Fix for OAPH can't be created outside of constructor

README.md

@@ -111,24 +111,29 @@ public partial class MyReactiveClass : ReactiveObject
 }
 ```
 
+### Usage ObservableAsPropertyHelper with Field and non readonly nullable OAPH field
+```csharp
+
 ### Usage ObservableAsPropertyHelper with Observable Property
 ```csharp
 using ReactiveUI.SourceGenerators;
 
-public partial class MyReactiveClass : ReactiveObject
-{    
+public partial class MyReactiveClass : ReactiveObject, IActivatableViewModel
+{
+    [ObservableAsProperty(ReadOnly = false)]
+    private string _myProperty = "Default Value";
+
     public MyReactiveClass()
     {
-        // default value for MyObservableProperty prior to initialization.
-        _myObservable = "Test Value Pre Init";
-
-        // Initialize generated _myObservablePropertyHelper
-        // for the generated MyObservableProperty
-        InitializeOAPH();
+        this.WhenActivated(disposables =>
+        {
+            _myPrpertyHelper = MyPropertyObservable()
+                .ToProperty(this, x => x.MyProperty)
+                .DisposeWith(disposables);
+        });
     }
 
-    [ObservableAsProperty]
-    IObservable<string> MyObservable => Observable.Return("Test Value");
+    IObservable<string> MyPropertyObservable() => Observable.Return("Test Value");
 }
 ```
 

src/ReactiveUI.SourceGenerators.Execute/TestViewModel.cs

@@ -4,6 +4,7 @@
 // See the LICENSE file in the project root for full license information.
 
 using System.Reactive;
+using System.Reactive.Disposables;
 using System.Reactive.Linq;
 using System.Reactive.Subjects;
 using System.Runtime.Serialization;
@@ -16,8 +17,11 @@ namespace SGReactiveUI.SourceGenerators.Test;
 /// <summary>
 /// TestClass.
 /// </summary>
+/// <seealso cref="ReactiveUI.ReactiveObject" />
+/// <seealso cref="ReactiveUI.IActivatableViewModel" />
+/// <seealso cref="System.IDisposable" />
 [DataContract]
-public partial class TestViewModel : ReactiveObject, IDisposable
+public partial class TestViewModel : ReactiveObject, IActivatableViewModel, IDisposable
 {
     private readonly IObservable<bool> _observable = Observable.Return(true);
     private readonly Subject<double?> _testSubject = new();
@@ -28,6 +32,12 @@ public partial class TestViewModel : ReactiveObject, IDisposable
     [ObservableAsProperty]
     private double? _test2Property = 1.1d;
 
+    [ObservableAsProperty(ReadOnly = false)]
+    private double? _test11Property = 11.1d;
+
+    [Reactive]
+    private double? _test12Property = 12.1d;
+
     [JsonInclude]
     [Reactive(SetModifier = AccessModifier.Protected)]
     [DataMember]
@@ -39,6 +49,12 @@ public partial class TestViewModel : ReactiveObject, IDisposable
     /// </summary>
     public TestViewModel()
     {
+        this.WhenActivated(disposables =>
+        {
+            Console.Out.WriteLine("Activated");
+            _test11PropertyHelper = this.WhenAnyValue(x => x.Test12Property).ToProperty(this, x => x.Test11Property, out _).DisposeWith(disposables);
+        });
+
         Console.Out.WriteLine("MyReadOnlyProperty before init");
 
         // only settable prior to init, after init it will be ignored.
@@ -201,6 +217,11 @@ public TestViewModel()
     [ObservableAsProperty]
     public IObservable<int> ObservableAsPropertyTest2 => Observable.Return(9);
 
+    /// <summary>
+    /// Gets the Activator which will be used by the View when Activation/Deactivation occurs.
+    /// </summary>
+    public ViewModelActivator Activator { get; } = new();
+
     /// <summary>
     /// Gets observables as property test.
     /// </summary>

src/ReactiveUI.SourceGenerators/Core/Helpers/AttributeDefinitions.cs

@@ -161,6 +161,14 @@ internal sealed class ObservableAsPropertyAttribute : Attribute
     /// The name of the property.
     /// </value>
     public string? PropertyName { get; init; }
+
+    /// <summary>
+    /// Gets the Readonly state of the OAPH property.
+    /// </summary>
+    /// <value>
+    /// The is read only of the OAPH property.
+    /// </value>
+    public bool ReadOnly { get; init; } = true;
 }
 #nullable restore
 #pragma warning restore

src/ReactiveUI.SourceGenerators/ObservableAsProperty/ObservableAsPropertyGenerator.Execute.cs

@@ -3,6 +3,7 @@
 // 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 System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
@@ -79,6 +80,19 @@ internal static ImmutableArray<MemberDeclarationSyntax> GetPropertySyntax(Proper
                 .Select(static a => AttributeList(SingletonSeparatedList(a.GetSyntax())))
                 .ToImmutableArray();
 
+            var modifiers = new List<SyntaxToken>();
+            var helperTypeName = $"ReactiveUI.ObservableAsPropertyHelper<{propertyType}>";
+            if (propertyInfo.AccessModifier == "readonly")
+            {
+                modifiers.Add(Token(SyntaxKind.PrivateKeyword));
+                modifiers.Add(Token(SyntaxKind.ReadOnlyKeyword));
+            }
+            else
+            {
+                helperTypeName = $"ReactiveUI.ObservableAsPropertyHelper<{propertyType}>?";
+                modifiers.Add(Token(SyntaxKind.PrivateKeyword));
+            }
+
             // Construct the generated property as follows:
             //
             // /// <inheritdoc cref="<FIELD_NAME>"/>
@@ -91,7 +105,7 @@ internal static ImmutableArray<MemberDeclarationSyntax> GetPropertySyntax(Proper
             // }
             return
                 ImmutableArray.Create<MemberDeclarationSyntax>(
-                    FieldDeclaration(VariableDeclaration(ParseTypeName($"ReactiveUI.ObservableAsPropertyHelper<{propertyType}>")))
+                    FieldDeclaration(VariableDeclaration(ParseTypeName(helperTypeName)))
                         .AddDeclarationVariables(VariableDeclarator(getterFieldIdentifierName + "Helper"))
                         .AddAttributeLists(
                             AttributeList(SingletonSeparatedList(
@@ -100,9 +114,7 @@ internal static ImmutableArray<MemberDeclarationSyntax> GetPropertySyntax(Proper
                                     AttributeArgument(LiteralExpression(SyntaxKind.StringLiteralExpression, Literal(typeof(ObservableAsPropertyGenerator).FullName))),
                                     AttributeArgument(LiteralExpression(SyntaxKind.StringLiteralExpression, Literal(typeof(ObservableAsPropertyGenerator).Assembly.GetName().Version.ToString()))))))
                             .WithOpenBracketToken(Token(TriviaList(Comment($"/// <inheritdoc cref=\"{propertyInfo.FieldName + "Helper"}\"/>")), SyntaxKind.OpenBracketToken, TriviaList())))
-                            .AddModifiers(
-                                Token(SyntaxKind.PrivateKeyword),
-                                Token(SyntaxKind.ReadOnlyKeyword)),
+                            .AddModifiers([.. modifiers]),
                     PropertyDeclaration(propertyType, Identifier(propertyInfo.PropertyName))
                         .AddAttributeLists(
                             AttributeList(SingletonSeparatedList(
@@ -124,6 +136,7 @@ internal static bool GetFieldInfoFromClass(
             FieldDeclarationSyntax fieldSyntax,
             IFieldSymbol fieldSymbol,
             SemanticModel semanticModel,
+            bool? isReadonly,
             CancellationToken token,
             [NotNullWhen(true)] out PropertyInfo? propertyInfo,
             out ImmutableArray<DiagnosticInfo> diagnostics)
@@ -282,7 +295,7 @@ internal static bool GetFieldInfoFromClass(
                 isReferenceTypeOrUnconstraindTypeParameter,
                 includeMemberNotNullOnSetAccessor,
                 forwardedAttributes.ToImmutable(),
-                "public");
+                isReadonly == false ? string.Empty : "readonly");
 
             diagnostics = builder.ToImmutable();
 

src/ReactiveUI.SourceGenerators/ObservableAsProperty/ObservableAsPropertyGenerator.cs

@@ -35,6 +35,18 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
                 static (node, _) => node is VariableDeclaratorSyntax { Parent: VariableDeclarationSyntax { Parent: FieldDeclarationSyntax { Parent: ClassDeclarationSyntax or RecordDeclarationSyntax, AttributeLists.Count: > 0 } } },
                 static (context, token) =>
                 {
+                    var symbol = ModelExtensions.GetDeclaredSymbol(context.SemanticModel, context.TargetNode, token)!;
+                    token.ThrowIfCancellationRequested();
+
+                    // Skip symbols without the target attribute
+                    if (!symbol.TryGetAttributeWithFullyQualifiedMetadataName(AttributeDefinitions.ObservableAsPropertyAttributeType, out var attributeData))
+                    {
+                        return default;
+                    }
+
+                    // Get the can PropertyName member, if any
+                    attributeData.TryGetNamedArgument("ReadOnly", out bool? isReadonly);
+
                     var fieldDeclaration = (FieldDeclarationSyntax)context.TargetNode.Parent!.Parent!;
                     var fieldSymbol = (IFieldSymbol)context.TargetSymbol;
 
@@ -43,7 +55,7 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
 
                     token.ThrowIfCancellationRequested();
 
-                    Execute.GetFieldInfoFromClass(fieldDeclaration, fieldSymbol, context.SemanticModel, token, out var propertyInfo, out var diagnostics);
+                    Execute.GetFieldInfoFromClass(fieldDeclaration, fieldSymbol, context.SemanticModel, isReadonly, token, out var propertyInfo, out var diagnostics);
 
                     token.ThrowIfCancellationRequested();
                     return (Hierarchy: hierarchy, new Result<PropertyInfo?>(propertyInfo, diagnostics));