Handle lazy fields too

And adds tests.

Author: AArnott

src/ImmutableObjectGraph.Generation.Tests/TestSources/FileSystem.Tests.cs

@@ -572,6 +572,45 @@ public void RootedStruct_EqualityOperators()
             Assert.True(r1a != r1Rerooted);
         }
 
+        [Fact]
+        public void Builder_INotifyPropertyChanged_SimpleValue()
+        {
+            var builder = FileSystemFile.CreateBuilder();
+            var raisedName = new Stack<string>();
+            builder.PropertyChanged += (s, e) => raisedName.Push(e.PropertyName);
+
+            // Set a value property and assert it raises the changed event once.
+            builder.PathSegment = "fo";
+            Assert.Equal(nameof(builder.PathSegment), raisedName.Pop());
+            Assert.Equal(0, raisedName.Count);
+
+            // Set it to the same value and assert no event is raised.
+            builder.PathSegment = builder.PathSegment;
+            Assert.Equal(0, raisedName.Count);
+        }
+
+        [Fact]
+        public void Builder_INotifyPropertyChanged_LazyImmutable()
+        {
+            var builder = FileSystemFile.CreateBuilder();
+            var raisedName = new Stack<string>();
+            builder.PropertyChanged += (s, e) => raisedName.Push(e.PropertyName);
+
+            // Set a Builder property and assert it raises the changed event once.
+            builder.Data = RichData.CreateBuilder();
+            Assert.Equal(nameof(builder.Data), raisedName.Pop());
+            Assert.Equal(0, raisedName.Count);
+
+            // Set it to the same value and assert no event is raised.
+            builder.Data = builder.Data;
+            Assert.Equal(0, raisedName.Count);
+
+            // Set it to another value.
+            builder.Data = RichData.CreateBuilder();
+            Assert.Equal(nameof(builder.Data), raisedName.Pop());
+            Assert.Equal(0, raisedName.Count);
+        }
+
         private static void VerifyDescendentsShareRoot(RootedFileSystemDirectory directory)
         {
             foreach (var child in directory)

src/ImmutableObjectGraph.Generation/CodeGen+BuilderGen.cs

@@ -193,11 +193,12 @@ protected IReadOnlyList<MemberDeclarationSyntax> CreateMutableProperties()
                 foreach (var field in this.generator.applyToMetaType.LocalFields)
                 {
                     var thisField = Syntax.ThisDot(field.NameAsField);
+                    var optionalFieldNotYetDefined = SyntaxFactory.PrefixUnaryExpression(SyntaxKind.LogicalNotExpression, Syntax.OptionalIsDefined(thisField));
                     var getterBlock = field.IsGeneratedImmutableType
                         ? SyntaxFactory.Block(
                             // if (!this.fieldName.IsDefined) {
                             SyntaxFactory.IfStatement(
-                                SyntaxFactory.PrefixUnaryExpression(SyntaxKind.LogicalNotExpression, Syntax.OptionalIsDefined(thisField)),
+                                optionalFieldNotYetDefined,
                                 SyntaxFactory.Block(
                                     // this.fieldName = this.immutable.fieldName?.ToBuilder();
                                     SyntaxFactory.ExpressionStatement(SyntaxFactory.AssignmentExpression(
@@ -213,17 +214,32 @@ protected IReadOnlyList<MemberDeclarationSyntax> CreateMutableProperties()
                                                 SyntaxFactory.ArgumentList())))))),
                             SyntaxFactory.ReturnStatement(Syntax.OptionalValue(thisField)))
                         : SyntaxFactory.Block(SyntaxFactory.ReturnStatement(thisField));
-                    var setterBlock = SyntaxFactory.Block(
-                        SyntaxFactory.ExpressionStatement(SyntaxFactory.AssignmentExpression(
-                            SyntaxKind.SimpleAssignmentExpression,
+                    var setterCondition = field.IsGeneratedImmutableType
+                        ? SyntaxFactory.BinaryExpression(
+                            SyntaxKind.LogicalOrExpression,
+                            optionalFieldNotYetDefined,
+                            SyntaxFactory.BinaryExpression(
+                                SyntaxKind.NotEqualsExpression,
+                                Syntax.OptionalValue(thisField),
+                                SyntaxFactory.IdentifierName("value")))
+                        : SyntaxFactory.BinaryExpression(
+                            SyntaxKind.NotEqualsExpression,
                             thisField,
-                            SyntaxFactory.IdentifierName("value"))),
-                        SyntaxFactory.ExpressionStatement(
-                            SyntaxFactory.InvocationExpression(
-                                SyntaxFactory.MemberAccessExpression(
-                                    SyntaxKind.SimpleMemberAccessExpression,
-                                    SyntaxFactory.ThisExpression(),
-                                    OnPropertyChangedMethodName))));
+                            SyntaxFactory.IdentifierName("value"));
+                    var setterBlock = SyntaxFactory.Block(
+                        SyntaxFactory.IfStatement(
+                            setterCondition,
+                            SyntaxFactory.Block(
+                                SyntaxFactory.ExpressionStatement(SyntaxFactory.AssignmentExpression(
+                                    SyntaxKind.SimpleAssignmentExpression,
+                                    thisField,
+                                    SyntaxFactory.IdentifierName("value"))),
+                                SyntaxFactory.ExpressionStatement(
+                                    SyntaxFactory.InvocationExpression(
+                                        SyntaxFactory.MemberAccessExpression(
+                                            SyntaxKind.SimpleMemberAccessExpression,
+                                            SyntaxFactory.ThisExpression(),
+                                            OnPropertyChangedMethodName))))));
 
                     var property = SyntaxFactory.PropertyDeclaration(
                         this.GetPropertyTypeForBuilder(field),