Add Required Init For Reactive Property

src/ReactiveUI.SourceGenerator.Tests/REACTIVE/ReactiveGeneratorTests.FromReactiveProperiesWithAttributes#ReactiveUI.SourceGenerators.AccessModifier.g.verified.cs

@@ -20,6 +20,7 @@ internal enum AccessModifier
     Private,
     InternalProtected,
     PrivateProtected,
+    Init,
 }
 
 /// <summary>

src/ReactiveUI.SourceGenerator.Tests/REACTIVE/ReactiveGeneratorTests.FromReactiveProperiesWithAttributes#ReactiveUI.SourceGenerators.ReactiveAttribute.g.verified.cs

@@ -30,6 +30,11 @@ internal sealed class ReactiveAttribute : Attribute
     /// Gets the InheritanceModifier of the property.
     /// </sumary>
     public InheritanceModifier Inheritance { get; init; }
+
+    /// <summary>
+    /// Use Required attribute to indicate that the property is required.
+    /// </summary>
+    public bool UseRequired { get; init; }
 }
 #nullable restore
 #pragma warning restore

src/ReactiveUI.SourceGenerator.Tests/REACTIVE/ReactiveGeneratorTests.FromReactiveProperties#ReactiveUI.SourceGenerators.AccessModifier.g.verified.cs

@@ -20,6 +20,7 @@ internal enum AccessModifier
     Private,
     InternalProtected,
     PrivateProtected,
+    Init,
 }
 
 /// <summary>

src/ReactiveUI.SourceGenerator.Tests/REACTIVE/ReactiveGeneratorTests.FromReactiveProperties#ReactiveUI.SourceGenerators.ReactiveAttribute.g.verified.cs

@@ -30,6 +30,11 @@ internal sealed class ReactiveAttribute : Attribute
     /// Gets the InheritanceModifier of the property.
     /// </sumary>
     public InheritanceModifier Inheritance { get; init; }
+
+    /// <summary>
+    /// Use Required attribute to indicate that the property is required.
+    /// </summary>
+    public bool UseRequired { get; init; }
 }
 #nullable restore
 #pragma warning restore

src/ReactiveUI.SourceGenerator.Tests/REACTIVE/ReactiveGeneratorTests.FromReactivePropertiesWithAccess#ReactiveUI.SourceGenerators.AccessModifier.g.verified.cs

@@ -20,6 +20,7 @@ internal enum AccessModifier
     Private,
     InternalProtected,
     PrivateProtected,
+    Init,
 }
 
 /// <summary>

src/ReactiveUI.SourceGenerator.Tests/REACTIVE/ReactiveGeneratorTests.FromReactivePropertiesWithAccess#ReactiveUI.SourceGenerators.ReactiveAttribute.g.verified.cs

@@ -30,6 +30,11 @@ internal sealed class ReactiveAttribute : Attribute
     /// Gets the InheritanceModifier of the property.
     /// </sumary>
     public InheritanceModifier Inheritance { get; init; }
+
+    /// <summary>
+    /// Use Required attribute to indicate that the property is required.
+    /// </summary>
+    public bool UseRequired { get; init; }
 }
 #nullable restore
 #pragma warning restore

src/ReactiveUI.SourceGenerator.Tests/REACTIVE/ReactiveGeneratorTests.FromReactivePropertiesWithAttributesAccessAndInheritance#ReactiveUI.SourceGenerators.AccessModifier.g.verified.cs

@@ -20,6 +20,7 @@ internal enum AccessModifier
     Private,
     InternalProtected,
     PrivateProtected,
+    Init,
 }
 
 /// <summary>

src/ReactiveUI.SourceGenerator.Tests/REACTIVE/ReactiveGeneratorTests.FromReactivePropertiesWithAttributesAccessAndInheritance#ReactiveUI.SourceGenerators.ReactiveAttribute.g.verified.cs

@@ -30,6 +30,11 @@ internal sealed class ReactiveAttribute : Attribute
     /// Gets the InheritanceModifier of the property.
     /// </sumary>
     public InheritanceModifier Inheritance { get; init; }
+
+    /// <summary>
+    /// Use Required attribute to indicate that the property is required.
+    /// </summary>
+    public bool UseRequired { get; init; }
 }
 #nullable restore
 #pragma warning restore

src/ReactiveUI.SourceGenerator.Tests/REACTIVE/ReactiveGeneratorTests.FromReactivePropertiesWithIdenticalClass#ReactiveUI.SourceGenerators.AccessModifier.g.verified.cs

@@ -20,6 +20,7 @@ internal enum AccessModifier
     Private,
     InternalProtected,
     PrivateProtected,
+    Init,
 }
 
 /// <summary>

src/ReactiveUI.SourceGenerator.Tests/REACTIVE/ReactiveGeneratorTests.FromReactivePropertiesWithIdenticalClass#ReactiveUI.SourceGenerators.ReactiveAttribute.g.verified.cs

@@ -30,6 +30,11 @@ internal sealed class ReactiveAttribute : Attribute
     /// Gets the InheritanceModifier of the property.
     /// </sumary>
     public InheritanceModifier Inheritance { get; init; }
+
+    /// <summary>
+    /// Use Required attribute to indicate that the property is required.
+    /// </summary>
+    public bool UseRequired { get; init; }
 }
 #nullable restore
 #pragma warning restore

src/ReactiveUI.SourceGenerator.Tests/REACTIVE/ReactiveGeneratorTests.FromReactivePropertiesWithInit#ReactiveUI.SourceGenerators.AccessModifier.g.verified.cs

@@ -0,0 +1,37 @@
+//HintName: ReactiveUI.SourceGenerators.AccessModifier.g.cs
+// Copyright (c) 2025 .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.
+
+// <auto-generated/>
+#pragma warning disable
+#nullable enable
+namespace ReactiveUI.SourceGenerators;
+
+/// <summary>
+/// AccessModifier.
+/// </summary>
+internal enum AccessModifier
+{
+    Public,
+    Protected,
+    Internal,
+    Private,
+    InternalProtected,
+    PrivateProtected,
+    Init,
+}
+
+/// <summary>
+/// InheritanceModifier.
+/// </summary>
+internal enum InheritanceModifier
+{
+    None,
+    Virtual,
+    Override,
+    New,
+}
+#nullable restore
+#pragma warning restore

src/ReactiveUI.SourceGenerator.Tests/REACTIVE/ReactiveGeneratorTests.FromReactivePropertiesWithInit#ReactiveUI.SourceGenerators.ReactiveAttribute.g.verified.cs

@@ -0,0 +1,40 @@
+//HintName: ReactiveUI.SourceGenerators.ReactiveAttribute.g.cs
+// Copyright (c) 2025 .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 System;
+
+// <auto-generated/>
+#pragma warning disable
+#nullable enable
+namespace ReactiveUI.SourceGenerators;
+
+/// <summary>
+/// ReactiveAttribute.
+/// </summary>
+/// <seealso cref="Attribute" />
+[AttributeUsage(AttributeTargets.Field, AllowMultiple = false, Inherited = false)]
+internal sealed class ReactiveAttribute : Attribute
+{
+    /// <summary>
+    /// Gets the AccessModifier of the set property.
+    /// </summary>
+    /// <value>
+    /// The AccessModifier of the set property.
+    /// </value>
+    public AccessModifier SetModifier { get; init; }
+
+    /// <summary>
+    /// Gets the InheritanceModifier of the property.
+    /// </sumary>
+    public InheritanceModifier Inheritance { get; init; }
+
+    /// <summary>
+    /// Use Required attribute to indicate that the property is required.
+    /// </summary>
+    public bool UseRequired { get; init; }
+}
+#nullable restore
+#pragma warning restore

src/ReactiveUI.SourceGenerator.Tests/REACTIVE/ReactiveGeneratorTests.FromReactivePropertiesWithInit#TestNs.TestVM.Properties.g.verified.cs

@@ -0,0 +1,26 @@
+//HintName: TestNs.TestVM.Properties.g.cs
+// <auto-generated/>
+using ReactiveUI;
+
+#pragma warning disable
+#nullable enable
+
+namespace TestNs
+{
+    /// <summary>
+    /// Partial class for the TestVM which contains ReactiveUI Reactive property initialization.
+    /// </summary>
+    public partial class TestVM
+    {
+        /// <inheritdoc cref="_mustBeSet"/>
+        [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
+        public string MustBeSet
+        { 
+            get => _mustBeSet;
+            [global::System.Diagnostics.CodeAnalysis.MemberNotNull("_mustBeSet")]
+            set => this.RaiseAndSetIfChanged(ref _mustBeSet, value);
+        }
+    }
+}
+#nullable restore
+#pragma warning restore

src/ReactiveUI.SourceGenerator.Tests/REACTIVE/ReactiveGeneratorTests.FromReactivePropertiesWithNestedClass#ReactiveUI.SourceGenerators.AccessModifier.g.verified.cs

@@ -20,6 +20,7 @@ internal enum AccessModifier
     Private,
     InternalProtected,
     PrivateProtected,
+    Init,
 }
 
 /// <summary>

src/ReactiveUI.SourceGenerator.Tests/REACTIVE/ReactiveGeneratorTests.FromReactivePropertiesWithNestedClass#ReactiveUI.SourceGenerators.ReactiveAttribute.g.verified.cs

@@ -30,6 +30,11 @@ internal sealed class ReactiveAttribute : Attribute
     /// Gets the InheritanceModifier of the property.
     /// </sumary>
     public InheritanceModifier Inheritance { get; init; }
+
+    /// <summary>
+    /// Use Required attribute to indicate that the property is required.
+    /// </summary>
+    public bool UseRequired { get; init; }
 }
 #nullable restore
 #pragma warning restore

src/ReactiveUI.SourceGenerator.Tests/ReactiveUI.SourceGenerators.Tests.csproj

@@ -4,7 +4,7 @@
     <TargetFramework>net8.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
-
+    <LangVersion>12.0</LangVersion>
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
     <NoWarn>$(NoWarn);CA1812;CA1001</NoWarn>

src/ReactiveUI.SourceGenerator.Tests/UnitTests/ReactiveGeneratorTests.cs

@@ -245,4 +245,31 @@ public partial class TestInnerClass3 : ReactiveObject
         // Act: Initialize the helper and run the generator. Assert: Verify the generated code.
         return TestHelper.TestPass(sourceCode);
     }
+
+    /// <summary>
+    /// Tests that the source generator correctly generates reactive properties.
+    /// </summary>
+    /// <returns>A task to monitor the async.</returns>
+    [Fact]
+    public Task FromReactivePropertiesWithInit()
+    {
+        // Arrange: Setup the source code that matches the generator input expectations.
+        const string sourceCode = """
+                using System;
+                using ReactiveUI;
+                using ReactiveUI.SourceGenerators;
+                using System.Reactive.Linq;
+
+                namespace TestNs;
+
+                public partial class TestVM : ReactiveObject
+                {
+                    [Reactive(SetModifier = AccessModifier.Init, UseRequired = true)]
+                    private string _mustBeSet;
+                }
+            """;
+
+        // Act: Initialize the helper and run the generator. Assert: Verify the generated code.
+        return TestHelper.TestPass(sourceCode);
+    }
 }

src/ReactiveUI.SourceGenerators.Execute/TestViewModel.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.Diagnostics.CodeAnalysis;
 using System.Reactive;
 using System.Reactive.Disposables;
 using System.Reactive.Linq;
@@ -56,11 +57,16 @@ public partial class TestViewModel : ReactiveObject, IActivatableViewModel, IDis
     [Reactive(Inheritance = InheritanceModifier.Virtual, SetModifier = AccessModifier.Protected)]
     private string? _name;
 
+    [Reactive(SetModifier = AccessModifier.Init, UseRequired = true)]
+    private string _mustBeSet;
+
     /// <summary>
     /// Initializes a new instance of the <see cref="TestViewModel"/> class.
     /// </summary>
+    [SetsRequiredMembers]
     public TestViewModel()
     {
+        MustBeSet = "Test";
         this.WhenActivated(disposables =>
         {
             Console.Out.WriteLine("Activated");

src/ReactiveUI.SourceGenerators/AttributeDefinitions.cs

@@ -39,6 +39,7 @@ internal enum AccessModifier
             Private,
             InternalProtected,
             PrivateProtected,
+            Init,
         }
 
         /// <summary>
@@ -155,6 +156,11 @@ internal sealed class ReactiveAttribute : Attribute
     /// Gets the InheritanceModifier of the property.
     /// </sumary>
     public InheritanceModifier Inheritance { get; init; }
+
+    /// <summary>
+    /// Use Required attribute to indicate that the property is required.
+    /// </summary>
+    public bool UseRequired { get; init; }
 }
 #nullable restore
 #pragma warning restore

src/ReactiveUI.SourceGenerators/Reactive/Models/PropertyInfo.cs

@@ -20,4 +20,5 @@ internal sealed record PropertyInfo(
     bool IncludeMemberNotNullOnSetAccessor,
     EquatableArray<string> ForwardedAttributes,
     string AccessModifier,
-    string Inheritance);
+    string Inheritance,
+    string UseRequired);

src/ReactiveUI.SourceGenerators/Reactive/ReactiveGenerator.Execute.cs

@@ -67,14 +67,17 @@ public sealed partial class ReactiveGenerator
         attributeData.TryGetNamedArgument("SetModifier", out int accessModifierArgument);
         var accessModifier = accessModifierArgument switch
         {
-            1 => "protected ",
-            2 => "internal ",
-            3 => "private ",
-            4 => "internal protected ",
-            5 => "private protected ",
-            _ => string.Empty,
+            1 => "protected set",
+            2 => "internal set",
+            3 => "private set",
+            4 => "internal protected set",
+            5 => "private protected set",
+            6 => "init",
+            _ => "set",
         };
 
+        token.ThrowIfCancellationRequested();
+
         // Get Inheritance value from the attribute
         attributeData.TryGetNamedArgument("Inheritance", out int inheritanceArgument);
         var inheritance = inheritanceArgument switch
@@ -87,6 +90,12 @@ public sealed partial class ReactiveGenerator
 
         token.ThrowIfCancellationRequested();
 
+        // Get Inheritance value from the attribute
+        attributeData.TryGetNamedArgument("UseRequired", out bool useRequiredArgument);
+        var useRequired = useRequiredArgument ? "required " : string.Empty;
+
+        token.ThrowIfCancellationRequested();
+
         // Get the property type and name
         var typeNameWithNullabilityAnnotations = fieldSymbol.Type.GetFullyQualifiedNameWithNullabilityAnnotations();
         var fieldName = fieldSymbol.Name;
@@ -219,7 +228,8 @@ public sealed partial class ReactiveGenerator
             includeMemberNotNullOnSetAccessor,
             forwardedAttributesString,
             accessModifier,
-            inheritance),
+            inheritance,
+            useRequired),
             builder.ToImmutable());
     }
 
@@ -303,11 +313,11 @@ private static string GetPropertySyntax(PropertyInfo propertyInfo)
 $$"""
         /// <inheritdoc cref="{{propertyInfo.FieldName}}"/>
         {{propertyAttributes}}
-        {{propertyInfo.TargetInfo.TargetVisibility}}{{propertyInfo.Inheritance}} {{propertyInfo.TypeNameWithNullabilityAnnotations}} {{propertyInfo.PropertyName}}
+        {{propertyInfo.TargetInfo.TargetVisibility}}{{propertyInfo.Inheritance}} {{propertyInfo.UseRequired}}{{propertyInfo.TypeNameWithNullabilityAnnotations}} {{propertyInfo.PropertyName}}
         { 
             get => {{propertyInfo.FieldName}};
             [global::System.Diagnostics.CodeAnalysis.MemberNotNull("{{propertyInfo.FieldName}}")]
-            {{propertyInfo.AccessModifier}}set => this.RaiseAndSetIfChanged(ref {{propertyInfo.FieldName}}, value);
+            {{propertyInfo.AccessModifier}} => this.RaiseAndSetIfChanged(ref {{propertyInfo.FieldName}}, value);
         }
 """;
         }
@@ -316,7 +326,7 @@ private static string GetPropertySyntax(PropertyInfo propertyInfo)
 $$"""
         /// <inheritdoc cref="{{propertyInfo.FieldName}}"/>
         {{propertyAttributes}}
-        {{propertyInfo.TargetInfo.TargetVisibility}}{{propertyInfo.Inheritance}} {{propertyInfo.TypeNameWithNullabilityAnnotations}} {{propertyInfo.PropertyName}} { get => {{propertyInfo.FieldName}}; {{propertyInfo.AccessModifier}}set => this.RaiseAndSetIfChanged(ref {{propertyInfo.FieldName}}, value); }
+        {{propertyInfo.TargetInfo.TargetVisibility}}{{propertyInfo.Inheritance}} {{propertyInfo.UseRequired}}{{propertyInfo.TypeNameWithNullabilityAnnotations}} {{propertyInfo.PropertyName}} { get => {{propertyInfo.FieldName}}; {{propertyInfo.AccessModifier}} => this.RaiseAndSetIfChanged(ref {{propertyInfo.FieldName}}, value); }
 """;
     }