Fix ReadOnly for OAPH

src/ReactiveUI.SourceGenerator.Tests/OAPH/OAPFromObservableGeneratorTests.FromPartialProperty#TestNs.TestVM.ObservableAsPropertyFromObservable.g.verified.cs

@@ -16,7 +16,7 @@ public partial class TestVM
         private double? _testProperty;
 
         /// <inheritdoc cref="_testPropertyHelper"/>
-        private ReactiveUI.ObservableAsPropertyHelper<double?>? _testPropertyHelper;
+        private readonly ReactiveUI.ObservableAsPropertyHelper<double?> _testPropertyHelper;
 
         /// <inheritdoc cref="_testProperty"/>
         [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]

src/ReactiveUI.SourceGenerators.Execute/TestClassOAPH_VM.cs

@@ -0,0 +1,51 @@
+// Copyright (c) 2024 .NET Foundation and Contributors. All rights reserved.
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for full license information.
+
+using ReactiveUI;
+using ReactiveUI.SourceGenerators;
+
+namespace SGReactiveUI.SourceGenerators.Test;
+
+/// <summary>
+/// TestClassOAPH VM.
+/// </summary>
+public partial class TestClassOAPH_VM : ReactiveObject
+{
+    [ObservableAsProperty]
+    private bool _observableTestField;
+
+    [Reactive]
+    private bool _reactiveTestField;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="TestClassOAPH_VM"/> class.
+    /// </summary>
+    public TestClassOAPH_VM()
+    {
+        _observableTestPropertyHelper = this.WhenAnyValue(x => x.ReactiveTestProperty)
+            .ToProperty(this, x => x.ObservableTestProperty);
+
+        _observableTestFieldHelper = this.WhenAnyValue(x => x.ReactiveTestField)
+            .ToProperty(this, x => x.ObservableTestField);
+    }
+
+    /// <summary>
+    /// Gets a value indicating whether [observable test property].
+    /// </summary>
+    /// <value>
+    ///   <c>true</c> if [observable test property]; otherwise, <c>false</c>.
+    /// </value>
+    [ObservableAsProperty]
+    public partial bool ObservableTestProperty { get; }
+
+    /// <summary>
+    /// Gets or sets a value indicating whether [reactive test property].
+    /// </summary>
+    /// <value>
+    ///   <c>true</c> if [reactive test property]; otherwise, <c>false</c>.
+    /// </value>
+    [Reactive]
+    public partial bool ReactiveTestProperty { get; set; }
+}

src/ReactiveUI.SourceGenerators.Execute/TestViewModel.cs

@@ -70,6 +70,10 @@ public partial class TestViewModel : ReactiveObject, IActivatableViewModel, IDis
 
     [Reactive]
     private IEnumerable<Person> _people = [new Person()];
+    [Reactive]
+    private double? _myDoubleProperty;
+    [Reactive]
+    private double _myDoubleNonNullProperty;
 
     [BindableDerivedList]
     private ReadOnlyObservableCollection<Person>? _visiblePeople;

src/ReactiveUI.SourceGenerators.Roslyn/ObservableAsProperty/Models/ObservableMethodInfo.cs

@@ -19,6 +19,7 @@ internal record ObservableMethodInfo(
         bool IsNullableType,
         bool IsProperty,
         EquatableArray<string> ForwardedPropertyAttributes,
+        string IsReadOnly,
         string AccessModifier,
         string? InitialValue)
     {

src/ReactiveUI.SourceGenerators.Roslyn/ObservableAsProperty/ObservableAsPropertyGenerator{FromObservable}.Execute.cs

@@ -41,6 +41,11 @@ public sealed partial class ObservableAsPropertyGenerator
         attributeData.TryGetNamedArgument("UseProtected", out bool useProtected);
         var useProtectedModifier = useProtected ? "protected" : "private";
 
+        token.ThrowIfCancellationRequested();
+
+        // Get the can ReadOnly member, if any
+        attributeData.TryGetNamedArgument("ReadOnly", out bool? isReadonly);
+
         token.ThrowIfCancellationRequested();
         var compilation = context.SemanticModel.Compilation;
 
@@ -106,6 +111,7 @@ public sealed partial class ObservableAsPropertyGenerator
                 isNullableType,
                 false,
                 propertyAttributes,
+                string.Empty,
                 useProtectedModifier,
                 initialValue),
                 diagnostics.ToImmutable());
@@ -131,6 +137,7 @@ public sealed partial class ObservableAsPropertyGenerator
 
             var observableType = string.Empty;
             var isNullableType = false;
+            var isPartialProperty = false;
 
             token.ThrowIfCancellationRequested();
             context.GetForwardedAttributes(
@@ -173,10 +180,9 @@ public sealed partial class ObservableAsPropertyGenerator
 
                 token.ThrowIfCancellationRequested();
 
-                var inheritance = propertySymbol.IsVirtual ? " virtual" : propertySymbol.IsOverride ? " override" : string.Empty;
+                isPartialProperty = true;
 
-                // Get the can ReadOnly member, if any
-                attributeData.TryGetNamedArgument("ReadOnly", out bool? isReadonly);
+                var inheritance = propertySymbol.IsVirtual ? " virtual" : propertySymbol.IsOverride ? " override" : string.Empty;
 
                 token.ThrowIfCancellationRequested();
 
@@ -215,6 +221,12 @@ public sealed partial class ObservableAsPropertyGenerator
             }
 #endif
 
+            var isReadOnlyString = string.Empty;
+            if (isPartialProperty)
+            {
+                isReadOnlyString = isReadonly == false ? string.Empty : "readonly";
+            }
+
             // Get the containing type info
             var targetInfo = TargetInfo.From(propertySymbol.ContainingType);
 
@@ -229,6 +241,7 @@ public sealed partial class ObservableAsPropertyGenerator
                 isNullableType,
                 true,
                 propertyAttributes,
+                isReadOnlyString,
                 useProtectedModifier,
                 initialValue),
                 diagnostics.ToImmutable());
@@ -307,12 +320,20 @@ private static string GetPropertySyntax(ObservableMethodInfo propertyInfo)
             propertyType = propertyInfo.PartialPropertyType;
         }
 
+        var helperTypeName = $"{propertyInfo.AccessModifier} ReactiveUI.ObservableAsPropertyHelper<{propertyType}>?";
+
+        // If the property is readonly, we need to change the helper to be non-nullable
+        if (propertyInfo.IsReadOnly == "readonly")
+        {
+            helperTypeName = $"{propertyInfo.AccessModifier} readonly ReactiveUI.ObservableAsPropertyHelper<{propertyType}>";
+        }
+
         return $$"""
 /// <inheritdoc cref="{{propertyInfo.PropertyName}}"/>
         private {{propertyType}} {{getterFieldIdentifierName}}{{initialValue}};
 
         /// <inheritdoc cref="{{getterFieldIdentifierName}}Helper"/>
-        {{propertyInfo.AccessModifier}} ReactiveUI.ObservableAsPropertyHelper<{{propertyType}}>? {{getterFieldIdentifierName}}Helper;
+        {{helperTypeName}} {{getterFieldIdentifierName}}Helper;
 
         /// <inheritdoc cref="{{getterFieldIdentifierName}}"/>
         {{propertyAttributes}}