TestRuntime testRuntime )
             await analyzerTest.RunAsync( TestContext.CancellationToken );
         }
 
+        [TestMethod]
+        [DataRow( TestRuntime.Net8_0 )]
+        [DataRow( TestRuntime.Net10_0 )]
+        public async Task FileValidation_with_wrong_type_produces_UNC002( TestRuntime testRuntime )
+        {
+            SourceText txt = GetSourceText( nameof(FileValidation_with_wrong_type_produces_UNC002), "input.cs" );
+            var analyzerTest = CreateTestRunner( txt, testRuntime );
+
+            // (9,6): error UNC003: Property attribute '{0}' requires a property of type '{1}'.
+            analyzerTest.ExpectedDiagnostics.AddRange(
+               [
+                   new DiagnosticResult("UNC003", DiagnosticSeverity.Error)
+                      .WithSpan(9, 6, 9, 51),
+               ]
+             );
+
+            await analyzerTest.RunAsync( TestContext.CancellationToken );
+        }
+
+        [TestMethod]
+        [DataRow( TestRuntime.Net8_0 )]
+        [DataRow( TestRuntime.Net10_0 )]
+        public async Task Required_nullable_types_produce_diagnostic( TestRuntime testRuntime )
+        {
+            SourceText txt = GetSourceText( nameof(Required_nullable_types_produce_diagnostic), "input.cs" );
+            var analyzerTest = CreateTestRunner( txt, testRuntime );
+
+            // (9,6): error UNC003: Property attribute '{0}' requires a property of type '{1}'.
+            analyzerTest.ExpectedDiagnostics.AddRange(
+               [
+                   new DiagnosticResult("UNC004", DiagnosticSeverity.Warning)
+                      .WithSpan(9, 6, 9, 127)
+                      .WithArguments("bool?", "OptionAttribute"),
+               ]
+             );
+
+            await analyzerTest.RunAsync( TestContext.CancellationToken );
+        }
+
         // TODO: Test that a nullable value is not marked as required. (That's a conflicting claim, if it's required it can't be null)
         //       A nullable type MAY have a default value handler to provide a null default. Additional test - anything with a default
         //       value provider shouldn't be "required" it's also nonsensical.
@@ -117,6 +168,13 @@ private AnalyzerTest<MsTestVerifier> CreateTestRunner( SourceText source, TestRu
             };
         }
 
+        // Sadly, at this time the test infrastructure doesn't provide support for
+        // testing the help URI
+        //private static string FormatHelpUri( string id )
+        //{
+        //    return $"https://ubiquitydotnet.github.io/Ubiquity.NET.Utils/CommandLine/diagnostics/{id}.html";
+        //}
+
         private static SourceText GetSourceText( params string[] nameParts )
         {
             return TestHelpers.GetTestText( nameof( CommandAnalyzerTests ), nameParts );
diff --git a/src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/CommandAnalyzerTests/FileValidation_attribute_without_command_triggers_diagnostic/input.cs b/src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/CommandAnalyzerTests/FileValidation_attribute_without_command_triggers_diagnostic/input.cs
index 51333e9..882afe1 100644
--- a/src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/CommandAnalyzerTests/FileValidation_attribute_without_command_triggers_diagnostic/input.cs
+++ b/src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/CommandAnalyzerTests/FileValidation_attribute_without_command_triggers_diagnostic/input.cs
@@ -6,7 +6,7 @@ namespace TestNamespace;
 
 internal class testInput1
 {
-    // This attribute alone should trigger UNC002 - Property attribute FileValidation is not allowed on a property independent of a qualifying attribute such as OptionAttribute.
-    [FileValidation( FileValidation.ExistingOnly )]
+    [Option( "-o", Description = "Test SomePath" )]
+    [FileValidation( FileValidation.ExistingOnly )] // UNC002 : Property attribute not allowed standalone
     public required FileInfo SomePath { get; init; }
 }
diff --git a/src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/CommandAnalyzerTests/FileValidation_with_wrong_type_produces_UNC002/input.cs b/src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/CommandAnalyzerTests/FileValidation_with_wrong_type_produces_UNC002/input.cs
new file mode 100644
index 0000000..caf9cf8
--- /dev/null
+++ b/src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/CommandAnalyzerTests/FileValidation_with_wrong_type_produces_UNC002/input.cs
@@ -0,0 +1,11 @@
+using Ubiquity.NET.CommandLine.GeneratorAttributes;
+
+namespace TestNamespace;
+
+[RootCommand( Description = "Root command for tests" )]
+internal class testInput1
+{
+    [Option( "-o", Description = "Test SomePath" )]
+    [FileValidation( FileValidation.ExistingOnly )] // UNC003 - Property attribute 'FileValidation' requires a property of type 'FileInfo'.
+    public required string SomePath { get; init; }
+}
diff --git a/src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/CommandAnalyzerTests/FolderValidation_attribute_without_command_triggers_diagnostic/input.cs b/src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/CommandAnalyzerTests/FolderValidation_attribute_without_command_triggers_diagnostic/input.cs
index 51333e9..4c6c928 100644
--- a/src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/CommandAnalyzerTests/FolderValidation_attribute_without_command_triggers_diagnostic/input.cs
+++ b/src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/CommandAnalyzerTests/FolderValidation_attribute_without_command_triggers_diagnostic/input.cs
@@ -6,7 +6,8 @@ namespace TestNamespace;
 
 internal class testInput1
 {
-    // This attribute alone should trigger UNC002 - Property attribute FileValidation is not allowed on a property independent of a qualifying attribute such as OptionAttribute.
-    [FileValidation( FileValidation.ExistingOnly )]
-    public required FileInfo SomePath { get; init; }
+    // UNC001 - Property attribute 'FolderValidation' is only allowed on a property in a type attributed with a command attribute. This use will be ignored by the generator.
+    // UNC002 - Property attribute 'FolderValidationAttribute' is not allowed on a property independent of a qualifying attribute such as OptionAttribute.
+    [FolderValidation( FolderValidation.ExistingOnly )]
+    public required DirectoryInfo SomePath { get; init; }
 }
diff --git a/src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/CommandAnalyzerTests/Required_nullable_types_produce_diagnostic/input.cs b/src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/CommandAnalyzerTests/Required_nullable_types_produce_diagnostic/input.cs
new file mode 100644
index 0000000..7d00c40
--- /dev/null
+++ b/src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/CommandAnalyzerTests/Required_nullable_types_produce_diagnostic/input.cs
@@ -0,0 +1,14 @@
+using Ubiquity.NET.CommandLine.GeneratorAttributes;
+
+namespace TestNamespace;
+
+[RootCommand( Description = "Root command for tests" )]
+internal partial class TestOptions
+{
+    // Warning : UNC004 : Type 'bool?' for property 'Thing1' is nullable but marked as required; These annotations conflict resulting in behavior that is explicitly UNDEFINED.
+    [Option( "--thing1", Aliases = [ "-t" ], Required = true, Description = "Test Thing1", HelpName = "Help name for thing1" )]
+    public bool? Thing1 { get; init; }
+
+    [Option( "--thing2", Aliases = [ "-t" ], Description = "Test Thing2", HelpName = "Help name for thing2" )]
+    public bool Thing2 { get; init; }
+}
diff --git a/src/Ubiquity.NET.CommandLine.SrcGen.UT/Ubiquity.NET.CommandLine.SrcGen.UT.csproj b/src/Ubiquity.NET.CommandLine.SrcGen.UT/Ubiquity.NET.CommandLine.SrcGen.UT.csproj
index 468c5e5..edd2aed 100644
--- a/src/Ubiquity.NET.CommandLine.SrcGen.UT/Ubiquity.NET.CommandLine.SrcGen.UT.csproj
+++ b/src/Ubiquity.NET.CommandLine.SrcGen.UT/Ubiquity.NET.CommandLine.SrcGen.UT.csproj
@@ -29,23 +29,27 @@
   </ItemGroup>
 
   <ItemGroup>
+    <Compile Remove="TestFiles\CommandAnalyzerTests\FileValidation_with_wrong_type_produces_UNC002\input.cs" />
     <Compile Remove="TestFiles\CommandAnalyzerTests\FolderValidation_attribute_without_command_triggers_diagnostic\input.cs" />
     <Compile Remove="TestFiles\CommandAnalyzerTests\FileValidation_attribute_without_command_triggers_diagnostic\input.cs" />
     <Compile Remove="TestFiles\CommandAnalyzerTests\GoldenPath_produces_no_diagnostics\input.cs" />
     <Compile Remove="TestFiles\CommandAnalyzerTests\Option_attribute_without_command_triggers_diagnostic\input.cs" />
     <Compile Remove="TestFiles\RootCommandAttributeTests\Basic_golden_path_succeeds\expected.cs" />
     <Compile Remove="TestFiles\RootCommandAttributeTests\Basic_golden_path_succeeds\Input.cs" />
+    <Compile Remove="TestFiles\CommandAnalyzerTests\Required_nullable_types_produce_diagnostic\input.cs" />
     <Compile Remove="TestFiles\RootCommandAttributeTests\Generator_handles_nullable_types\expected.cs" />
     <Compile Remove="TestFiles\RootCommandAttributeTests\Generator_handles_nullable_types\input.cs" />
   </ItemGroup>
 
   <ItemGroup>
+    <EmbeddedResource Include="TestFiles\CommandAnalyzerTests\FileValidation_with_wrong_type_produces_UNC002\input.cs" />
     <EmbeddedResource Include="TestFiles\CommandAnalyzerTests\FolderValidation_attribute_without_command_triggers_diagnostic\input.cs" />
     <EmbeddedResource Include="TestFiles\CommandAnalyzerTests\FileValidation_attribute_without_command_triggers_diagnostic\input.cs" />
     <EmbeddedResource Include="TestFiles\CommandAnalyzerTests\GoldenPath_produces_no_diagnostics\input.cs" />
     <EmbeddedResource Include="TestFiles\CommandAnalyzerTests\Option_attribute_without_command_triggers_diagnostic\input.cs" />
     <EmbeddedResource Include="TestFiles\RootCommandAttributeTests\Basic_golden_path_succeeds\expected.cs" />
     <EmbeddedResource Include="TestFiles\RootCommandAttributeTests\Basic_golden_path_succeeds\input.cs" />
+    <EmbeddedResource Include="TestFiles\CommandAnalyzerTests\Required_nullable_types_produce_diagnostic\input.cs" />
     <EmbeddedResource Include="TestFiles\RootCommandAttributeTests\Generator_handles_nullable_types\expected.cs" />
     <EmbeddedResource Include="TestFiles\RootCommandAttributeTests\Generator_handles_nullable_types\input.cs" />
   </ItemGroup>
diff --git a/src/Ubiquity.NET.CommandLine.SrcGen/AnalyzerReleases.Unshipped.md b/src/Ubiquity.NET.CommandLine.SrcGen/AnalyzerReleases.Unshipped.md
index 43c19e2..42acf4f 100644
--- a/src/Ubiquity.NET.CommandLine.SrcGen/AnalyzerReleases.Unshipped.md
+++ b/src/Ubiquity.NET.CommandLine.SrcGen/AnalyzerReleases.Unshipped.md
@@ -6,6 +6,7 @@
 Rule ID | Category | Severity | Notes
 --------|----------|----------|-------
 UNC000 | Internal | Error | Diagnostics, [Documentation](NotConfigurable)
-UNC001 | Usage | Warning | Diagnostics, [Documentation](Unnecessary)
+UNC001 | Usage | Error | Diagnostics
 UNC002 | Usage | Error | Diagnostics, [Documentation](NotConfigurable)
-UNC003 | Usage | Error | Diagnostics, [Documentation](NotConfigurable)
\ No newline at end of file
+UNC003 | Usage | Error | Diagnostics, [Documentation](NotConfigurable)
+UNC004 | Usage | Warning | Diagnostics, [Documentation](Compiler)
diff --git a/src/Ubiquity.NET.CommandLine.SrcGen/CommandLineAnalyzer.cs b/src/Ubiquity.NET.CommandLine.SrcGen/CommandLineAnalyzer.cs
index 1cefca8..0320148 100644
--- a/src/Ubiquity.NET.CommandLine.SrcGen/CommandLineAnalyzer.cs
+++ b/src/Ubiquity.NET.CommandLine.SrcGen/CommandLineAnalyzer.cs
@@ -104,18 +104,29 @@ private static void OnProperty( SymbolAnalysisContext context )
             // not the behavior intended...
         }
 
-        private static void OnOptionAttribute( SymbolAnalysisContext context, IPropertySymbol symbol, Location? attribLoc, EquatableAttributeDataCollection _ )
+        private static void OnOptionAttribute(
+            SymbolAnalysisContext context,
+            IPropertySymbol symbol,
+            Location? attribLoc,
+            EquatableAttributeDataCollection attributes
+            )
         {
             // Verify it is applied to a property in a type with a command attribute
             VerifyCommandAttribute( context, attribLoc, Constants.OptionAttribute.SimpleName );
 
+            EquatableAttributeData attribute = attributes[Constants.OptionAttribute];
+            VerifyNotNullableRequired( context, symbol, attribute, attribLoc, Constants.OptionAttribute.SimpleName );
+
             // Additional validations...
         }
 
-        private static void OnFileValidationAttribute( SymbolAnalysisContext context, IPropertySymbol symbol, Location? attribLoc, EquatableAttributeDataCollection attribs )
+        private static void OnFileValidationAttribute(
+            SymbolAnalysisContext context,
+            IPropertySymbol symbol,
+            Location? attribLoc,
+            EquatableAttributeDataCollection attribs
+            )
         {
-            Location? propertyLoc = symbol.Locations.Length < 1 ? default : symbol.Locations[0];
-
             // Verify it is applied to a property in a type with a command attribute
             VerifyCommandAttribute( context, attribLoc, Constants.FileValidationAttribute.SimpleName );
 
@@ -123,23 +134,26 @@ private static void OnFileValidationAttribute( SymbolAnalysisContext context, IP
             VerifyHasConstrainedAttribute( context, attribLoc, attribs );
 
             // Verify type of the property is System.IO.FileInfo.
-            VerifyPropertyType( context, symbol, Constants.FileInfo, propertyLoc, Constants.FileValidationAttribute.SimpleName );
+            VerifyRequiredPropertyType( context, symbol, Constants.FileInfo, attribLoc, Constants.FileValidationAttribute.SimpleName );
 
             // Additional validations...
         }
 
-        private static void OnFolderValidationAttribute( SymbolAnalysisContext context, IPropertySymbol symbol, Location? attribLoc, EquatableAttributeDataCollection attribs )
+        private static void OnFolderValidationAttribute(
+            SymbolAnalysisContext context,
+            IPropertySymbol symbol,
+            Location? attribLoc,
+            EquatableAttributeDataCollection attribs
+            )
         {
-            Location? propertyLoc = symbol.Locations.Length < 1 ? default : symbol.Locations[0];
-
             // Verify it is applied to a property in a type with a command attribute
             VerifyCommandAttribute( context, attribLoc, Constants.FolderValidationAttribute.SimpleName );
 
             // Verify an Option property (or maybe an argument attribute once supported) exists
-            VerifyHasConstrainedAttribute( context, propertyLoc, attribs );
+            VerifyHasConstrainedAttribute( context, attribLoc, attribs );
 
             // Verify type of the property is System.IO.FileInfo.
-            VerifyPropertyType( context, symbol, Constants.DirectoryInfo, propertyLoc, Constants.FolderValidationAttribute.SimpleName );
+            VerifyRequiredPropertyType( context, symbol, Constants.DirectoryInfo, attribLoc, Constants.FolderValidationAttribute.SimpleName );
 
             // Additional validations...
         }
@@ -167,19 +181,19 @@ private static void VerifyCommandAttribute( SymbolAnalysisContext context, Locat
 
         /// <summary>Verifies (and reports a diagnostic if not) that a set of attributes contains a required constraint</summary>
         /// <param name="context">Context to use for reporting diagnostics</param>
-        /// <param name="diagnosticLoc">Location to use for any diagnostics reported</param>
+        /// <param name="attribLoc">Location to use for any diagnostics reported</param>
         /// <param name="attribs">Set of attributes to check</param>
         /// <remarks>
-        /// The <paramref name="diagnosticLoc"/> is used to report diagnostics that normally references the attribute
+        /// The <paramref name="attribLoc"/> is used to report diagnostics that normally references the attribute
         /// that is missing the constrained attribute. (ex. The `FileValidation` or `FolderValidation` that does NOT
         /// have an `OptionAttribute` that it validates.
         /// </remarks>
-        private static void VerifyHasConstrainedAttribute( SymbolAnalysisContext context, Location? diagnosticLoc, EquatableAttributeDataCollection attribs )
+        private static void VerifyHasConstrainedAttribute( SymbolAnalysisContext context, Location? attribLoc, EquatableAttributeDataCollection attribs )
         {
             // Verify an Option property (or maybe an argument attribute once supported) exists
             if(!attribs.TryGetValue( Constants.OptionAttribute, out _ ))
             {
-                ReportDiagnostic( context, Diagnostics.MissingConstraintAttribute, diagnosticLoc, Constants.FileValidationAttribute.SimpleName );
+                ReportDiagnostic( context, Diagnostics.MissingConstraintAttribute, attribLoc, Constants.FileValidationAttribute.SimpleName );
             }
         }
 
@@ -194,7 +208,13 @@ private static void VerifyHasConstrainedAttribute( SymbolAnalysisContext context
         /// reported. That is, they reference the attribute that requires a specific type. It is debatable
         /// if the location of the return type is wrong or the attribute is wrong...
         /// </remarks>
-        private static void VerifyPropertyType( SymbolAnalysisContext context, IPropertySymbol symbol, NamespaceQualifiedName expectedType, Location? attribLoc, string attribName )
+        private static void VerifyRequiredPropertyType(
+            SymbolAnalysisContext context,
+            IPropertySymbol symbol,
+            NamespaceQualifiedName expectedType,
+            Location? attribLoc,
+            string attribName
+            )
         {
             if(symbol.Type.GetNamespaceQualifiedName() != expectedType)
             {
@@ -202,6 +222,25 @@ private static void VerifyPropertyType( SymbolAnalysisContext context, IProperty
             }
         }
 
+        private static void VerifyNotNullableRequired(
+            SymbolAnalysisContext context,
+            IPropertySymbol property,
+            EquatableAttributeData attribute,
+            Location? attribLoc,
+            string simpleName
+            )
+        {
+            if(attribute.NamedArguments.TryGetValue( Constants.CommonAttributeNamedArgs.Required, out StructurallyEquatableTypedConstant tc ))
+            {
+                bool isRequired = !tc.IsNull && (bool)tc.Value!;
+                NamespaceQualifiedTypeName propType = property.Type.GetNamespaceQualifiedName();
+                if(isRequired && propType.IsNullable)
+                {
+                    ReportDiagnostic( context, Diagnostics.RequiredNullableType, attribLoc, $"{propType:A}", simpleName );
+                }
+            }
+        }
+
         private static void ReportDiagnostic( SymbolAnalysisContext context, DiagnosticDescriptor descriptor, Location? loc, params object[] args )
         {
             context.ReportDiagnostic( Diagnostic.Create( descriptor, loc, args ) );
diff --git a/src/Ubiquity.NET.CommandLine.SrcGen/Diagnostics.cs b/src/Ubiquity.NET.CommandLine.SrcGen/Diagnostics.cs
index c08411c..d12e795 100644
--- a/src/Ubiquity.NET.CommandLine.SrcGen/Diagnostics.cs
+++ b/src/Ubiquity.NET.CommandLine.SrcGen/Diagnostics.cs
@@ -15,6 +15,7 @@ internal static class IDs
             internal const string MissingCommandAttribute = "UNC001";
             internal const string MissingConstraintAttribute = "UNC002";
             internal const string IncorrectPropertyType = "UNC003";
+            internal const string RequiredNullableType = "UNC004";
         }
 
         private static LocalizableResourceString Localized( string resName )
@@ -27,7 +28,8 @@ internal static ImmutableArray<DiagnosticDescriptor> CommandLineAnalyzerDiagnost
                 InternalError,
                 MissingCommandAttribute,
                 MissingConstraintAttribute,
-                IncorrectPropertyType
+                IncorrectPropertyType,
+                RequiredNullableType,
             ];
 
         internal static readonly DiagnosticDescriptor InternalError = new(
@@ -38,6 +40,7 @@ internal static ImmutableArray<DiagnosticDescriptor> CommandLineAnalyzerDiagnost
             defaultSeverity: DiagnosticSeverity.Error,
             isEnabledByDefault: true,
             description: Localized(nameof(Resources.InternalError_Description)),
+            helpLinkUri: FormatHelpUri(IDs.InternalError),
             WellKnownDiagnosticTags.NotConfigurable
             );
 
@@ -46,9 +49,10 @@ internal static ImmutableArray<DiagnosticDescriptor> CommandLineAnalyzerDiagnost
             title: Localized(nameof(Resources.MissingCommandAttribute_Title)),
             messageFormat: Localized(nameof(Resources.MissingCommandAttribute_MessageFormat)),
             category: "Usage",
-            defaultSeverity: DiagnosticSeverity.Warning,
+            defaultSeverity: DiagnosticSeverity.Error,
             isEnabledByDefault: true,
             description: Localized(nameof(Resources.MissingCommandAttribute_Description)),
+            helpLinkUri: FormatHelpUri(IDs.MissingCommandAttribute),
             WellKnownDiagnosticTags.Unnecessary,
             WellKnownDiagnosticTags.Compiler
             );
@@ -61,6 +65,7 @@ internal static ImmutableArray<DiagnosticDescriptor> CommandLineAnalyzerDiagnost
             defaultSeverity: DiagnosticSeverity.Error,
             isEnabledByDefault: true,
             description: Localized(nameof(Resources.MissingConstraintAttribute_Description)),
+            helpLinkUri: FormatHelpUri(IDs.MissingConstraintAttribute),
             WellKnownDiagnosticTags.Compiler
             );
 
@@ -72,8 +77,27 @@ internal static ImmutableArray<DiagnosticDescriptor> CommandLineAnalyzerDiagnost
             defaultSeverity: DiagnosticSeverity.Error,
             isEnabledByDefault: true,
             description: Localized(nameof(Resources.IncorrectPropertyType_Description)),
+            helpLinkUri: FormatHelpUri(IDs.IncorrectPropertyType),
             WellKnownDiagnosticTags.Compiler,
             WellKnownDiagnosticTags.NotConfigurable
             );
+
+        // Type '{0}' for property '{1}' is nullable but marked as required; These annotations conflict resulting in behavior that is explicitly UNDEFINED.
+        internal static readonly DiagnosticDescriptor RequiredNullableType = new(
+            id: IDs.RequiredNullableType,
+            title: Localized(nameof(Resources.RequiredNullableType_Title)),
+            messageFormat: Localized(nameof(Resources.RequiredNullableType_MessageFormat)),
+            category: "Usage",
+            defaultSeverity: DiagnosticSeverity.Warning,
+            isEnabledByDefault: true,
+            description: Localized(nameof(Resources.RequiredNullableType_Description)),
+            helpLinkUri: FormatHelpUri(IDs.RequiredNullableType),
+            WellKnownDiagnosticTags.Compiler
+            );
+
+        private static string FormatHelpUri(string id)
+        {
+            return $"https://ubiquitydotnet.github.io/Ubiquity.NET.Utils/CommandLine/diagnostics/{id}.html";
+        }
     }
 }
diff --git a/src/Ubiquity.NET.CommandLine.SrcGen/Properties/Resources.Designer.cs b/src/Ubiquity.NET.CommandLine.SrcGen/Properties/Resources.Designer.cs
index 39a5749..67b3508 100644
--- a/src/Ubiquity.NET.CommandLine.SrcGen/Properties/Resources.Designer.cs
+++ b/src/Ubiquity.NET.CommandLine.SrcGen/Properties/Resources.Designer.cs
@@ -167,5 +167,32 @@ internal static string MissingConstraintAttribute_Title {
                 return ResourceManager.GetString("MissingConstraintAttribute_Title", resourceCulture);
             }
         }
+        
+        /// <summary>
+        ///   Looks up a localized string similar to Property type is nullable but marked as required; These annotations conflict resulting in behavior that is explicitly UNDEFINED..
+        /// </summary>
+        internal static string RequiredNullableType_Description {
+            get {
+                return ResourceManager.GetString("RequiredNullableType_Description", resourceCulture);
+            }
+        }
+        
+        /// <summary>
+        ///   Looks up a localized string similar to attrib.NamedArguments.
+        /// </summary>
+        internal static string RequiredNullableType_MessageFormat {
+            get {
+                return ResourceManager.GetString("RequiredNullableType_MessageFormat", resourceCulture);
+            }
+        }
+        
+        /// <summary>
+        ///   Looks up a localized string similar to Property type is nullable but marked as required..
+        /// </summary>
+        internal static string RequiredNullableType_Title {
+            get {
+                return ResourceManager.GetString("RequiredNullableType_Title", resourceCulture);
+            }
+        }
     }
 }
diff --git a/src/Ubiquity.NET.CommandLine.SrcGen/Properties/Resources.resx b/src/Ubiquity.NET.CommandLine.SrcGen/Properties/Resources.resx
index 4f328de..7a8f367 100644
--- a/src/Ubiquity.NET.CommandLine.SrcGen/Properties/Resources.resx
+++ b/src/Ubiquity.NET.CommandLine.SrcGen/Properties/Resources.resx
@@ -1,17 +1,17 @@
 <?xml version="1.0" encoding="utf-8"?>
 <root>
-  <!-- 
-    Microsoft ResX Schema 
-    
+  <!--
+    Microsoft ResX Schema
+
     Version 2.0
-    
-    The primary goals of this format is to allow a simple XML format 
-    that is mostly human readable. The generation and parsing of the 
-    various data types are done through the TypeConverter classes 
+
+    The primary goals of this format is to allow a simple XML format
+    that is mostly human readable. The generation and parsing of the
+    various data types are done through the TypeConverter classes
     associated with the data types.
-    
+
     Example:
-    
+
     ... ado.net/XML headers & schema ...
     <resheader name="resmimetype">text/microsoft-resx</resheader>
     <resheader name="version">2.0</resheader>
@@ -26,36 +26,36 @@
         <value>[base64 mime encoded string representing a byte array form of the .NET Framework object]</value>
         <comment>This is a comment</comment>
     </data>
-                
-    There are any number of "resheader" rows that contain simple 
+
+    There are any number of "resheader" rows that contain simple
     name/value pairs.
-    
-    Each data row contains a name, and value. The row also contains a 
-    type or mimetype. Type corresponds to a .NET class that support 
-    text/value conversion through the TypeConverter architecture. 
-    Classes that don't support this are serialized and stored with the 
+
+    Each data row contains a name, and value. The row also contains a
+    type or mimetype. Type corresponds to a .NET class that support
+    text/value conversion through the TypeConverter architecture.
+    Classes that don't support this are serialized and stored with the
     mimetype set.
-    
-    The mimetype is used for serialized objects, and tells the 
-    ResXResourceReader how to depersist the object. This is currently not 
+
+    The mimetype is used for serialized objects, and tells the
+    ResXResourceReader how to depersist the object. This is currently not
     extensible. For a given mimetype the value must be set accordingly:
-    
-    Note - application/x-microsoft.net.object.binary.base64 is the format 
-    that the ResXResourceWriter will generate, however the reader can 
+
+    Note - application/x-microsoft.net.object.binary.base64 is the format
+    that the ResXResourceWriter will generate, however the reader can
     read any of the formats listed below.
-    
+
     mimetype: application/x-microsoft.net.object.binary.base64
-    value   : The object must be serialized with 
+    value   : The object must be serialized with
             : System.Runtime.Serialization.Formatters.Binary.BinaryFormatter
             : and then encoded with base64 encoding.
-    
+
     mimetype: application/x-microsoft.net.object.soap.base64
-    value   : The object must be serialized with 
+    value   : The object must be serialized with
             : System.Runtime.Serialization.Formatters.Soap.SoapFormatter
             : and then encoded with base64 encoding.
 
     mimetype: application/x-microsoft.net.object.bytearray.base64
-    value   : The object must be serialized into a byte array 
+    value   : The object must be serialized into a byte array
             : using a System.ComponentModel.TypeConverter
             : and then encoded with base64 encoding.
     -->
@@ -154,4 +154,13 @@
   <data name="IncorrectPropertyType_Title" xml:space="preserve">
     <value>Property has incorrect type for attribute</value>
   </data>
-</root>
\ No newline at end of file
+  <data name="RequiredNullableType_Description" xml:space="preserve">
+    <value>Property type is nullable but marked as required; These annotations conflict resulting in behavior that is explicitly UNDEFINED.</value>
+  </data>
+  <data name="RequiredNullableType_MessageFormat" xml:space="preserve">
+    <value>Type '{0}' for property '{1}' is nullable but marked as required; These annotations conflict resulting in behavior that is explicitly UNDEFINED.</value>
+  </data>
+  <data name="RequiredNullableType_Title" xml:space="preserve">
+    <value>Property type is nullable but marked as required.</value>
+  </data>
+</root>