Fixed generated properties within regions (#4227)

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## Fixes #4225

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## PR Type

What kind of change does this PR introduce?

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- Bugfix
<!-- - Feature -->
<!-- - Code style update (formatting) -->
<!-- - Refactoring (no functional changes, no api changes) -->
<!-- - Build or CI related changes -->
<!-- - Documentation content changes -->
<!-- - Sample app changes -->
<!-- - Other... Please describe: -->

## What is the current behavior?
Compilation fails If the Microsoft MVVM Toolkit source generator attribute `[ObservableProperty]` is in a `#region` block.
<!-- Please describe the current behavior that you are modifying, or link to a relevant issue. -->

## What is the new behavior?
The generated code is now correct when regions are used.
<!-- Describe how was this issue resolved or changed? -->

## PR Checklist

Please check if your PR fulfills the following requirements: <!-- and remove the ones that are not applicable to the current PR -->

- [X] Tested code with current [supported SDKs](../#supported)
- [X] New component
  - [X] Pull Request has been submitted to the documentation repository [instructions](../blob/main/Contributing.md#docs). Link: <!-- docs PR link -->
  - [X] Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
  - [X] If control, added to Visual Studio Design project
- [X] Sample in sample app has been added / updated (for bug fixes / features)
  - [X] Icon has been created (if new sample) following the [Thumbnail Style Guide and templates](https://github.com/CommunityToolkit/WindowsCommunityToolkit-design-assets)
- [X] New major technical changes in the toolkit have or will be added to the [Wiki](https://github.com/CommunityToolkit/WindowsCommunityToolkit/wiki) e.g. build changes, source generators, testing infrastructure, sample creation changes, etc...
- [X] Tests for the changes have been added (for bug fixes / features) (if applicable)
- [X] Header has been added to all new source files (run _build/UpdateHeaders.bat_)
- [X] Contains **NO** breaking changes

Author: msftbot[bot]

Microsoft.Toolkit.Mvvm.SourceGenerators/ComponentModel/ObservablePropertyGenerator.cs

@@ -443,7 +443,7 @@ private static PropertyDeclarationSyntax CreatePropertyDeclaration(
                     AttributeList(SingletonSeparatedList(Attribute(IdentifierName("global::System.Diagnostics.DebuggerNonUserCode")))),
                     AttributeList(SingletonSeparatedList(Attribute(IdentifierName("global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage")))))
                 .AddAttributeLists(validationAttributes.Select(static a => AttributeList(SingletonSeparatedList(a))).ToArray())
-                .WithLeadingTrivia(leadingTrivia)
+                .WithLeadingTrivia(leadingTrivia.Where(static trivia => !trivia.IsKind(SyntaxKind.RegionDirectiveTrivia) && !trivia.IsKind(SyntaxKind.EndRegionDirectiveTrivia)))
                 .AddModifiers(Token(SyntaxKind.PublicKeyword))
                 .AddAccessorListAccessors(
                     AccessorDeclaration(SyntaxKind.GetAccessorDeclaration)

UnitTests/UnitTests.NetCore/Mvvm/Test_ICommandAttribute.cs

@@ -8,6 +8,8 @@
 using Microsoft.Toolkit.Mvvm.Input;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 
+#pragma warning disable SA1124
+
 namespace UnitTests.Mvvm
 {
     [SuppressMessage("StyleCop.CSharp.DocumentationRules", "SA1601", Justification = "Type only used for testing")]
@@ -78,6 +80,8 @@ private async Task DelayAndIncrementCounterAsync()
                 Counter += 1;
             }
 
+            #region Test region
+
             /// <summary>
             /// This is multi line with also other stuff below
             /// </summary>
@@ -100,6 +104,8 @@ private async Task DelayAndIncrementCounterWithValueAsync(int count)
                 Counter += count;
             }
 
+            #endregion
+
             [ICommand]
             private async Task DelayAndIncrementCounterWithValueAndTokenAsync(int count, CancellationToken token)
             {

UnitTests/UnitTests.NetCore/Mvvm/Test_ObservablePropertyAttribute.cs

@@ -11,6 +11,8 @@
 using Microsoft.Toolkit.Mvvm.ComponentModel;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 
+#pragma warning disable SA1124
+
 #nullable enable
 
 namespace UnitTests.Mvvm
@@ -58,6 +60,86 @@ public void Test_ObservablePropertyAttribute_Events()
             Assert.AreEqual(changed.Item2, 42);
         }
 
+        // See https://github.com/CommunityToolkit/WindowsCommunityToolkit/issues/4225
+        [TestCategory("Mvvm")]
+        [TestMethod]
+        public void Test_ObservablePropertyAttributeWithinRegion_Events()
+        {
+            var model = new SampleModel();
+
+            (PropertyChangingEventArgs, int) changing = default;
+            (PropertyChangedEventArgs, int) changed = default;
+
+            model.PropertyChanging += (s, e) =>
+            {
+                Assert.IsNull(changing.Item1);
+                Assert.IsNull(changed.Item1);
+                Assert.AreSame(model, s);
+                Assert.IsNotNull(s);
+                Assert.IsNotNull(e);
+
+                changing = (e, model.Counter);
+            };
+
+            model.PropertyChanged += (s, e) =>
+            {
+                Assert.IsNotNull(changing.Item1);
+                Assert.IsNull(changed.Item1);
+                Assert.AreSame(model, s);
+                Assert.IsNotNull(s);
+                Assert.IsNotNull(e);
+
+                changed = (e, model.Counter);
+            };
+
+            model.Counter = 42;
+
+            Assert.AreEqual(changing.Item1?.PropertyName, nameof(SampleModel.Counter));
+            Assert.AreEqual(changing.Item2, 0);
+            Assert.AreEqual(changed.Item1?.PropertyName, nameof(SampleModel.Counter));
+            Assert.AreEqual(changed.Item2, 42);
+        }
+
+        // See https://github.com/CommunityToolkit/WindowsCommunityToolkit/issues/4225
+        [TestCategory("Mvvm")]
+        [TestMethod]
+        public void Test_ObservablePropertyAttributeRightBelowRegion_Events()
+        {
+            var model = new SampleModel();
+
+            (PropertyChangingEventArgs, string?) changing = default;
+            (PropertyChangedEventArgs, string?) changed = default;
+
+            model.PropertyChanging += (s, e) =>
+            {
+                Assert.IsNull(changing.Item1);
+                Assert.IsNull(changed.Item1);
+                Assert.AreSame(model, s);
+                Assert.IsNotNull(s);
+                Assert.IsNotNull(e);
+
+                changing = (e, model.Name);
+            };
+
+            model.PropertyChanged += (s, e) =>
+            {
+                Assert.IsNotNull(changing.Item1);
+                Assert.IsNull(changed.Item1);
+                Assert.AreSame(model, s);
+                Assert.IsNotNull(s);
+                Assert.IsNotNull(e);
+
+                changed = (e, model.Name);
+            };
+
+            model.Name = "Bob";
+
+            Assert.AreEqual(changing.Item1?.PropertyName, nameof(SampleModel.Name));
+            Assert.AreEqual(changing.Item2, null);
+            Assert.AreEqual(changed.Item1?.PropertyName, nameof(SampleModel.Name));
+            Assert.AreEqual(changed.Item2, "Bob");
+        }
+
         [TestCategory("Mvvm")]
         [TestMethod]
         public void Test_AlsoNotifyChangeForAttribute_Events()
@@ -162,6 +244,16 @@ public partial class SampleModel : ObservableObject
             /// </summary>
             [ObservableProperty]
             private int data;
+
+            #region More properties
+
+            [ObservableProperty]
+            private int counter;
+
+            #endregion
+
+            [ObservableProperty]
+            private string? name;
         }
 
         [INotifyPropertyChanged]