Merge branch 'maintenance/code-analysis' into develop

Introduced polyfills for the new .NET 5 `[MemberNotNull()]` and `[MemberNotNullWhen()]` nullability attributes, and applied them to `Topic` in order to get rid of an irritating hack. Migrated the `CompatiblePropertyTopicViewModel` class, from the test project, to use an `init` setter, which not only alleviates a CA2227 warning, but is also more accurate in terms of how the property should be used.

Author: JeremyCaney

OnTopic.Tests/ViewModels/CompatiblePropertyTopicViewModel.cs

@@ -20,12 +20,11 @@ namespace OnTopic.Tests.ViewModels {
   /// <remarks>
   ///   This is a sample class intended for test purposes only; it is not designed for use in a production environment.
   /// </remarks>
-  [SuppressMessage("Usage", "CA2227", Justification = "This is intended to be initialized by the mapping service.")]
   public class CompatiblePropertyTopicViewModel {
 
     public ModelType ModelType { get; set; }
 
-    public Collection<DateTime>? VersionHistory { get; set; }
+    public Collection<DateTime>? VersionHistory { get; init; }
 
   } //Class
 } //Namespace

OnTopic/Internal/Diagnostics/MemberNotNullAttribute.cs

@@ -0,0 +1,45 @@
+/*==============================================================================================================================
+| Author        Ignia, LLC
+| Client        Ignia, LLC
+| Project       Topics Library
+\=============================================================================================================================*/
+namespace System.Diagnostics.CodeAnalysis {
+
+  /*============================================================================================================================
+  | CLASS: MEMBER NOT NULL (ATTRIBUTE)
+  \---------------------------------------------------------------------------------------------------------------------------*/
+  /// <summary>
+  ///   Specifies that the method or property will ensure that the listed field and property members have not-null values.
+  /// </summary>
+  [AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true)]
+  internal sealed class MemberNotNullAttribute : Attribute {
+
+    /*==========================================================================================================================
+    | CONSTRUCTOR
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <summary>Initializes the attribute with a field or property member.</summary>
+    /// <param name="member">
+    /// The field or property member that is promised to be not-null.
+    /// </param>
+    #pragma warning disable CA1019 // Define accessors for attribute arguments
+    public MemberNotNullAttribute(string member) {
+      Members = new[] { member };
+      }
+    #pragma warning restore CA1019 // Define accessors for attribute arguments
+
+    /// <summary>Initializes the attribute with the list of field and property members.</summary>
+    /// <param name="members">
+    /// The list of field and property members that are promised to be not-null.
+    /// </param>
+    public MemberNotNullAttribute(params string[] members) {
+      Members = members;
+    }
+
+    /*==========================================================================================================================
+    | PROPERTY: MEMBERS
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <summary>Gets field or property member names.</summary>
+    public string[] Members { get; }
+
+  }
+}

OnTopic/Internal/Diagnostics/MemberNotNullWhenAttribute.cs

@@ -0,0 +1,62 @@
+/*==============================================================================================================================
+| Author        Ignia, LLC
+| Client        Ignia, LLC
+| Project       Topics Library
+\=============================================================================================================================*/
+namespace System.Diagnostics.CodeAnalysis {
+
+  /*============================================================================================================================
+  | CLASS: MEMBER NOT NULL (ATTRIBUTE)
+  \---------------------------------------------------------------------------------------------------------------------------*/
+  /// <summary>
+  ///   Specifies that the method or property will ensure that the listed field and property members have not-null values when
+  ///   returning with the specified return value condition.
+  /// </summary>
+  [AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true)]
+  internal sealed class MemberNotNullWhenAttribute: Attribute {
+
+    /*==========================================================================================================================
+    | CONSTRUCTOR
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <summary>Initializes the attribute with the specified return value condition and a field or property member.</summary>
+    /// <param name="returnValue">
+    /// The return value condition. If the method returns this value, the associated parameter will not be null.
+    /// </param>
+    /// <param name="member">
+    /// The field or property member that is promised to be not-null.
+    /// </param>
+    #pragma warning disable CA1019 // Define accessors for attribute arguments
+    public MemberNotNullWhenAttribute(bool returnValue, string member) {
+      ReturnValue = returnValue;
+      Members = new[] { member };
+    }
+    #pragma warning restore CA1019 // Define accessors for attribute arguments
+
+    /// <summary>
+    ///   Initializes the attribute with the specified return value condition and list of field and property members.
+    /// </summary>
+    /// <param name="returnValue">
+    /// The return value condition. If the method returns this value, the associated parameter will not be null.
+    /// </param>
+    /// <param name="members">
+    /// The list of field and property members that are promised to be not-null.
+    /// </param>
+    public MemberNotNullWhenAttribute(bool returnValue, params string[] members) {
+      ReturnValue = returnValue;
+      Members = members;
+    }
+
+    /*==========================================================================================================================
+    | PROPERTY: RETURN VALUE
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <summary>Gets the return value condition.</summary>
+    public bool ReturnValue { get; }
+
+    /*==========================================================================================================================
+    | PROPERTY: MEMBERS
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <summary>Gets field or property member names.</summary>
+    public string[] Members { get; }
+
+  }
+}

OnTopic/Topic.cs

@@ -87,15 +87,6 @@ public Topic(string key, string contentType, Topic? parent = null, int id = -1)
         Parent                  = parent;
       }
 
-      /*------------------------------------------------------------------------------------------------------------------------
-      | Initialize key fields
-      \-----------------------------------------------------------------------------------------------------------------------*/
-      //###HACK JJC20190924: The local backing fields _key and _contentType are always initialized at this point. But Roslyn's
-      //flow analysis isn't smart enough to detect this. As such, the following effectively sets _key and _contentType to
-      //themselves.
-      _key                      = Key;
-      _contentType              = ContentType;
-
     }
 
     #region Core Properties
@@ -183,6 +174,7 @@ public Topic? Parent {
     /// </value>
     public string ContentType {
       get => _contentType;
+      [MemberNotNull(nameof(_contentType))]
       set {
         TopicFactory.ValidateKey(value);
         if (_contentType == value) {
@@ -215,6 +207,7 @@ public string ContentType {
     /// </requires>
     public string Key {
       get => _key;
+      [MemberNotNull(nameof(_key))]
       set {
         TopicFactory.ValidateKey(value);
         if (_key == value) {